HOME

TheInfoList



OR:

Sea urchins () are spiny, globular
echinoderm An echinoderm () is any member of the phylum Echinodermata (). The adults are recognisable by their (usually five-point) radial symmetry, and include starfish, brittle stars, sea urchins, sand dollars, and sea cucumbers, as well as the ...
s in the class Echinoidea. About 950 species of sea urchin live on the seabed of every ocean and inhabit every depth zone from the intertidal seashore down to . The spherical, hard shells (
tests Test(s), testing, or TEST may refer to: * Test (assessment), an educational assessment intended to measure the respondents' knowledge or other abilities Arts and entertainment * ''Test'' (2013 film), an American film * ''Test'' (2014 film), ...
) of sea urchins are round and spiny, ranging in diameter from . Sea urchins move slowly, crawling with
tube feet Tube feet (technically podia) are small active tubular projections on the oral face of an echinoderm, whether the arms of a starfish, or the undersides of sea urchins, sand dollars and sea cucumbers; they are more discreet though present on br ...
, and also propel themselves with their spines. Although
algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic eukaryotic organisms. It is a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from unicellular micr ...
are the primary diet, sea urchins also eat slow-moving ( sessile) animals. Predators that eat sea urchins include a wide variety of fish,
starfish Starfish or sea stars are star-shaped echinoderms belonging to the class Asteroidea (). Common usage frequently finds these names being also applied to ophiuroids, which are correctly referred to as brittle stars or basket stars. Starfish a ...
, crabs,
marine mammals Marine mammals are aquatic mammals that rely on the ocean and other marine ecosystems for their existence. They include animals such as seals, whales, manatees, sea otters and polar bears. They are an informal group, unified only by their ...
. Sea urchins are also used as food especially in
Japan Japan ( ja, 日本, or , and formally , ''Nihonkoku'') is an island country in East Asia. It is situated in the northwest Pacific Ocean, and is bordered on the west by the Sea of Japan, while extending from the Sea of Okhotsk in the n ...
. Adult sea urchins have fivefold symmetry, but their pluteus larvae feature bilateral (mirror) symmetry, indicating that the sea urchin belongs to the
Bilateria The Bilateria or bilaterians are animals with bilateral symmetry as an embryo, i.e. having a left and a right side that are mirror images of each other. This also means they have a head and a tail (anterior-posterior axis) as well as a belly and ...
group of animal
phyla Phyla, the plural of ''phylum'', may refer to: * Phylum, a biological taxon between Kingdom and Class * by analogy, in linguistics, a large division of possibly related languages, or a major language family which is not subordinate to another Phy ...
, which also comprises the
chordate A chordate () is an animal of the phylum Chordata (). All chordates possess, at some point during their larval or adult stages, five synapomorphies, or primary physical characteristics, that distinguish them from all the other taxa. These fi ...
s and the
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 chiti ...
s, the
annelid The annelids (Annelida , from Latin ', "little ring"), also known as the segmented worms, are a large phylum, with over 22,000 extant species including ragworms, earthworms, and leeches. The species exist in and have adapted to various ecol ...
s and the
mollusc Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000  extant species of molluscs are recognized. The number of fossil species is est ...
s, and are found in every ocean and in every climate, from the
tropics The tropics are the regions of Earth surrounding the Equator. They are defined in latitude by the Tropic of Cancer in the Northern Hemisphere at N and the Tropic of Capricorn in the Southern Hemisphere at S. The tropics are also refer ...
to the polar regions, and inhabit marine benthic (sea bed) habitats, from rocky shores to hadal zone depths. The fossil record of the ''Echinoids'' dates from the Ordovician period, some 450 million years ago. The closest echinoderm relatives of the sea urchin are the
sea cucumber Sea cucumbers are echinoderms from the class Holothuroidea (). They are marine animals with a leathery skin and an elongated body containing a single, branched gonad. Sea cucumbers are found on the sea floor worldwide. The number of holothuri ...
s (Holothuroidea), both of which are
deuterostome Deuterostomia (; in Greek) are animals typically characterized by their anus forming before their mouth during embryonic development. The group's sister clade is Protostomia, animals whose digestive tract development is more varied. Some ...
s, a clade that includes the
chordate A chordate () is an animal of the phylum Chordata (). All chordates possess, at some point during their larval or adult stages, five synapomorphies, or primary physical characteristics, that distinguish them from all the other taxa. These fi ...
s. The animals have been studied since the 19th century as
model organism A model organism (often shortened to model) is a non-human species that is extensively studied to understand particular biological phenomena, with the expectation that discoveries made in the model organism will provide insight into the workin ...
s in
developmental biology Developmental biology is the study of the process by which animals and plants grow and develop. Developmental biology also encompasses the biology of regeneration, asexual reproduction, metamorphosis, and the growth and differentiation of st ...
, as their embryos were easy to observe. That has continued with studies of their
genome In the fields of molecular biology and genetics, a genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding ...
s because of their unusual fivefold symmetry and relationship to chordates. Species such as the slate pencil urchin are popular in aquariums, where they are useful for controlling algae. Fossil urchins have been used as protective
amulet An amulet, also known as a good luck charm or phylactery, is an object believed to confer protection upon its possessor. The word "amulet" comes from the Latin word amuletum, which Pliny's ''Natural History'' describes as "an object that protect ...
s.


