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Amphibians are four-limbed and
ectotherm An ectotherm (from the Greek () "outside" and () "heat") is an organism in which internal physiological sources of heat are of relatively small or of quite negligible importance in controlling body temperature.Davenport, John. Animal Life a ...
ic vertebrates of the
class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Class (philosophy), an analytical concept used differently ...
Amphibia. All living amphibians belong to the group
Lissamphibia The Lissamphibia is a group of tetrapods that includes all modern amphibians. Lissamphibians consist of three living groups: the Salientia (frogs, toads, and their extinct relatives), the Caudata (salamanders, newts, and their extinct relatives), ...
. They inhabit a wide variety of habitats, with most species living within terrestrial, fossorial,
arboreal Arboreal locomotion is the locomotion of animals in trees. In habitats in which trees are present, animals have evolved to move in them. Some animals may scale trees only occasionally, but others are exclusively arboreal. The habitats pose nu ...
or freshwater
aquatic ecosystems An aquatic ecosystem is an ecosystem formed by surrounding a body of water, in contrast to land-based terrestrial ecosystems. Aquatic ecosystems contain communities of organisms that are dependent on each other and on their environment. The tw ...
. Thus amphibians typically start out as larvae living in water, but some species have developed behavioural adaptations to bypass this. The young generally undergo metamorphosis from larva with gills to an adult air-breathing form with lungs. Amphibians use their skin as a secondary respiratory surface and some small terrestrial salamanders and frogs lack lungs and rely entirely on their skin. They are superficially similar to reptiles like
lizard Lizards are a widespread group of squamate reptiles, with over 7,000 species, ranging across all continents except Antarctica, as well as most oceanic island chains. The group is paraphyletic since it excludes the snakes and Amphisbaenia althou ...
s but, along with mammals and birds, reptiles are
amniote Amniotes are a clade of tetrapod vertebrates that comprises sauropsids (including all reptiles and birds, and extinct parareptiles and non-avian dinosaurs) and synapsids (including pelycosaurs and therapsids such as mammals). They are distin ...
s and do not require water bodies in which to breed. With their complex reproductive needs and permeable skins, amphibians are often
ecological indicator Ecological indicators are used to communicate information about ecosystems and the impact human activity has on ecosystems to groups such as the public or government policy makers. Ecosystems are complex and ecological indicators can help describe ...
s; in recent decades there has been a dramatic
decline in amphibian populations Since the 1980s, decreases in amphibian populations, including population decline and localized mass extinctions, have been observed in locations all over the world. These declines are known as one of the most critical threats to global biodivers ...
for many species around the globe. The earliest amphibians
evolved Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variati ...
in the Devonian period from
sarcopterygian Sarcopterygii (; ) — sometimes considered synonymous with Crossopterygii () — is a taxon (traditionally a class or subclass) of the bony fishes known as the lobe-finned fishes. The group Tetrapoda, a mostly terrestrial superclass includ ...
fish with lungs and bony-limbed fins, features that were helpful in adapting to dry land. They diversified and became dominant during the
Carboniferous The Carboniferous ( ) is a geologic period and system of the Paleozoic that spans 60 million years from the end of the Devonian Period million years ago ( Mya), to the beginning of the Permian Period, million years ago. The name ''Carboniferous ...
and Permian periods, but were later displaced by reptiles and other vertebrates. The origin of modern amphibians belonging to Lissamphibia, which first appeared during the Early Triassic, around 250 million years ago, has long been contentious. However the emerging consensus is that they likely originated from
temnospondyls Temnospondyli (from Greek τέμνειν, ''temnein'' 'to cut' and σπόνδυλος, ''spondylos'' 'vertebra') is a diverse order of small to giant tetrapods—often considered primitive amphibians—that flourished worldwide during the Carb ...
, the most diverse group of prehistoric amphibians, during the Permian period. The three modern orders of amphibians are Anura (the frogs),
Urodela Salamanders are a group of amphibians typically characterized by their lizard-like appearance, with slender bodies, blunt snouts, short limbs projecting at right angles to the body, and the presence of a tail in both larvae and adults. All ten ...
(the salamanders), and Apoda (the caecilians), a fourth group, the
Albanerpetontidae The Albanerpetontidae are an extinct family of small amphibians, native to the Northern Hemisphere during the Mesozoic and Cenozoic. The only members of the order Allocaudata, they are thought to be allied with living amphibians belonging to Lissa ...
, became extinct around 2 million years ago. The number of known amphibian species is approximately 8,000, of which nearly 90% are frogs. The smallest amphibian (and vertebrate) in the world is a frog from New Guinea (''
Paedophryne amauensis ''Paedophryne amauensis'' is a species of microhylid frog endemic to eastern Papua New Guinea. At in snout-to-vent length, it is considered the world's smallest known vertebrate. (See also Ecological guild.) The species was listed in the ...
'') with a length of just . The largest living amphibian is the South China giant salamander (''Andrias sligoi''), but this is dwarfed by prehistoric temnospondyls such as '' Mastodonsaurus'' which could reach up to 6 metres in length. The study of amphibians is called
batrachology Batrachology is the branch of zoology concerned with the study of amphibians including frogs and toads, salamanders, newts, and caecilians. It is a sub-discipline of herpetology, which also includes non-avian reptiles (snakes, lizards, amphisbaen ...
, while the study of both reptiles and amphibians is called herpetology.


Classification

The word ''amphibian'' is derived from the Ancient Greek term (), which means 'both kinds of life', meaning 'of both kinds' and meaning 'life'. The term was initially used as a general adjective for animals that could live on land or in water, including seals and otters. Traditionally, the class Amphibia includes all tetrapod vertebrates that are not amniotes. Amphibia in its widest sense () was divided into three
subclasses Subclass may refer to: * Subclass (taxonomy), a taxonomic rank below "class" * Subclass (computer science) * Subclass (set theory) In set theory and its applications throughout mathematics, a subclass is a class contained in some other class in ...
, two of which are extinct: *Subclass
Lepospondyli Lepospondyli is a diverse taxon of early tetrapods. With the exception of one late-surviving lepospondyl from the Late Permian of Morocco ('' Diplocaulus minumus''), lepospondyls lived from the Early Carboniferous ( Mississippian) to the Early ...
† (small Paleozoic group, which are more closely related to amniotes than Lissamphibia) * Subclass
Temnospondyli Temnospondyli (from Greek τέμνειν, ''temnein'' 'to cut' and σπόνδυλος, ''spondylos'' 'vertebra') is a diverse order of small to giant tetrapods—often considered primitive amphibians—that flourished worldwide during the Carb ...
† (diverse Paleozoic and early Mesozoic grade) * Subclass
Lissamphibia The Lissamphibia is a group of tetrapods that includes all modern amphibians. Lissamphibians consist of three living groups: the Salientia (frogs, toads, and their extinct relatives), the Caudata (salamanders, newts, and their extinct relatives), ...
(all modern amphibians, including frogs, toads, salamanders, newts and caecilians) ** Salientia (frogs, toads and relatives): Jurassic to present—7,360 current species in 53 families ** Caudata ( salamanders, newts and relatives): Jurassic to present—764 current species in 9 families ** Gymnophiona (
caecilian Caecilians (; ) are a group of limbless, vermiform or serpentine amphibians. They mostly live hidden in the ground and in stream substrates, making them the least familiar order of amphibians. Caecilians are mostly distributed in the tropics of ...
s and relatives): Jurassic to present—215 current species in 10 families **Allocaudata† (
Albanerpetontidae The Albanerpetontidae are an extinct family of small amphibians, native to the Northern Hemisphere during the Mesozoic and Cenozoic. The only members of the order Allocaudata, they are thought to be allied with living amphibians belonging to Lissa ...
) Middle Jurassic – Early Pleistocene The actual number of species in each group depends on the taxonomic classification followed. The two most common systems are the classification adopted by the website AmphibiaWeb, University of California, Berkeley, and the classification by
herpetologist Herpetology (from Greek ἑρπετόν ''herpetón'', meaning "reptile" or "creeping animal") is the branch of zoology concerned with the study of amphibians (including frogs, toads, salamanders, newts, and caecilians (gymnophiona)) and rept ...
Darrel Frost Darrel Richmond Frost (born 1951) is an American herpetologist and systematist. He was previously head curator of herpetology at the American Museum of Natural History, as well as president of both the Society for the Study of Amphibians and Rep ...
and the
American Museum of Natural History The American Museum of Natural History (abbreviated as AMNH) is a natural history museum on the Upper West Side of Manhattan in New York City. In Theodore Roosevelt Park, across the street from Central Park, the museum complex comprises 26 inte ...
, available as the online reference database "Amphibian Species of the World". The numbers of species cited above follows Frost and the total number of known amphibian species as of March 31, 2019, is exactly 8,000, of which nearly 90% are frogs. With the
phylogenetic In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups o ...
classification, the taxon
Labyrinthodontia "Labyrinthodontia" (Greek, 'maze-toothed') is an informal grouping of extinct predatory amphibians which were major components of ecosystems in the late Paleozoic and early Mesozoic eras (about 390 to 150 million years ago). Traditionally conside ...
has been discarded as it is a polyparaphyletic group without unique defining features apart from shared primitive characteristics. Classification varies according to the preferred phylogeny of the author and whether they use a stem-based or a node-based classification. Traditionally, amphibians as a class are defined as all tetrapods with a larval stage, while the group that includes the common ancestors of all living amphibians (frogs, salamanders and caecilians) and all their descendants is called Lissamphibia. The phylogeny of Paleozoic amphibians is uncertain, and Lissamphibia may possibly fall within extinct groups, like the Temnospondyli (traditionally placed in the subclass Labyrinthodontia) or the Lepospondyli, and in some analyses even in the amniotes. This means that advocates of phylogenetic nomenclature have removed a large number of basal Devonian and Carboniferous amphibian-type tetrapod groups that were formerly placed in Amphibia in Linnaean taxonomy, and included them elsewhere under
cladistic taxonomy Cladistics (; ) is an approach to biological classification in which organisms are categorized in groups ("clades") based on hypotheses of most recent common ancestry. The evidence for hypothesized relationships is typically shared derived char ...
. If the common ancestor of amphibians and amniotes is included in Amphibia, it becomes a paraphyletic group. All modern amphibians are included in the subclass Lissamphibia, which is usually considered a clade, a group of species that have evolved from a common ancestor. The three modern orders are Anura (the frogs), Caudata (or Urodela, the salamanders), and Gymnophiona (or Apoda, the caecilians). It has been suggested that salamanders arose separately from a Temnospondyl-like ancestor, and even that caecilians are the sister group of the advanced reptiliomorph amphibians, and thus of amniotes. Although the fossils of several older proto-frogs with primitive characteristics are known, the oldest "true frog" is ''
Prosalirus bitis ''Prosalirus'' is the name given to a fossilised prehistoric frog found in the Kayenta Formation of Arizona in 1981 by Farish Jenkins.''Geographica''; "Jurassic Frog hops into the record book", National Geographic; August 1996, pg.5. The type, a ...
'', from the
Early Jurassic The Early Jurassic Epoch (in chronostratigraphy corresponding to the Lower Jurassic Series) is the earliest of three epochs of the Jurassic Period. The Early Jurassic starts immediately after the Triassic-Jurassic extinction event, 201.3 Ma&nb ...
Kayenta Formation The Kayenta Formation is a geological formation in the Glen Canyon Group that is spread across the Colorado Plateau province of the United States, including northern Arizona, northwest Colorado, Nevada, and Utah. Traditionally has been suggested ...
of Arizona. It is anatomically very similar to modern frogs. The oldest known caecilian is another Early Jurassic species, ''
Eocaecilia micropodia ''Eocaecilia'' is an extinct genus of gymnophionan amphibian from the early Jurassic Kayenta Formation of Arizona, United States. One species is described, ''Eocaecilia micropodia''. ''Eocaecilia'' shared some characteristics with salamanders ...
'', also from Arizona. The earliest salamander is ''
Beiyanerpeton jianpingensis ''Beiyanerpeton'' is an extinct genus of salamandroid amphibians known from the Late Jurassic of western Liaoning Province, China. It contains a single species, ''B. jianpingensis''. Discovery ''Beiyanerpeton'' is known from the holotype speci ...
'' from the Late Jurassic of northeastern China. Authorities disagree as to whether Salientia is a superorder that includes the order Anura, or whether Anura is a sub-order of the order Salientia. The Lissamphibia are traditionally divided into three
orders Order, ORDER or Orders may refer to: * Categorization, the process in which ideas and objects are recognized, differentiated, and understood * Heterarchy, a system of organization wherein the elements have the potential to be ranked a number of ...
, but an extinct salamander-like family, the Albanerpetontidae, is now considered part of Lissamphibia alongside the superorder Salientia. Furthermore, Salientia includes all three recent orders plus the Triassic proto-frog, ''
Triadobatrachus ''Triadobatrachus'' is an extinct genus of salientian frog-like amphibians, including only one known species, ''Triadobatrachus massinoti''. It is the oldest member of the frog lineage known, and an excellent example of a transitional fossil. It ...
''.


Evolutionary history

The first major groups of amphibians developed in the Devonian period, around 370 million years ago, from lobe-finned fish which were similar to the modern
coelacanth The coelacanths ( ) are fish belonging to the order Actinistia that includes two extant species in the genus ''Latimeria'': the West Indian Ocean coelacanth (''Latimeria chalumnae''), primarily found near the Comoro Islands off the east coast ...
and lungfish. These ancient lobe-finned fish had evolved multi-jointed leg-like fins with digits that enabled them to crawl along the sea bottom. Some fish had developed primitive lungs that help them breathe air when the stagnant pools of the Devonian swamps were low in oxygen. They could also use their strong fins to hoist themselves out of the water and onto dry land if circumstances so required. Eventually, their bony fins would evolve into limbs and they would become the ancestors to all
tetrapod Tetrapods (; ) are four-limbed vertebrate animals constituting the superclass Tetrapoda (). It includes extant and extinct amphibians, sauropsids (reptiles, including dinosaurs and therefore birds) and synapsids (pelycosaurs, extinct therapsid ...
s, including modern amphibians, reptiles, birds, and mammals. Despite being able to crawl on land, many of these prehistoric
tetrapodomorph The Tetrapodomorpha (also known as Choanata) are a clade of vertebrates consisting of tetrapods (four-limbed vertebrates) and their closest sarcopterygian relatives that are more closely related to living tetrapods than to living lungfish. Advan ...
fish still spent most of their time in the water. They had started to develop lungs, but still breathed predominantly with gills. Many examples of species showing transitional features have been discovered. ''
Ichthyostega ''Ichthyostega'' (from el, ἰχθῦς , 'fish' and el, στέγη , 'roof') is an extinct genus of limbed tetrapodomorphs from the Late Devonian of Greenland. It was among the earliest four-limbed vertebrates in the fossil record, and was on ...
'' was one of the first primitive amphibians, with nostrils and more efficient lungs. It had four sturdy limbs, a neck, a tail with fins and a skull very similar to that of the lobe-finned fish, ''
Eusthenopteron ''Eusthenopteron'' (from el, εὖ , 'good', el, σθένος , 'strength', and el, πτερόν 'wing' or 'fin') is a genus of prehistoric sarcopterygian (often called lobe-finned fishes) which has attained an iconic status from its clos ...
''. Amphibians evolved adaptations that allowed them to stay out of the water for longer periods. Their lungs improved and their skeletons became heavier and stronger, better able to support the weight of their bodies on land. They developed "hands" and "feet" with five or more digits; the skin became more capable of retaining body fluids and resisting desiccation. The fish's
hyomandibula The hyomandibula, commonly referred to as hyomandibular one( la, os hyomandibulare, from el, hyoeides, "upsilon-shaped" (υ), and Latin: mandibula, "jawbone") is a set of bones that is found in the hyoid region in most fishes. It usually plays ...
bone in the
hyoid The hyoid bone (lingual bone or tongue-bone) () is a horseshoe-shaped bone situated in the anterior midline of the neck between the chin and the thyroid cartilage. At rest, it lies between the base of the mandible and the third cervical vertebra ...
region behind the gills diminished in size and became the stapes of the amphibian ear, an adaptation necessary for hearing on dry land. An affinity between the amphibians and the teleost fish is the multi-folded structure of the teeth and the paired supra-occipital bones at the back of the head, neither of these features being found elsewhere in the animal kingdom. At the end of the Devonian period (360 million years ago), the seas, rivers and lakes were teeming with life while the land was the realm of early plants and devoid of vertebrates, though some, such as ''Ichthyostega'', may have sometimes hauled themselves out of the water. It is thought they may have propelled themselves with their forelimbs, dragging their hindquarters in a similar manner to that used by the elephant seal. In the early
Carboniferous The Carboniferous ( ) is a geologic period and system of the Paleozoic that spans 60 million years from the end of the Devonian Period million years ago ( Mya), to the beginning of the Permian Period, million years ago. The name ''Carboniferous ...
(360 to 345 million years ago), the climate became wet and warm. Extensive swamps developed with mosses, ferns,
horsetail ''Equisetum'' (; horsetail, snake grass, puzzlegrass) is the only living genus in Equisetaceae, a family of ferns, which reproduce by spores rather than seeds. ''Equisetum'' is a "living fossil", the only living genus of the entire subclass E ...
s and
calamites ''Calamites'' is a genus of extinct arborescent (tree-like) horsetails to which the modern horsetails (genus '' Equisetum'') are closely related. Unlike their herbaceous modern cousins, these plants were medium-sized trees, growing to heights o ...
. Air-breathing
arthropod Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and cuticle made of chitin, o ...
s evolved and invaded the land where they provided food for the
carnivorous A carnivore , or meat-eater (Latin, ''caro'', genitive ''carnis'', meaning meat or "flesh" and ''vorare'' meaning "to devour"), is an animal or plant whose food and energy requirements derive from animal tissues (mainly muscle, fat and other so ...
amphibians that began to adapt to the terrestrial environment. There were no other tetrapods on the land and the amphibians were at the top of the food chain, occupying the ecological position currently held by the crocodile. Though equipped with limbs and the ability to breathe air, most still had a long tapering body and strong tail. They were the top land predators, sometimes reaching several metres in length, preying on the large insects of the period and the many types of fish in the water. They still needed to return to water to lay their shell-less eggs, and even most modern amphibians have a fully aquatic larval stage with gills like their fish ancestors. It was the development of the amniotic egg, which prevents the developing embryo from drying out, that enabled the reptiles to reproduce on land and which led to their dominance in the period that followed. After the Carboniferous rainforest collapse amphibian dominance gave way to reptiles, and amphibians were further devastated by the Permian–Triassic extinction event. During the
Triassic Period The Triassic ( ) is a geologic period and system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.36 Mya. The Triassic is the first and shortest period ...
(250 to 200 million years ago), the reptiles continued to out-compete the amphibians, leading to a reduction in both the amphibians' size and their importance in the
biosphere The biosphere (from Greek βίος ''bíos'' "life" and σφαῖρα ''sphaira'' "sphere"), also known as the ecosphere (from Greek οἶκος ''oîkos'' "environment" and σφαῖρα), is the worldwide sum of all ecosystems. It can also be ...
. According to the fossil record,
Lissamphibia The Lissamphibia is a group of tetrapods that includes all modern amphibians. Lissamphibians consist of three living groups: the Salientia (frogs, toads, and their extinct relatives), the Caudata (salamanders, newts, and their extinct relatives), ...
, which includes all modern amphibians and is the only surviving lineage, may have branched off from the extinct groups
Temnospondyli Temnospondyli (from Greek τέμνειν, ''temnein'' 'to cut' and σπόνδυλος, ''spondylos'' 'vertebra') is a diverse order of small to giant tetrapods—often considered primitive amphibians—that flourished worldwide during the Carb ...
and
Lepospondyli Lepospondyli is a diverse taxon of early tetrapods. With the exception of one late-surviving lepospondyl from the Late Permian of Morocco ('' Diplocaulus minumus''), lepospondyls lived from the Early Carboniferous ( Mississippian) to the Early ...
at some period between the Late Carboniferous and the Early Triassic. The relative scarcity of fossil evidence precludes precise dating, but the most recent molecular study, based on
multilocus sequence typing Multilocus sequence typing (MLST) is a technique in molecular biology for the typing of multiple loci, using DNA sequences of internal fragments of multiple housekeeping genes to characterize isolates of microbial species. The first MLST scheme ...
, suggests a Late Carboniferous/
Early Permian 01 or '01 may refer to: * The year 2001, or any year ending with 01 * The month of January * 1 (number) Music * 01'' (Richard Müller album), 2001 * ''01'' (Son of Dave album), 2000 * ''01'' (Urban Zakapa album), 2011 * ''O1'' (Hiroyuki Sawano ...
origin for extant amphibians. The origins and evolutionary relationships between the three main groups of amphibians is a matter of debate. A 2005 molecular phylogeny, based on rDNA analysis, suggests that salamanders and caecilians are more closely related to each other than they are to frogs. It also appears that the divergence of the three groups took place in the Paleozoic or early Mesozoic (around 250 million years ago), before the breakup of the supercontinent
Pangaea Pangaea or Pangea () was a supercontinent that existed during the late Paleozoic and early Mesozoic eras. It assembled from the earlier continental units of Gondwana, Euramerica and Siberia during the Carboniferous approximately 335 million y ...
and soon after their divergence from the lobe-finned fish. The briefness of this period, and the swiftness with which radiation took place, would help account for the relative scarcity of primitive amphibian fossils. There are large gaps in the
fossil record A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved in ...
, the discovery of the
dissorophoid Dissorophoideans are a clade of medium-sized, temnospondyl amphibians that appeared during the Moscovian in Euramerica, and continued through to the Late Permian and the Early Triassic of Gondwana. They are distinguished by various details of t ...
temnospondyl ''Gerobatrachus'' from the Early Permian in Texas in 2008 provided a missing link with many of the characteristics of modern frogs. Molecular analysis suggests that the frog–salamander divergence took place considerably earlier than the
palaeontological Paleontology (), also spelled palaeontology or palæontology, is the scientific study of life that existed prior to, and sometimes including, the start of the Holocene epoch (roughly 11,700 years before present). It includes the study of foss ...
evidence indicates. One study suggested suggested that the last common ancestor of all modern amphibians lived about 315 million years ago, and that
stereospondyl The Stereospondyli are a group of extinct temnospondyl amphibians that existed primarily during the Mesozoic period. They are known from all seven continents and were common components of many Triassic ecosystems, likely filling a similar ecologi ...
temnospondyls are the closest relatives to the caecilians. However, most studies support a single monophyletic origin of all modern amphibians within the dissorophoid temnospondyls. As they evolved from lunged fish, amphibians had to make certain adaptations for living on land, including the need to develop new means of locomotion. In the water, the sideways thrusts of their tails had propelled them forward, but on land, quite different mechanisms were required. Their vertebral columns, limbs, limb girdles and musculature needed to be strong enough to raise them off the ground for locomotion and feeding. Terrestrial adults discarded their
lateral line The lateral line, also called the lateral line organ (LLO), is a system of sensory organs found in fish, used to detect movement, vibration, and pressure gradients in the surrounding water. The sensory ability is achieved via modified epithelial ...
systems and adapted their sensory systems to receive stimuli via the medium of the air. They needed to develop new methods to regulate their body heat to cope with fluctuations in ambient temperature. They developed behaviours suitable for reproduction in a terrestrial environment. Their skins were exposed to harmful ultraviolet rays that had previously been absorbed by the water. The skin changed to become more protective and prevent excessive water loss.