Diversity

Sea urchins are members of the
phylum In biology, a phylum (; plural: phyla) is a level of classification or taxonomic rank below kingdom and above class. Traditionally, in botany the term division has been used instead of phylum, although the International Code of Nomenclature ...
Echinoderm An echinoderm () is any member of the phylum Echinodermata (). The adults are recognisable by their (usually five-point) radial symmetry, and include starfish, brittle stars, sea urchins, sand dollars, and sea cucumbers, as well as the ...
ata, which also includes sea stars, sea cucumbers, brittle stars, and crinoids. Like other echinoderms, they have five-fold symmetry (called pentamerism) and move by means of hundreds of tiny, transparent, adhesive "
tube feet Tube feet (technically podia) are small active tubular projections on the oral face of an echinoderm, whether the arms of a starfish, or the undersides of sea urchins, sand dollars and sea cucumbers; they are more discreet though present on br ...
". The symmetry is not obvious in the living animal, but is easily visible in the dried
test Test(s), testing, or TEST may refer to: * Test (assessment), an educational assessment intended to measure the respondents' knowledge or other abilities Arts and entertainment * ''Test'' (2013 film), an American film * ''Test'' (2014 film), ...
. Specifically, the term "sea urchin" refers to the "regular echinoids", which are symmetrical and globular, and includes several different taxonomic groups, with two subclasses :
Euechinoidea The subclass Euechinoidea includes almost all living species of sea urchin, and fossil forms going back as far as the Triassic. Taxonomy List of orders according to World Register of Marine Species : * Infra-classis '' Acroechinoidea'' ** ...
("modern" sea urchins, including irregular ones) and Cidaroidea or "slate-pencil urchins", which have very thick, blunt spines, with algae and sponges growing on them. The "irregular" sea urchins are an infra-class inside the Euechinoidea, called
Irregularia Irregularia is an extant infraclass of sea urchins that first appeared in the Lower Jurassic. Description and characteristics These particular sea urchins are distinguished from other sea urchins by their irregular shape: the anus and often e ...
, and include
Atelostomata The Atelostomata are a type of sea urchins. They are distinguished from other sea urchins by their irregular shape and the absence of a feeding lantern. The group includes the well known heart urchins, as well as some less familiar and extinct ...
and
Neognathostomata The Neognathostomata are a superorder of sea urchins. Description and characteristics They are distinguished from other sea urchins by their irregular shape and a highly modified feeding lantern. The group includes the well known sand dollars, ...
. Irregular echinoids include: flattened sand dollars, sea biscuits, and heart urchins. Together with sea cucumbers ( Holothuroidea), they make up the subphylum Echinozoa, which is characterized by a globoid shape without arms or projecting rays. Sea cucumbers and the irregular echinoids have secondarily evolved diverse shapes. Although many sea cucumbers have branched
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 surrounding their oral openings, these have originated from modified tube feet and are not homologous to the arms of the crinoids, sea stars, and brittle stars. File:Paracentrotus lividus profil.JPG, '' Paracentrotus lividus'', a regular sea urchin (
Euechinoidea The subclass Euechinoidea includes almost all living species of sea urchin, and fossil forms going back as far as the Triassic. Taxonomy List of orders according to World Register of Marine Species : * Infra-classis '' Acroechinoidea'' ** ...
, infraclass Carinacea) File:Live Sand Dollar trying to bury itself in beach sand.jpg, A sand dollar, an irregular sea urchin (
Irregularia Irregularia is an extant infraclass of sea urchins that first appeared in the Lower Jurassic. Description and characteristics These particular sea urchins are distinguished from other sea urchins by their irregular shape: the anus and often e ...
) File:Phyllacanthus.jpg, '' Phyllacanthus imperialis'', a cidaroid sea urchin ( Cidaroidea)