Characteristics

The superclass Tetrapoda is divided into four classes of vertebrate animals with four limbs. Reptiles, birds and mammals are amniotes, the eggs of which are either laid or carried by the female and are surrounded by several membranes, some of which are impervious. Lacking these membranes, amphibians require water bodies for reproduction, although some species have developed various strategies for protecting or bypassing the vulnerable aquatic larval stage. They are not found in the sea with the exception of one or two frogs that live in
brackish water Brackish water, sometimes termed brack water, is water occurring in a natural environment that has more salinity than freshwater, but not as much as seawater. It may result from mixing seawater (salt water) and fresh water together, as in estuari ...
in mangrove swamps; the
Anderson's salamander Anderson's salamander (''Ambystoma andersoni'') is a neotenic salamander from Zacapu Lagoon in the Mexican state of Michoacán. This salamander is a relatively recent discovery, first described by Branden and Krebs in 1984. ''Ambystoma anderso ...
meanwhile occurs in brackish or salt water lakes. On land, amphibians are restricted to moist habitats because of the need to keep their skin damp. Modern amphibians have a simplified anatomy compared to their ancestors due to
paedomorphosis Neoteny (), also called juvenilization,Montagu, A. (1989). Growing Young. Bergin & Garvey: CT. is the delaying or slowing of the physiological, or somatic, development of an organism, typically an animal. Neoteny is found in modern humans compare ...
, caused by two evolutionary trends: miniaturization and an unusually large genome, which result in a slower growth and development rate compared to other vertebrates. Another reason for their size is associated with their rapid metamorphosis, which seems to have evolved only in the ancestors of lissamphibia; in all other known lines the development was much more gradual. Because a remodeling of the feeding apparatus means they don't eat during the metamorphosis, the metamorphosis has to go faster the smaller the individual is, so it happens at an early stage when the larvae are still small. (The largest species of salamanders don't go through a metamorphosis.) Amphibians that lay eggs on land often go through the whole metamorphosis inside the egg. An anamniotic terrestrial egg is less than 1 cm in diameter due to diffusion problems, a size which puts a limit on the amount of posthatching growth. The smallest amphibian (and vertebrate) in the world is a
microhylid The Microhylidae, commonly known as narrow-mouthed frogs, are a geographically widespread family of frogs. The 683 species are in 63 genera and 11 subfamilies, which is the largest number of genera of any frog family. Evolution A molecular phylo ...
frog from New Guinea (''
Paedophryne amauensis ''Paedophryne amauensis'' is a species of microhylid frog endemic to eastern Papua New Guinea. At in snout-to-vent length, it is considered the world's smallest known vertebrate. (See also Ecological guild.) The species was listed in the ...
'') first discovered in 2012. It has an average length of and is part of a genus that contains four of the world's ten smallest frog species. The largest living amphibian is the
Chinese giant salamander The Chinese giant salamander (''Andrias davidianus'') is one of the largest salamanders and one of the largest amphibians in the world. but this is a great deal smaller than the largest amphibian that ever existed—the extinct ''
Prionosuchus ''Prionosuchus'' is an extinct genus of large temnospondyl. A single species ''P. plummeri'', is recognized from the Early Permian (some time between 299 and 272 million years ago). Its fossils have been found in what is now northeastern Brazil. ...
'', a crocodile-like temnospondyl dating to 270 million years ago from the middle Permian of Brazil. The largest frog is the African
Goliath frog The goliath frog (''Conraua goliath''), otherwise known commonly as the giant slippery frog and the goliath bullfrog, is a species of frog in the family Conrauidae. The goliath frog is the largest living frog. Specimens can grow up to in lengt ...
(''Conraua goliath''), which can reach and weigh . Amphibians are ectothermic (cold-blooded) vertebrates that do not maintain their body temperature through internal
physiological Physiology (; ) is the scientific study of functions and mechanisms in a living system. As a sub-discipline of biology, physiology focuses on how organisms, organ systems, individual organs, cells, and biomolecules carry out the chemical ...
processes. Their
metabolic rate Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run cell ...
is low and as a result, their food and energy requirements are limited. In the adult state, they have tear ducts and movable eyelids, and most species have ears that can detect airborne or ground vibrations. They have muscular tongues, which in many species can be protruded. Modern amphibians have fully
ossified Ossification (also called osteogenesis or bone mineralization) in bone remodeling is the process of laying down new bone material by cells named osteoblasts. It is synonymous with bone tissue formation. There are two processes resulting in th ...
vertebrae with
articular processes The articular processes or zygapophyses (Greek ζυγον = "yoke" (because it links two vertebrae) + απο = "away" + φυσις = "process") of a vertebra are projections of the vertebra that serve the purpose of fitting with an adjacent vertebr ...
. Their ribs are usually short and may be fused to the vertebrae. Their skulls are mostly broad and short, and are often incompletely ossified. Their skin contains little keratin and lacks scales, apart from a few fish-like scales in certain caecilians. The skin contains many mucous glands and in some species, poison glands (a type of granular gland). The hearts of amphibians have three chambers, two atria and one ventricle. They have a
urinary bladder The urinary bladder, or simply bladder, is a hollow organ in humans and other vertebrates that stores urine from the kidneys before disposal by urination. In humans the bladder is a distensible organ that sits on the pelvic floor. Urine enters ...
and nitrogenous waste products are excreted primarily as
urea Urea, also known as carbamide, is an organic compound with chemical formula . This amide has two amino groups (–) joined by a carbonyl functional group (–C(=O)–). It is thus the simplest amide of carbamic acid. Urea serves an important ...
. Most amphibians lay their eggs in water and have aquatic larvae that undergo metamorphosis to become terrestrial adults. Amphibians breathe by means of a pump action in which air is first drawn into the buccopharyngeal region through the nostrils. These are then closed and the air is forced into the lungs by contraction of the throat. They supplement this with gas exchange through the skin.


Anura

The order Anura (from the Ancient Greek '' a(n)-'' meaning "without" and ''oura'' meaning "tail") comprises the frogs and toads. They usually have long hind limbs that fold underneath them, shorter forelimbs, webbed toes with no claws, no tails, large eyes and glandular moist skin. Members of this order with smooth skins are commonly referred to as frogs, while those with warty skins are known as toads. The difference is not a formal one taxonomically and there are numerous exceptions to this rule. Members of the family
Bufonidae {{Cat main, Toad This category Category, plural categories, may refer to: Philosophy and general uses *Categorization, categories in cognitive science, information science and generally *Category of being * ''Categories'' (Aristotle) *Category ...
are known as the "true toads". Frogs range in size from the
Goliath frog The goliath frog (''Conraua goliath''), otherwise known commonly as the giant slippery frog and the goliath bullfrog, is a species of frog in the family Conrauidae. The goliath frog is the largest living frog. Specimens can grow up to in lengt ...
(''Conraua goliath'') of West Africa to the ''
Paedophryne amauensis ''Paedophryne amauensis'' is a species of microhylid frog endemic to eastern Papua New Guinea. At in snout-to-vent length, it is considered the world's smallest known vertebrate. (See also Ecological guild.) The species was listed in the ...
'', first described in Papua New Guinea in 2012, which is also the smallest known vertebrate. Although most species are associated with water and damp habitats, some are specialised to live in trees or in deserts. They are found worldwide except for polar areas. Anura is divided into three suborders that are broadly accepted by the scientific community, but the relationships between some families remain unclear. Future molecular studies should provide further insights into their evolutionary relationships. The suborder
Archaeobatrachia Archaeobatrachia (New Latin ''archaeo-'' ("old") + ''batrachia'' ("frog")) is a suborder of the order Anura containing various primitive frogs and toads. As the name suggests, these are the most primitive frogs. Many of the species (28 in total) ...
contains four families of primitive frogs. These are
Ascaphidae The tailed frogs are two species of frogs in the genus ''Ascaphus'', the only taxon in the family Ascaphidae . The "tail" in the name is actually an extension of the male cloaca. The tail is one of two distinctive anatomical features adapting the ...
,
Bombinatoridae Bombinatoridae is a family of toads found in Eurasia. Species of the family have flattened bodies and some are highly toxic. Taxonomy and systematics Fossil specimens of the genus ''Bombina'' are known from the Pliocene to the Pleistocene. The ...
,
Discoglossidae The Alytidae are a family of primitive frogs. Their common name is painted frogs or midwife toads. Most are endemic to Europe, but three species occur in northwest Africa, and a species formerly thought to be extinct is found in Israel. This fam ...
and
Leiopelmatidae __NOTOC__ ''Leiopelma'' is a genus of New Zealand primitive frogs, belonging to the suborder Archaeobatrachia. It is the only genus in the monotypic family Leiopelmatidae. The leiopelmatids' relatively basal form indicates they have an ancient l ...
which have few derived features and are probably paraphyletic with regard to other frog lineages. The six families in the more evolutionarily advanced suborder
Mesobatrachia The Mesobatrachia ( Ancient Greek ''μέσος'' (mésos, "middle") + ''batrachia'' ("frogs")) is a paraphyletic group of relatively primitive frogs. At the end of 2016, it contained 3 superfamilies ( Pelobatoidea, Pelodytoidea and Pipoidea), 6 ...
are the fossorial
Megophryidae Megophryidae, commonly known as goose frogs, is a large family of frogs native to the warm southeast of Asia, from the Himalayan foothills eastwards, south to Indonesia and the Greater Sunda Islands in Maritime Southeast Asia, and extending to th ...
,
Pelobatidae The European spadefoot toads are a family of frogs, the Pelobatidae, with only one extant genus ''Pelobates'', containing six species. They are native to Europe, the Mediterranean, northwestern Africa, and western Asia. Description The Europe ...
,
Pelodytidae Pelodytidae, also known as the parsley frogs, or rarely, mud divers, is a family of frogs. It contains a single extant genus, '' Pelodytes'', and two genera only known from fossils. The extant species are found in southwestern Europe and the Cauc ...
, Scaphiopodidae and
Rhinophrynidae The Rhinophrynidae are a family of frogs containing one extant genus, the monotypic '' Rhinophrynus'', and a number of fossil genera. The family is sometimes known as the Mexican burrowing toads or simply burrowing toads. ''Rhinophrynus'' occur ...
and the obligatorily aquatic
Pipidae The Pipidae are a family of primitive, tongueless frogs. The 41 species in the family Pipidae are found in tropical South America (genus ''Pipa'') and sub-Saharan Africa (the three other genera). Description Pipid frogs are highly aquatic and ...
. These have certain characteristics that are intermediate between the two other suborders.
Neobatrachia The Neobatrachia (New Latin ''neo-'' ("new") + ''batrachia'' ("frogs")) are a suborder of the Anura, the order of frogs and toads. This suborder is the most advanced and apomorphic of the three anuran suborders alive today, hence its name, w ...
is by far the largest suborder and includes the remaining families of modern frogs, including most common species. Ninety-six percent of the over 5,000 extant species of frog are neobatrachians.


Caudata

The order
Caudata The Caudata are a group of amphibians containing the extant salamanders (Urodela) and all extinct species of amphibians more closely related to salamanders than to frogs. They are typically characterized by a superficially lizard-like appearance ...
(from the Latin ''cauda'' meaning "tail") consists of the salamanders—elongated, low-slung animals that mostly resemble lizards in form. This is a symplesiomorphic trait and they are no more closely related to lizards than they are to mammals. Salamanders lack claws, have scale-free skins, either smooth or covered with
tubercle In anatomy, a tubercle (literally 'small tuber', Latin for 'lump') is any round nodule, small eminence, or warty outgrowth found on external or internal organs of a plant or an animal. In plants A tubercle is generally a wart-like projectio ...
s, and tails that are usually flattened from side to side and often finned. They range in size from the
Chinese giant salamander The Chinese giant salamander (''Andrias davidianus'') is one of the largest salamanders and one of the largest amphibians in the world.Thorius pennatulus ''Thorius pennatulus'' is a species of salamander in the family Plethodontidae and one of the smallest tetrapods, with adults reaching a length of about . It is endemic to the mountains of Veracruz, Mexico. Description ''Thorius pennatulus'' ...
'' from Mexico which seldom exceeds in length. Salamanders have a mostly
Laurasia Laurasia () was the more northern of two large landmasses that formed part of the Pangaea supercontinent from around ( Mya), the other being Gondwana. It separated from Gondwana (beginning in the late Triassic period) during the breakup of Pan ...
n distribution, being present in much of the Holarctic region of the northern hemisphere. The family
Plethodontidae Plethodontidae, or lungless salamanders, are a family of salamanders. Most species are native to the Western Hemisphere, from British Columbia to Brazil, although a few species are found in Sardinia, Europe south of the Alps, and South Korea. In ...
is also found in Central America and South America north of the
Amazon basin The Amazon basin is the part of South America drained by the Amazon River and its tributaries. The Amazon drainage basin covers an area of about , or about 35.5 percent of the South American continent. It is located in the countries of Boliv ...
; South America was apparently invaded from Central America by about the start of the Miocene, 23 million years ago. Urodela is a name sometimes used for all the
extant Extant is the opposite of the word extinct. It may refer to: * Extant hereditary titles * Extant literature, surviving literature, such as ''Beowulf'', the oldest extant manuscript written in English * Extant taxon, a taxon which is not extin ...
species of salamanders. Members of several salamander families have become paedomorphic and either fail to complete their metamorphosis or retain some larval characteristics as adults. Most salamanders are under long. They may be terrestrial or aquatic and many spend part of the year in each habitat. When on land, they mostly spend the day hidden under stones or logs or in dense vegetation, emerging in the evening and night to forage for worms, insects and other invertebrates. The suborder
Cryptobranchoidea The Cryptobranchoidea are a suborder of salamanders found in Asia, European Russia, and the United States. They are known as primitive salamanders, in contrast to Salamandroidea, the advanced salamanders. It has two living subdivisions, Cryptobr ...
contains the primitive salamanders. A number of fossil cryptobranchids have been found, but there are only three living species, the Chinese giant salamander (''Andrias davidianus''), the Japanese giant salamander (''Andrias japonicus'') and the hellbender (''Cryptobranchus alleganiensis'') from North America. These large amphibians retain several larval characteristics in their adult state; gills slits are present and the eyes are unlidded. A unique feature is their ability to feed by suction, depressing either the left side of their lower jaw or the right. The males excavate nests, persuade females to lay their egg strings inside them, and guard them. As well as breathing with lungs, they respire through the many folds in their thin skin, which has
capillaries A capillary is a small blood vessel from 5 to 10 micrometres (μm) in diameter. Capillaries are composed of only the tunica intima, consisting of a thin wall of simple squamous endothelial cells. They are the smallest blood vessels in the body: ...
close to the surface. The suborder
Salamandroidea The Salamandroidea are a suborder of salamanders, referred to as advanced salamanders. The members of the suborder are found worldwide except for Antarctica, sub-Saharan Africa, and Oceania. They differ from suborder Cryptobranchoidea as the an ...
contains the advanced salamanders. They differ from the cryptobranchids by having fused prearticular bones in the lower jaw, and by using internal fertilisation. In salamandrids, the male deposits a bundle of sperm, the
spermatophore A spermatophore or sperm ampulla is a capsule or mass containing spermatozoa created by males of various animal species, especially salamanders and arthropods, and transferred in entirety to the female's ovipore during reproduction. Spermatophore ...
, and the female picks it up and inserts it into her cloaca where the sperm is stored until the eggs are laid. The largest family in this group is Plethodontidae, the lungless salamanders, which includes 60% of all salamander species. The family
Salamandridae Salamandridae is a family of salamanders consisting of true salamanders and newts. Salamandrids are distinguished from other salamanders by the lack of rib or costal grooves along the sides of their bodies and by their rough skin. Their skin is v ...
includes the true salamanders and the name " newt" is given to members of its subfamily Pleurodelinae. The third suborder,
Sirenoidea Sirenidae, the sirens, are a family of neotenic aquatic salamanders. Family members have very small fore limbs and lack hind limbs altogether. In one species, the skeleton in their fore limbs is made of only cartilage. In contrast to most other ...
, contains the four species of sirens, which are in a single family,
Sirenidae Sirenidae, the sirens, are a family of neotenic aquatic salamanders. Family members have very small fore limbs and lack hind limbs altogether. In one species, the skeleton in their fore limbs is made of only cartilage. In contrast to most other ...
. Members of this order are
eel Eels are ray-finned fish belonging to the order Anguilliformes (), which consists of eight suborders, 19 families, 111 genera, and about 800 species. Eels undergo considerable development from the early larval stage to the eventual adult stage ...
-like aquatic salamanders with much reduced forelimbs and no hind limbs. Some of their features are primitive while others are derived. Fertilisation is likely to be external as sirenids lack the cloacal glands used by male salamandrids to produce spermatophores and the females lack spermathecae for sperm storage. Despite this, the eggs are laid singly, a behaviour not conducive for external fertilisation.


Gymnophiona

The order
Gymnophiona Caecilians (; ) are a group of limbless, vermiform or serpentine amphibians. They mostly live hidden in the ground and in stream substrates, making them the least familiar order of amphibians. Caecilians are mostly distributed in the tropics of ...
(from the Greek ''gymnos'' meaning "naked" and ''ophis'' meaning "serpent") or Apoda comprises the caecilians. These are long, cylindrical, limbless animals with a snake- or worm-like form. The adults vary in length from 8 to 75 centimetres (3 to 30 inches) with the exception of Thomson's caecilian (''Caecilia thompsoni''), which can reach . A caecilian's skin has a large number of transverse folds and in some species contains tiny embedded dermal scales. It has rudimentary eyes covered in skin, which are probably limited to discerning differences in light intensity. It also has a pair of short
tentacle In zoology, a tentacle is a flexible, mobile, and elongated organ present in some species of animals, most of them invertebrates. In animal anatomy, tentacles usually occur in one or more pairs. Anatomically, the tentacles of animals work main ...
s near the eye that can be extended and which have
tactile Tactile may refer to: * Tactile, related to the sense of touch * Haptics (disambiguation) * Tactile (device), a text-to-braille translation device See also * Tangibility, in law * Somatosensory system In physiology, the somatosensory system ...
and olfactory functions. Most caecilians live underground in burrows in damp soil, in rotten wood and under plant debris, but some are aquatic. Most species lay their eggs underground and when the larvae hatch, they make their way to adjacent bodies of water. Others brood their eggs and the larvae undergo metamorphosis before the eggs hatch. A few species give birth to live young, nourishing them with glandular secretions while they are in the oviduct. Caecilians have a mostly Gondwanan distribution, being found in tropical regions of Africa, Asia and Central and South America.


Anatomy and physiology


Skin

The integumentary structure contains some typical characteristics common to terrestrial vertebrates, such as the presence of highly
cornified Keratin () is one of a family of structural fibrous proteins also known as ''scleroproteins''. Alpha-keratin (α-keratin) is a type of keratin found in vertebrates. It is the key structural material making up scales, hair, nails, feathers, horn ...
outer layers, renewed periodically through a moulting process controlled by the pituitary and thyroid glands. Local thickenings (often called warts) are common, such as those found on toads. The outside of the skin is shed periodically mostly in one piece, in contrast to mammals and birds where it is shed in flakes. Amphibians often eat the sloughed skin. Caecilians are unique among amphibians in having mineralized dermal scales embedded in the
dermis The dermis or corium is a layer of skin between the epidermis (with which it makes up the cutis) and subcutaneous tissues, that primarily consists of dense irregular connective tissue and cushions the body from stress and strain. It is divided i ...
between the furrows in the skin. The similarity of these to the scales of bony fish is largely superficial. Lizards and some frogs have somewhat similar
osteoderm Osteoderms are bony deposits forming scales, plates, or other structures based in the dermis. Osteoderms are found in many groups of extant and extinct reptiles and amphibians, including lizards, crocodilians, frogs, temnospondyls (extinct amp ...
s forming bony deposits in the dermis, but this is an example of convergent evolution with similar structures having arisen independently in diverse vertebrate lineages. Amphibian skin is permeable to water. Gas exchange can take place through the skin (
cutaneous respiration Cutaneous respiration, or cutaneous gas exchange (sometimes called, skin breathing), is a form of respiration in which gas exchange occurs across the skin or outer integument of an organism rather than gills or lungs. Cutaneous respiration may be t ...
) and this allows adult amphibians to respire without rising to the surface of water and to hibernate at the bottom of ponds. To compensate for their thin and delicate skin, amphibians have evolved mucous glands, principally on their heads, backs and tails. The secretions produced by these help keep the skin moist. In addition, most species of amphibian have granular glands that secrete distasteful or poisonous substances. Some amphibian toxins can be lethal to humans while others have little effect. The main poison-producing glands, the parotoids, produce the neurotoxin
bufotoxin Bufotoxins are a family of toxic steroid lactones or substituted tryptamines of which some may or may not be toxic. They occur in the parotoid glands, skin, and poison of many toads (genus ''Bufo'') and other amphibians, and in some plants and mus ...
and are located behind the ears of toads, along the backs of frogs, behind the eyes of salamanders and on the upper surface of caecilians. The skin colour of amphibians is produced by three layers of pigment cells called
chromatophore Chromatophores are cells that produce color, of which many types are pigment-containing cells, or groups of cells, found in a wide range of animals including amphibians, fish, reptiles, crustaceans and cephalopods. Mammals and birds, in contrast ...
s. These three cell layers consist of the melanophores (occupying the deepest layer), the guanophores (forming an intermediate layer and containing many granules, producing a blue-green colour) and the lipophores (yellow, the most superficial layer). The colour change displayed by many species is initiated by hormones secreted by the pituitary gland. Unlike bony fish, there is no direct control of the pigment cells by the nervous system, and this results in the colour change taking place more slowly than happens in fish. A vividly coloured skin usually indicates that the species is toxic and is a warning sign to predators.


Skeletal system and locomotion

Amphibians have a skeletal system that is structurally homologous to other tetrapods, though with a number of variations. They all have four limbs except for the legless caecilians and a few species of salamander with reduced or no limbs. The bones are hollow and lightweight. The musculoskeletal system is strong to enable it to support the head and body. The bones are fully
ossified Ossification (also called osteogenesis or bone mineralization) in bone remodeling is the process of laying down new bone material by cells named osteoblasts. It is synonymous with bone tissue formation. There are two processes resulting in th ...
and the vertebrae interlock with each other by means of overlapping processes. The
pectoral girdle The shoulder girdle or pectoral girdle is the set of bones in the appendicular skeleton which connects to the arm on each side. In humans it consists of the clavicle and scapula; in those species with three bones in the shoulder, it consists of ...
is supported by muscle, and the well-developed pelvic girdle is attached to the backbone by a pair of sacral ribs. The ilium slopes forward and the body is held closer to the ground than is the case in mammals. In most amphibians, there are four digits on the fore foot and five on the hind foot, but no claws on either. Some salamanders have fewer digits and the
amphiuma ''Amphiuma'' is a genus of aquatic salamanders from the United States, the only extant genus within the family Amphiumidae . They are colloquially known as amphiumas. They are also known to fishermen as "conger eels" or "Congo snakes", which are ...
s are eel-like in appearance with tiny, stubby legs. The
sirens Siren or sirens may refer to: Common meanings * Siren (alarm), a loud acoustic alarm used to alert people to emergencies * Siren (mythology), an enchanting but dangerous monster in Greek mythology Places * Siren (town), Wisconsin * Siren, Wisc ...
are aquatic salamanders with stumpy forelimbs and no hind limbs. The caecilians are limbless. They burrow in the manner of earthworms with zones of muscle contractions moving along the body. On the surface of the ground or in water they move by undulating their body from side to side. In frogs, the hind legs are larger than the fore legs, especially so in those species that principally move by jumping or swimming. In the walkers and runners the hind limbs are not so large, and the burrowers mostly have short limbs and broad bodies. The feet have adaptations for the way of life, with webbing between the toes for swimming, broad adhesive toe pads for climbing, and keratinised tubercles on the hind feet for digging (frogs usually dig backwards into the soil). In most salamanders, the limbs are short and more or less the same length and project at right angles from the body. Locomotion on land is by walking and the tail often swings from side to side or is used as a prop, particularly when climbing. In their normal gait, only one leg is advanced at a time in the manner adopted by their ancestors, the lobe-finned fish. Some salamanders in the genus ''
Aneides Climbing salamanders is the common name for plethodontid (lungless) salamanders of the genus ''Aneides''. It contains 10 species native to North America, distributed between the Pacific Coast (7 species), Sacramento Mountains (1 species), and App ...
'' and certain plethodontids climb trees and have long limbs, large toepads and prehensile tails. In aquatic salamanders and in frog tadpoles, the tail has
dorsal Dorsal (from Latin ''dorsum'' ‘back’) may refer to: * Dorsal (anatomy), an anatomical term of location referring to the back or upper side of an organism or parts of an organism * Dorsal, positioned on top of an aircraft's fuselage * Dorsal co ...
and
ventral Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...
fins and is moved from side to side as a means of propulsion. Adult frogs do not have tails and caecilians have only very short ones. Salamanders use their tails in defence and some are prepared to jettison them to save their lives in a process known as
autotomy Autotomy (from the Greek ''auto-'', "self-" and ''tome'', "severing", αὐτοτομία) or self-amputation, is the behaviour whereby an animal sheds or discards one or more of its own appendages, usually as a self-defense mechanism to elude ...
. Certain species in the Plethodontidae have a weak zone at the base of the tail and use this strategy readily. The tail often continues to twitch after separation which may distract the attacker and allow the salamander to escape. Both tails and limbs can be regenerated. Adult frogs are unable to regrow limbs but tadpoles can do so.


Circulatory system

Amphibians have a juvenile stage and an adult stage, and the circulatory systems of the two are distinct. In the juvenile (or tadpole) stage, the circulation is similar to that of a fish; the two-chambered heart pumps the blood through the gills where it is oxygenated, and is spread around the body and back to the heart in a single loop. In the adult stage, amphibians (especially frogs) lose their gills and develop lungs. They have a heart that consists of a single ventricle and two atria. When the ventricle starts contracting, deoxygenated blood is pumped through the
pulmonary artery A pulmonary artery is an artery in the pulmonary circulation that carries deoxygenated blood from the right side of the heart to the lungs. The largest pulmonary artery is the ''main pulmonary artery'' or ''pulmonary trunk'' from the heart, and ...
to the lungs. Continued contraction then pumps oxygenated blood around the rest of the body. Mixing of the two bloodstreams is minimized by the anatomy of the chambers.


Nervous and sensory systems

The nervous system is basically the same as in other vertebrates, with a central brain, a spinal cord, and nerves throughout the body. The amphibian brain is relatively simple but broadly the same structurally as in reptiles, birds and mammals. Their brains are elongated, except in caecilians, and contain the usual motor and sensory areas of tetrapods. The pineal body, known to regulate sleep patterns in humans, is thought to produce the hormones involved in
hibernation Hibernation is a state of minimal activity and metabolic depression undergone by some animal species. Hibernation is a seasonal heterothermy characterized by low body-temperature, slow breathing and heart-rate, and low metabolic rate. It most ...
and
aestivation Aestivation ( la, aestas (summer); also spelled estivation in American English) is a state of animal dormancy, similar to hibernation, although taking place in the summer rather than the winter. Aestivation is characterized by inactivity and a ...
in amphibians. Tadpoles retain the lateral line system of their ancestral fishes, but this is lost in terrestrial adult amphibians. Some caecilians possess
electroreceptors Electroreception and electrogenesis are the closely-related biological abilities to perceive electrical stimuli and to generate electric fields. Both are used to locate prey; stronger electric discharges are used in a few groups of fishes to stu ...
that allow them to locate objects around them when submerged in water. The ears are well developed in frogs. There is no external ear, but the large circular eardrum lies on the surface of the head just behind the eye. This vibrates and sound is transmitted through a single bone, the stapes, to the inner ear. Only high-frequency sounds like mating calls are heard in this way, but low-frequency noises can be detected through another mechanism. There is a patch of specialized haircells, called ''papilla amphibiorum'', in the inner ear capable of detecting deeper sounds. Another feature, unique to frogs and salamanders, is the columella-operculum complex adjoining the auditory capsule which is involved in the transmission of both airborne and seismic signals. The ears of salamanders and caecilians are less highly developed than those of frogs as they do not normally communicate with each other through the medium of sound. The eyes of tadpoles lack lids, but at metamorphosis, the
cornea The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Along with the anterior chamber and lens, the cornea refracts light, accounting for approximately two-thirds of the eye's total optical powe ...
becomes more dome-shaped, the
lens A lens is a transmissive optical device which focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements''), ...
becomes flatter, and eyelids and associated glands and ducts develop. The adult eyes are an improvement on invertebrate eyes and were a first step in the development of more advanced vertebrate eyes. They allow colour vision and depth of focus. In the retinas are green rods, which are receptive to a wide range of wavelengths.


Digestive and excretory systems

Many amphibians catch their prey by flicking out an elongated tongue with a sticky tip and drawing it back into the mouth before seizing the item with their jaws. Some use inertial feeding to help them swallow the prey, repeatedly thrusting their head forward sharply causing the food to move backwards in their mouth by inertia. Most amphibians swallow their prey whole without much chewing so they possess voluminous stomachs. The short
oesophagus The esophagus (American English) or oesophagus (British English; both ), non-technically known also as the food pipe or gullet, is an organ in vertebrates through which food passes, aided by peristaltic contractions, from the pharynx to the ...
is lined with cilia that help to move the food to the stomach and mucus produced by glands in the mouth and pharynx eases its passage. The enzyme
chitinase Chitinases (EC 3.2.1.14, chitodextrinase, 1,4-β-poly-N-acetylglucosaminidase, poly-β-glucosaminidase, β-1,4-poly-N-acetyl glucosamidinase, poly ,4-(N-acetyl-β-D-glucosaminide)glycanohydrolase, (1→4)-2-acetamido-2-deoxy-β-D-glucan glycano ...
produced in the stomach helps digest the chitinous cuticle of arthropod prey. Amphibians possess a
pancreas The pancreas is an organ of the digestive system and endocrine system of vertebrates. In humans, it is located in the abdomen behind the stomach and functions as a gland. The pancreas is a mixed or heterocrine gland, i.e. it has both an ...
, liver and gall bladder. The liver is usually large with two lobes. Its size is determined by its function as a glycogen and fat storage unit, and may change with the seasons as these reserves are built or used up.
Adipose tissue Adipose tissue, body fat, or simply fat is a loose connective tissue composed mostly of adipocytes. In addition to adipocytes, adipose tissue contains the stromal vascular fraction (SVF) of cells including preadipocytes, fibroblasts, vascular e ...
is another important means of storing energy and this occurs in the abdomen (in internal structures called fat bodies), under the skin and, in some salamanders, in the tail. There are two kidneys located dorsally, near the roof of the body cavity. Their job is to filter the blood of metabolic waste and transport the urine via ureters to the urinary bladder where it is stored before being passed out periodically through the cloacal vent. Larvae and most aquatic adult amphibians excrete the nitrogen as ammonia in large quantities of dilute urine, while terrestrial species, with a greater need to conserve water, excrete the less toxic product urea. Some tree frogs with limited access to water excrete most of their metabolic waste as uric acid.


Respiratory system

The lungs in amphibians are primitive compared to those of amniotes, possessing few internal septa and large
alveoli Alveolus (; pl. alveoli, adj. alveolar) is a general anatomical term for a concave cavity or pit. Uses in anatomy and zoology * Pulmonary alveolus, an air sac in the lungs ** Alveolar cell or pneumocyte ** Alveolar duct ** Alveolar macrophage * M ...
, and consequently having a comparatively slow diffusion rate for oxygen entering the blood. Ventilation is accomplished by
buccal pumping Buccal pumping is "breathing with one's cheeks": a method of ventilation used in respiration in which the animal moves the floor of its mouth in a rhythmic manner that is externally apparent.Brainerd, E. L. (1999). New perspectives on the evolution ...
. Most amphibians, however, are able to exchange gases with the water or air via their skin. To enable sufficient
cutaneous respiration Cutaneous respiration, or cutaneous gas exchange (sometimes called, skin breathing), is a form of respiration in which gas exchange occurs across the skin or outer integument of an organism rather than gills or lungs. Cutaneous respiration may be t ...
, the surface of their highly vascularised skin must remain moist to allow the oxygen to diffuse at a sufficiently high rate. Because oxygen concentration in the water increases at both low temperatures and high flow rates, aquatic amphibians in these situations can rely primarily on cutaneous respiration, as in the
Titicaca water frog ''Telmatobius culeus'', commonly known as the Titicaca water frog, is a medium-large to very large and endangered species of frog in the family Telmatobiidae. It is entirely aquatic and only found in the Lake Titicaca basin, including rivers tha ...
and the
hellbender salamander The hellbender (''Cryptobranchus alleganiensis''), also known as the hellbender salamander, is a species of aquatic giant salamander endemic to the eastern and central United States. It is the largest salamander in North America. A member of ...
. In air, where oxygen is more concentrated, some small species can rely solely on cutaneous gas exchange, most famously the plethodontid salamanders, which have neither lungs nor gills. Many aquatic salamanders and all tadpoles have gills in their larval stage, with some (such as the
axolotl The axolotl (; from nci, āxōlōtl ), ''Ambystoma mexicanum'', is a paedomorphic salamander closely related to the tiger salamander. Axolotls are unusual among amphibians in that they reach adulthood without undergoing metamorphosis. Instea ...
) retaining gills as aquatic adults.


Reproduction

For the purpose of reproduction most amphibians require fresh water although some lay their eggs on land and have developed various means of keeping them moist. A few (e.g. ''
Fejervarya raja The crab-eating frog (''Fejervarya cancrivora'') is a frog native to south-eastern Asia including Taiwan, China, the Philippines and more rarely as far west as Orissa in India. It has also been introduced to Guam, and was most likely introduced f ...
'') can inhabit brackish water, but there are no true
marine Marine is an adjective meaning of or pertaining to the sea or ocean. Marine or marines may refer to: Ocean * Maritime (disambiguation) * Marine art * Marine biology * Marine debris * Marine habitats * Marine life * Marine pollution Military ...
amphibians. There are reports, however, of particular amphibian populations unexpectedly invading marine waters. Such was the case with the Black Sea invasion of the natural hybrid ''
Pelophylax esculentus The edible frog (''Pelophylax'' kl. ''esculentus'') is a species of common European frog, also known as the common water frog or green frog (however, this latter term is also used for the North American species ''Rana clamitans''). It is used ...
'' reported in 2010. Several hundred frog species in
adaptive radiation In evolutionary biology, adaptive radiation is a process in which organisms diversify rapidly from an ancestral species into a multitude of new forms, particularly when a change in the environment makes new resources available, alters biotic in ...
s (e.g., ''
Eleutherodactylus ''Eleutherodactylus'' is a genus of frogs in the family Eleutherodactylidae.Hedges, S. B., W. E. Duellman, and M. P. Heinicke . 2008. New World direct-developing frogs (Anura: Terrarana): molecular phylogeny, classification, biogeography, and co ...
'', the Pacific ''
Platymantis ''Platymantis'' is a genus of frogs in the family Ceratobatrachidae. They are commonly known as wrinkled ground frogs, ground frogs, and forest frogs. As currently defined, ''Platymantis'' is a large genus with up to as many as 60 species fou ...
'', the Australo-Papuan
microhylid The Microhylidae, commonly known as narrow-mouthed frogs, are a geographically widespread family of frogs. The 683 species are in 63 genera and 11 subfamilies, which is the largest number of genera of any frog family. Evolution A molecular phylo ...
s, and many other tropical frogs), however, do not need any water for
breeding in the wild Breeding in the wild is the natural process of animal reproduction occurring in the natural habitat of a given species. This terminology is distinct from animal husbandry or breeding of species in captivity. Breeding locations are often chosen for ...
. They reproduce via direct development, an ecological and evolutionary adaptation that has allowed them to be completely independent from free-standing water. Almost all of these frogs live in wet
tropical rainforests Tropical rainforests are rainforests that occur in areas of tropical rainforest climate in which there is no dry season – all months have an average precipitation of at least 60 mm – and may also be referred to as ''lowland equatori ...
and their eggs hatch directly into miniature versions of the adult, passing through the tadpole stage within the egg. Reproductive success of many amphibians is dependent not only on the quantity of rainfall, but the seasonal timing. In the tropics, many amphibians breed continuously or at any time of year. In temperate regions, breeding is mostly seasonal, usually in the spring, and is triggered by increasing day length, rising temperatures or rainfall. Experiments have shown the importance of temperature, but the trigger event, especially in arid regions, is often a storm. In anurans, males usually arrive at the breeding sites before females and the vocal chorus they produce may stimulate ovulation in females and the endocrine activity of males that are not yet reproductively active. In caecilians, fertilisation is internal, the male extruding an intromittent organ, the , and inserting it into the female cloaca. The paired Müllerian glands inside the male cloaca secrete a fluid which resembles that produced by mammalian prostate glands and which may transport and nourish the sperm. Fertilisation probably takes place in the oviduct. The majority of salamanders also engage in internal fertilisation. In most of these, the male deposits a spermatophore, a small packet of sperm on top of a gelatinous cone, on the substrate either on land or in the water. The female takes up the sperm packet by grasping it with the lips of the cloaca and pushing it into the vent. The spermatozoa move to the spermatheca in the roof of the cloaca where they remain until ovulation which may be many months later. Courtship rituals and methods of transfer of the spermatophore vary between species. In some, the spermatophore may be placed directly into the female cloaca while in others, the female may be guided to the spermatophore or restrained with an embrace called
amplexus Amplexus (Latin "embrace") is a type of mating behavior exhibited by some externally fertilizing species (chiefly amphibians and horseshoe crabs) in which a male grasps a female with his front legs as part of the mating process, and at the same ...
. Certain primitive salamanders in the families Sirenidae,
Hynobiidae The Asiatic salamanders (family Hynobiidae) are primitive salamanders found all over Asia, and in European Russia. They are closely related to the giant salamanders (family Cryptobranchidae), with which they form the suborder Cryptobranchoidea. A ...
and
Cryptobranchidae The Cryptobranchidae are a family of fully aquatic salamanders commonly known as the giant salamanders. They include the largest living amphibians. The family is native to China, Japan, and the eastern United States. They constitute one of two li ...
practice external fertilisation in a similar manner to frogs, with the female laying the eggs in water and the male releasing sperm onto the egg mass. With a few exceptions, frogs use external fertilisation. The male grasps the female tightly with his forelimbs either behind the arms or in front of the back legs, or in the case of ''
Epipedobates tricolor The phantasmal poison frog or phantasmal poison-arrow frog (''Epipedobates tricolor'') is a species of poison dart frog. It is endemic to Ecuador and known from the Andean slopes of central Ecuador in Bolívar Province. They have radiant colo ...
'', around the neck. They remain in amplexus with their cloacae positioned close together while the female lays the eggs and the male covers them with sperm. Roughened nuptial pads on the male's hands aid in retaining grip. Often the male collects and retains the egg mass, forming a sort of basket with the hind feet. An exception is the
granular poison frog The granular poison frog or granular poison arrow frog (''Oophaga granulifera'') is a species of frog in the family Dendrobatidae, found in Costa Rica and Panama. Originally described as ''Dendrobates granuliferus'', it was moved to ''Oophaga' ...
(''Oophaga granulifera'') where the male and female place their cloacae in close proximity while facing in opposite directions and then release eggs and sperm simultaneously. The
tailed frog The tailed frogs are two species of frogs in the genus ''Ascaphus'', the only taxon in the family Ascaphidae . The "tail" in the name is actually an extension of the male cloaca. The tail is one of two distinctive anatomical features adapting the ...
(''Ascaphus truei'') exhibits internal fertilisation. The "tail" is only possessed by the male and is an extension of the cloaca and used to inseminate the female. This frog lives in fast-flowing streams and internal fertilisation prevents the sperm from being washed away before fertilisation occurs. The sperm may be retained in storage tubes attached to the
oviduct The oviduct in mammals, is the passageway from an ovary. In human females this is more usually known as the Fallopian tube or uterine tube. The eggs travel along the oviduct. These eggs will either be fertilized by spermatozoa to become a zygote, o ...
until the following spring. Most frogs can be classified as either prolonged or explosive breeders. Typically, prolonged breeders congregate at a breeding site, the males usually arriving first, calling and setting up territories. Other satellite males remain quietly nearby, waiting for their opportunity to take over a territory. The females arrive sporadically, mate selection takes place and eggs are laid. The females depart and territories may change hands. More females appear and in due course, the breeding season comes to an end. Explosive breeders on the other hand are found where temporary pools appear in dry regions after rainfall. These frogs are typically fossorial species that emerge after heavy rains and congregate at a breeding site. They are attracted there by the calling of the first male to find a suitable place, perhaps a pool that forms in the same place each rainy season. The assembled frogs may call in unison and frenzied activity ensues, the males scrambling to mate with the usually smaller number of females. There is a direct competition between males to win the attention of the females in salamanders and newts, with elaborate courtship displays to keep the female's attention long enough to get her interested in choosing him to mate with. Some species store
sperm Sperm is the male reproductive cell, or gamete, in anisogamous forms of sexual reproduction (forms in which there is a larger, female reproductive cell and a smaller, male one). Animals produce motile sperm with a tail known as a flagellum, wh ...
through long breeding seasons, as the extra time may allow for interactions with rival sperm.


Life cycle

Most amphibians go through metamorphosis, a process of significant morphological change after birth. In typical amphibian development, eggs are laid in water and larvae are adapted to an aquatic lifestyle. Frogs, toads and salamanders all hatch from the egg as larvae with external gills. Metamorphosis in amphibians is regulated by thyroxine concentration in the blood, which stimulates metamorphosis, and
prolactin Prolactin (PRL), also known as lactotropin, is a protein best known for its role in enabling mammals to produce milk. It is influential in over 300 separate processes in various vertebrates, including humans. Prolactin is secreted from the p ...
, which counteracts thyroxine's effect. Specific events are dependent on threshold values for different tissues. Because most embryonic development is outside the parental body, it is subject to many adaptations due to specific environmental circumstances. For this reason tadpoles can have horny ridges instead of
Teeth A tooth ( : teeth) is a hard, calcified structure found in the jaws (or mouths) of many vertebrates and used to break down food. Some animals, particularly carnivores and omnivores, also use teeth to help with capturing or wounding prey, tea ...
, whisker-like skin extensions or fins. They also make use of a sensory lateral line organ similar to that of fish. After metamorphosis, these organs become redundant and will be reabsorbed by controlled cell death, called
apoptosis Apoptosis (from grc, ἀπόπτωσις, apóptōsis, 'falling off') is a form of programmed cell death that occurs in multicellular organisms. Biochemical events lead to characteristic cell changes (morphology) and death. These changes includ ...
. The variety of adaptations to specific environmental circumstances among amphibians is wide, with many discoveries still being made.


Eggs

In the egg, the embryo is suspended in perivitelline fluid and surrounded by semi-permeable gelatinous capsules, with the yolk mass providing nutrients. As the larvae hatch, the capules are dissolved by enzymes secreted from gland at the tip of the snout. The eggs of some salamanders and frogs contain unicellular green algae. These penetrate the jelly envelope after the eggs are laid and may increase the supply of oxygen to the embryo through photosynthesis. They seem to both speed up the development of the larvae and reduce mortality. In the
wood frog ''Lithobates sylvaticus'' or ''Rana sylvatica'', commonly known as the wood frog, is a frog species that has a broad distribution over North America, extending from the boreal forest of the north to the southern Appalachians, with several notab ...
(''Rana sylvatica''), the interior of the globular egg cluster has been found to be up to warmer than its surroundings, which is an advantage in its cool northern habitat. The eggs may be deposited singly, in cluster or in long strands. Sites for laying eggs include water, mud, burrows, debris and on plants or under logs or stones. The greenhouse frog (''Eleutherodactylus planirostris'') lays eggs in small groups in the soil where they develop in about two weeks directly into juvenile frogs without an intervening larval stage. The tungara frog (''Physalaemus pustulosus'') builds a floating nest from foam to protect its eggs. First a raft is built, then eggs are laid in the centre, and finally a foam cap is overlaid. The foam has anti-microbial properties. It contains no
detergent A detergent is a surfactant or a mixture of surfactants with cleansing properties when in dilute solutions. There are a large variety of detergents, a common family being the alkylbenzene sulfonates, which are soap-like compounds that are mor ...
s but is created by whipping up proteins and
lectin Lectins are carbohydrate-binding proteins that are highly specific for sugar groups that are part of other molecules, so cause agglutination of particular cells or precipitation of glycoconjugates and polysaccharides. Lectins have a role in r ...
s secreted by the female.


Larvae

The eggs of amphibians are typically laid in water and hatch into free-living larvae that complete their development in water and later transform into either aquatic or terrestrial adults. In many species of frog and in most lungless salamanders (Plethodontidae), direct development takes place, the larvae growing within the eggs and emerging as miniature adults. Many caecilians and some other amphibians lay their eggs on land, and the newly hatched larvae wriggle or are transported to water bodies. Some caecilians, the
alpine salamander The alpine salamander (''Salamandra atra'') is a black salamander that can be found in the French Alps, and through the mountainous range in Europe. It is a member of the genus '' salamandra''. Their species name, ''atra'', may be derived from the ...
(''Salamandra atra'') and some of the African live-bearing toads (''Nectophrynoides spp.'') are viviparous. Their larvae feed on glandular secretions and develop within the female's oviduct, often for long periods. Other amphibians, but not caecilians, are ovoviviparous. The eggs are retained in or on the parent's body, but the larvae subsist on the yolks of their eggs and receive no nourishment from the adult. The larvae emerge at varying stages of their growth, either before or after metamorphosis, according to their species. The toad genus ''Nectophrynoides'' exhibits all of these developmental patterns among its dozen or so members. Amphibian larvae are known as tadpoles. They have thick, rounded bodies with powerful muscular tails.


Frogs

Unlike in other amphibians, frog tadpoles do not resemble adults. The free-living larvae are normally fully aquatic, but the tadpoles of some species (such as ''
Nannophrys ceylonensis ''Nannophrys ceylonensis'', commonly known as the Sri Lanka rock frog or the Ceylon streamlined frog, is a species of frog. It used to be placed in the large frog family Ranidae but a phylogenetic study was undertaken using DNA sequences and it ...
'') are semi-terrestrial and live among wet rocks. Tadpoles have cartilaginous skeletons, gills for respiration (external gills at first, internal gills later),
lateral line system The lateral line, also called the lateral line organ (LLO), is a system of sensory organs found in fish, used to detect movement, vibration, and pressure gradients in the surrounding water. The sensory ability is achieved via modified epithelial ...
s and large tails that they use for swimming. Newly hatched tadpoles soon develop gill pouches that cover the gills. The lungs develop early and are used as accessory breathing organs, the tadpoles rising to the water surface to gulp air. Some species complete their development inside the egg and hatch directly into small frogs. These larvae do not have gills but instead have specialised areas of skin through which respiration takes place. While tadpoles do not have true teeth, in most species, the jaws have long, parallel rows of small keratinized structures called keradonts surrounded by a horny beak. Front legs are formed under the gill sac and hind legs become visible a few days later. Iodine and T4 (over stimulate the spectacular
apoptosis Apoptosis (from grc, ἀπόπτωσις, apóptōsis, 'falling off') is a form of programmed cell death that occurs in multicellular organisms. Biochemical events lead to characteristic cell changes (morphology) and death. These changes includ ...
rogrammed cell deathof the cells of the larval gills, tail and fins) also stimulate the
evolution of nervous systems The evolution of nervous systems dates back to the first development of nervous systems in animals (or metazoans). Neurons developed as specialized electrical signaling cells in multicellular animals, adapting the mechanism of action potentials ...
transforming the aquatic, vegetarian tadpole into the terrestrial, carnivorous frog with better neurological, visuospatial, olfactory and cognitive abilities for hunting. In fact, tadpoles developing in ponds and streams are typically herbivorous. Pond tadpoles tend to have deep bodies, large caudal fins and small mouths; they swim in the quiet waters feeding on growing or loose fragments of vegetation. Stream dwellers mostly have larger mouths, shallow bodies and caudal fins; they attach themselves to plants and stones and feed on the surface films of
algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthesis, photosynthetic eukaryotic organisms. It is a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from u ...
and bacteria. They also feed on
diatom A diatom ( Neo-Latin ''diatoma''), "a cutting through, a severance", from el, διάτομος, diátomos, "cut in half, divided equally" from el, διατέμνω, diatémno, "to cut in twain". is any member of a large group comprising se ...
s, filtered from the water through the gills, and stir up the sediment at bottom of the pond, ingesting edible fragments. They have a relatively long, spiral-shaped gut to enable them to digest this diet. Some species are carnivorous at the tadpole stage, eating insects, smaller tadpoles and fish. Young of the
Cuban tree frog The Cuban tree frog (''Osteopilus septentrionalis'') is a large species of tree frog that is native to Cuba, the Bahamas, and the Cayman Islands; but has become invasive in several other places around the Americas. Its wide diet and ability to t ...
(''Osteopilus septentrionalis'') can occasionally be
cannibalistic Cannibalism is the act of consuming another individual of the same species as food. Cannibalism is a common ecological interaction in the animal kingdom and has been recorded in more than 1,500 species. Human cannibalism is well documented, bo ...
, the younger tadpoles attacking a larger, more developed tadpole when it is undergoing metamorphosis. At metamorphosis, rapid changes in the body take place as the lifestyle of the frog changes completely. The spiral‐shaped mouth with horny tooth ridges is reabsorbed together with the spiral gut. The animal develops a large jaw, and its gills disappear along with its gill sac. Eyes and legs grow quickly, and a tongue is formed. There are associated changes in the neural networks such as development of stereoscopic vision and loss of the lateral line system. All this can happen in about a day. A few days later, the tail is reabsorbed, due to the higher thyroxine concentration required for this to take place.


Salamanders

At hatching, a typical salamander larva has eyes without lids, teeth in both upper and lower jaws, three pairs of feathery external gills, and a long tail with
dorsal Dorsal (from Latin ''dorsum'' ‘back’) may refer to: * Dorsal (anatomy), an anatomical term of location referring to the back or upper side of an organism or parts of an organism * Dorsal, positioned on top of an aircraft's fuselage * Dorsal co ...
and
ventral Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...
fins. The forelimbs may be partially developed and the hind limbs are rudimentary in pond-living species but may be rather more developed in species that reproduce in moving water. Pond-type larvae often have a pair of balancers, rod-like structures on either side of the head that may prevent the gills from becoming clogged up with sediment. Both of these are able to breed. Some have larvae that never fully develop into the adult form, a condition known as neoteny. Neoteny occurs when the animal's growth rate is very low and is usually linked to adverse conditions such as low water temperatures that may change the response of the tissues to the hormone thyroxine. as well as lack of food. There are fifteen species of
obligate {{wiktionary, obligate As an adjective, obligate means "by necessity" (antonym '' facultative'') and is used mainly in biology in phrases such as: * Obligate aerobe, an organism that cannot survive without oxygen * Obligate anaerobe, an organism tha ...
neotenic salamanders, including species of ''
Necturus ''Necturus'' is a genus of aquatic salamanders native to the eastern United States and Canada. They are commonly known as waterdogs and mudpuppies. The common mudpuppy ''(N. maculosus)'' is probably the best-known species – as an amphibian wi ...
'', ''
Proteus In Greek mythology, Proteus (; Ancient Greek: Πρωτεύς, ''Prōteus'') is an early prophetic sea-god or god of rivers and oceanic bodies of water, one of several deities whom Homer calls the "Old Man of the Sea" ''(hálios gérôn)''. ...
'' and ''Amphiuma'', and many examples of
facultative {{wiktionary, facultative Facultative means "optional" or "discretionary" (antonym '' obligate''), used mainly in biology in phrases such as: * Facultative (FAC), facultative wetland (FACW), or facultative upland (FACU): wetland indicator statuses ...
ones, such as the
northwestern salamander The northwestern salamander (''Ambystoma gracile'') is a species of mole salamander that inhabits the northwest Pacific coast of North America. These fairly large salamanders grow to 8.7 in (220 mm) in length. It is found from southeastern ...
(''Ambystoma gracile'') and the
tiger salamander The tiger salamander (''Ambystoma tigrinum'') is a species of mole salamander and one of the largest terrestrial salamanders in North America. Description These salamanders usually grow to a length of with a lifespan of around 12–15 years. ...
(''A. tigrinum'') that adopt this strategy under appropriate environmental circumstances. Lungless salamanders in the family Plethodontidae are terrestrial and lay a small number of unpigmented eggs in a cluster among damp leaf litter. Each egg has a large yolk sac and the larva feeds on this while it develops inside the egg, emerging fully formed as a juvenile salamander. The female salamander often broods the eggs. In the genus ''
Ensatinas The ensatina (''Ensatina eschscholtzii'') is a species complex of plethodontid (lungless) salamanders found in coniferous forests, oak woodland and chaparral
'', the female has been observed to coil around them and press her throat area against them, effectively massaging them with a mucous secretion. In newts and salamanders, metamorphosis is less dramatic than in frogs. This is because the larvae are already carnivorous and continue to feed as predators when they are adults so few changes are needed to their digestive systems. Their lungs are functional early, but the larvae do not make as much use of them as do tadpoles. Their gills are never covered by gill sacs and are reabsorbed just before the animals leave the water. Other changes include the reduction in size or loss of tail fins, the closure of gill slits, thickening of the skin, the development of eyelids, and certain changes in dentition and tongue structure. Salamanders are at their most vulnerable at metamorphosis as swimming speeds are reduced and transforming tails are encumbrances on land. Adult salamanders often have an aquatic phase in spring and summer, and a land phase in winter. For adaptation to a water phase, prolactin is the required hormone, and for adaptation to the land phase, thyroxine. External gills do not return in subsequent aquatic phases because these are completely absorbed upon leaving the water for the first time.


Caecilians

Most terrestrial caecilians that lay eggs do so in burrows or moist places on land near bodies of water. The development of the young of ''
Ichthyophis glutinosus ''Ichthyophis glutinosus'', the Ceylon caecilian or common yellow-banded caecilian, is a species of caecilian in the family Ichthyophiidae endemic to Sri Lanka. Its natural habitats are moist tropical and subtropical forests and pastures. Descri ...
'', a species from Sri Lanka, has been much studied. The eel-like larvae hatch out of the eggs and make their way to water. They have three pairs of external red feathery gills, a blunt head with two rudimentary eyes, a lateral line system and a short tail with fins. They swim by undulating their body from side to side. They are mostly active at night, soon lose their gills and make sorties onto land. Metamorphosis is gradual. By the age of about ten months they have developed a pointed head with sensory tentacles near the mouth and lost their eyes, lateral line systems and tails. The skin thickens, embedded scales develop and the body divides into segments. By this time, the caecilian has constructed a burrow and is living on land. In the majority of species of caecilians, the young are produced by viviparity. ''
Typhlonectes compressicauda ''Typhlonectes compressicauda'', the Cayenne caecilian, is a species of amphibian in the family Typhlonectidae that lives in water. It is found in Amazonian Brazil, Peru, and Colombia as well as in Guyana and French Guiana, and likely Suriname, ...
'', a species from South America, is typical of these. Up to nine larvae can develop in the oviduct at any one time. They are elongated and have paired sac-like gills, small eyes and specialised scraping teeth. At first, they feed on the yolks of the eggs, but as this source of nourishment declines they begin to rasp at the ciliated epithelial cells that line the oviduct. This stimulates the secretion of fluids rich in lipids and mucoproteins on which they feed along with scrapings from the oviduct wall. They may increase their length sixfold and be two-fifths as long as their mother before being born. By this time they have undergone metamorphosis, lost their eyes and gills, developed a thicker skin and mouth tentacles, and reabsorbed their teeth. A permanent set of teeth grow through soon after birth. The ringed caecilian (''
Siphonops annulatus ''Siphonops annulatus'', the ringed caecilian, is a species of caecilian in the family Siphonopidae from South America. It might have the broadest known distribution among terrestrial caecilian species. Description Ringed caecilian measures in ...
'') has developed a unique adaptation for the purposes of reproduction. The progeny feed on a skin layer that is specially developed by the adult in a phenomenon known as maternal dermatophagy. The brood feed as a batch for about seven minutes at intervals of approximately three days which gives the skin an opportunity to regenerate. Meanwhile, they have been observed to ingest fluid exuded from the maternal cloaca.


Parental care

The care of offspring among amphibians has been little studied but, in general, the larger the number of eggs in a batch, the less likely it is that any degree of parental care takes place. Nevertheless, it is estimated that in up to 20% of amphibian species, one or both adults play some role in the care of the young. Those species that breed in smaller water bodies or other specialised habitats tend to have complex patterns of behaviour in the care of their young. Many woodland salamanders lay clutches of eggs under dead logs or stones on land. The black mountain salamander (''Desmognathus welteri'') does this, the mother brooding the eggs and guarding them from predation as the embryos feed on the yolks of their eggs. When fully developed, they break their way out of the egg capsules and disperse as juvenile salamanders. The male hellbender, a primitive salamander, excavates an underwater nest and encourages females to lay there. The male then guards the site for the two or three months before the eggs hatch, using body undulations to fan the eggs and increase their supply of oxygen. The male ''
Colostethus subpunctatus The cream-backed poison frog (''Hyloxalus subpunctatus'') is a species of frog in the family Dendrobatidae endemic to Colombia. Its natural habitats are subtropical or tropical, high-altitude grassland, shrub-dominated wetlands, swamps, interm ...
'', a tiny frog, protects the egg cluster which is hidden under a stone or log. When the eggs hatch, the male transports the tadpoles on his back, stuck there by a mucous secretion, to a temporary pool where he dips himself into the water and the tadpoles drop off. The male
midwife toad Midwife toads are a genus (''Alytes'') of frogs in the family Alytidae (formerly Discoglossidae), and are found in most of Europe and northwestern Africa. Characteristic of these toad-like frogs is their parental care; the males carry a string of ...
(''Alytes obstetricans'') winds egg strings round his thighs and carries the eggs around for up to eight weeks. He keeps them moist and when they are ready to hatch, he visits a pond or ditch and releases the tadpoles. The female
gastric-brooding frog The gastric-brooding frogs or platypus frogs (''Rheobatrachus'') is a genus of extinct ground-dwelling frogs native to Queensland in eastern Australia. The genus consisted of only two species, both of which became extinct in the mid-1980s. The ...
(''Rheobatrachus spp.'') reared larvae in her stomach after swallowing either the eggs or hatchlings; however, this stage was never observed before the species became extinct. The tadpoles secrete a hormone that inhibits digestion in the mother whilst they develop by consuming their very large yolk supply. The
pouched frog The pouched frog (''Assa darlingtoni''), or hip pocket frog, is a small, terrestrial frog found in rainforests in mountain areas of south-eastern Queensland and northern New South Wales, Australia. It is one of two species within the genus '' A ...
(''Assa darlingtoni'') lays eggs on the ground. When they hatch, the male carries the tadpoles around in brood pouches on his hind legs. The aquatic
Surinam toad The common Surinam toad or star-fingered toad (''Pipa pipa'') is an purely aquatic species of frog in the family ''Pipidae'' with a widespread distribution in South America. The species is known for incubating its eggs in honeycombed chambers i ...
(''Pipa pipa'') raises its young in pores on its back where they remain until metamorphosis. The granular poison frog (''Oophaga granulifera'') is typical of a number of tree frogs in the poison dart frog family
Dendrobatidae Poison dart frog (also known as dart-poison frog, poison frog or formerly known as poison arrow frog) is the common name of a group of frogs in the family Dendrobatidae which are native to tropical Central and South America. These species are ...
. Its eggs are laid on the forest floor and when they hatch, the tadpoles are carried one by one on the back of an adult to a suitable water-filled crevice such as the
axil A leaf ( : leaves) is any of the principal appendages of a vascular plant stem, usually borne laterally aboveground and specialized for photosynthesis. Leaves are collectively called foliage, as in "autumn foliage", while the leaves, ste ...
of a leaf or the rosette of a
bromeliad The Bromeliaceae (the bromeliads) are a family of monocot flowering plants of about 80 genera and 3700 known species, native mainly to the tropical Americas, with several species found in the American subtropics and one in tropical west Africa, ...
. The female visits the nursery sites regularly and deposits unfertilised eggs in the water and these are consumed by the tadpoles.


Genetics and genomics

Amphibians are notable among vertebrates for their diversity of chromosomes and genomes. The
karyotype A karyotype is the general appearance of the complete set of metaphase chromosomes in the cells of a species or in an individual organism, mainly including their sizes, numbers, and shapes. Karyotyping is the process by which a karyotype is disce ...
s ( chromosomes) have been determined for at least 1,193 (14.5%) of the ≈8,200 known ( diploid) species, including 963
anurans A frog is any member of a diverse and largely carnivorous group of short-bodied, tailless amphibians composing the order Anura (ανοὐρά, literally ''without tail'' in Ancient Greek). The oldest fossil "proto-frog" ''Triadobatrachus'' ...
, 209 salamanders, and 21
caecilian Caecilians (; ) are a group of limbless, vermiform or serpentine amphibians. They mostly live hidden in the ground and in stream substrates, making them the least familiar order of amphibians. Caecilians are mostly distributed in the tropics of ...
s. Generally, the karyotypes of diploid amphibians are characterized by 20–26 bi-armed chromosomes. Amphibians have also very large genomes compared to other taxa of vertebrates and corresponding variation in genome size (
C-value C-value is the amount, in picograms, of DNA contained within a haploid nucleus (e.g. a gamete) or one half the amount in a diploid somatic cell of a eukaryotic organism. In some cases (notably among diploid organisms), the terms C-value and geno ...
: picograms of DNA in haploid nuclei). The genome sizes range from 0.95 to 11.5 pg in frogs, from 13.89 to 120.56 pg in salamanders, and from 2.94 to 11.78 pg in caecilians. The large genome sizes have prevented whole- genome sequencing of amphibians although a number of genomes have been published recently. The 1.7GB draft genome of ''
Xenopus tropicalis The western clawed frog (''Xenopus tropicalis'') is a species of frog in the family Pipidae, also known as tropical clawed frog. It is the only species in the genus ''Xenopus'' to have a diploid genome. Its genome has been sequenced, making it a ...
'' was the first to be reported for amphibians in 2010. Compared to some salamanders this frog genome is tiny. For instance, the genome of the Mexican axolotl turned out to be 32 Gb, which is more than 10 times larger than the human genome (3GB).


Feeding and diet

With a few exceptions, adult amphibians are predators, feeding on virtually anything that moves that they can swallow. The diet mostly consists of small prey that do not move too fast such as beetles, caterpillars, earthworms and spiders. The sirens ('' Siren spp.'') often ingest aquatic plant material with the invertebrates on which they feed and a Brazilian tree frog (''
Xenohyla truncata ''Xenohyla truncata'', the Izecksohn's Brazilian treefrog, is a species of frugivorous tree frog in the family Hylidae. It is endemic to the State of Rio de Janeiro, Brazil. The range of this frog ranges from the southern border of Espírito San ...
'') includes a large quantity of fruit in its diet. The
Mexican burrowing toad This frog belongs to the Family ''Rhinophrynidae'' and Genus Rhinophrynus. Rhinophrynus dorsalis is the only living representation of the family Rhinophrynidae. This family is also known as the burrowing toads, and is evidently a small and specia ...
(''Rhinophrynus dorsalis'') has a specially adapted tongue for picking up ants and termites. It projects it with the tip foremost whereas other frogs flick out the rear part first, their tongues being hinged at the front. Food is mostly selected by sight, even in conditions of dim light. Movement of the prey triggers a feeding response. Frogs have been caught on fish hooks baited with red flannel and green frogs (''Rana clamitans'') have been found with stomachs full of elm seeds that they had seen floating past. Toads, salamanders and caecilians also use smell to detect prey. This response is mostly secondary because salamanders have been observed to remain stationary near odoriferous prey but only feed if it moves. Cave-dwelling amphibians normally hunt by smell. Some salamanders seem to have learned to recognize immobile prey when it has no smell, even in complete darkness. Amphibians usually swallow food whole but may chew it lightly first to subdue it. They typically have small hinged
pedicellate teeth Pedicellate teeth are a tooth morphology today unique to modern amphibians, but also seen in a variety of extinct labyrinthodonts "Labyrinthodontia" (Greek, 'maze-toothed') is an informal grouping of extinct predatory amphibians which were major ...
, a feature unique to amphibians. The base and crown of these are composed of dentine separated by an