Description

Urchins typically range in size from , although the largest species can reach up to . They have a rigid, usually spherical body bearing moveable spines, which gives 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 ...
the name ''Echinoidea'' (from the Greek 'spine'). The name ''urchin'' is an old word for hedgehog, which sea urchins resemble; they have archaically been called sea hedgehogs. The name is derived from Old French , from Latin ('hedgehog'). Like other echinoderms, sea urchin early larvae have bilateral symmetry, but they develop five-fold symmetry as they mature. This is most apparent in the "regular" sea urchins, which have roughly spherical bodies with five equally sized parts radiating out from their central axes. The mouth is at the base of the animal and the anus at the top; the lower surface is described as "oral" and the upper surface as "aboral". Several sea urchins, however, including the sand dollars, are oval in shape, with distinct front and rear ends, giving them a degree of bilateral symmetry. In these urchins, the upper surface of the body is slightly domed, but the underside is flat, while the sides are devoid of tube feet. This "irregular" body form has evolved to allow the animals to burrow through sand or other soft materials.


Systems


Musculoskeletal

Sea urchins might appear to be incapable of moving but this is a false impression. In some cases, the most visible sign of life is the spines, which are attached to ball-and-socket joints and can point in any direction; in most urchins, touch elicits a prompt reaction from the spines, which converge toward the touched point. Sea urchins have no visible eyes, legs, or means of propulsion, but can move freely but slowly over hard surfaces using adhesive tube feet, working in conjunction with the spines. The internal organs are enclosed in a hard shell or test composed of fused plates of calcium carbonate covered by a thin
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 ...
and
epidermis The epidermis is the outermost of the three layers that comprise the skin, the inner layers being the dermis and hypodermis. The epidermis layer provides a barrier to infection from environmental pathogens and regulates the amount of water rel ...
. The test is rigid, and divides into five ambulacral grooves separated by five interambulacral areas. Each of these areas consists of two rows of plates, so the sea urchin test includes 20 rows of plates in total. The plates are covered in rounded tubercles which contain the sockets to which the spines are attached by
ball and socket joint The ball-and-socket joint (or spheroid joint) is a type of synovial joint in which the ball-shaped surface of one rounded bone fits into the cup-like depression of another bone. The distal bone is capable of motion around an indefinite number o ...
s. The inner surface of the test is lined by
peritoneum The peritoneum is the serous membrane forming the lining of the abdominal cavity or coelom in amniotes and some invertebrates, such as annelids. It covers most of the intra-abdominal (or coelomic) organs, and is composed of a layer of mes ...
. Sea urchins convert aqueous
carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is t ...
using a catalytic process involving
nickel Nickel is a chemical element with symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive but large pieces are slow t ...
into the calcium carbonate portion of the test. Most species have two series of spines, primary (long) and secondary (short), distributed over the surface of the body, with the shortest at the poles and the longest at the equator. The spines are usually hollow and cylindrical. Contraction of the muscular sheath that covers the test causes the spines to lean in one direction or another, while an inner sheath of collagen fibres can reversibly change from soft to rigid which can lock the spine in one position. Located among the spines are several types of pedicellaria, moveable stalked structures with jaws. Sea urchins move by walking, using their many flexible tube feet in a way similar to that of starfish; regular sea urchins do not have any favourite walking direction. The tube feet protrude through pairs of pores in the test, and are operated by a water vascular system; this works through
hydraulic pressure Hydraulics (from Greek: Υδραυλική) is a technology and applied science using engineering, chemistry, and other sciences involving the mechanical properties and use of liquids. At a very basic level, hydraulics is the liquid counte ...
, allowing the sea urchin to pump water into and out of the tube feet. During locomotion, the tube feet are assisted by the spines which can be used for pushing the body along or to lift the test off the substrate. Movement is generally related to feeding, with the
red sea urchin The red sea urchin (''Mesocentrotus franciscanus'') is a sea urchin found in the northeastern Pacific Ocean from Alaska to Baja California. It lives in shallow waters from the low-tide line to greater than deep, and is typically found on rocky ...
(''Mesocentrotus franciscanus'') managing about a day when there is ample food, and up to a day where there is not. An inverted sea urchin can right itself by progressively attaching and detaching its tube feet and manipulating its spines to roll its body upright. Some species bury themselves in soft sediment using their spines, and '' Paracentrotus lividus'' uses its jaws to burrow into soft rocks. File:Sea Urchin test 5629 03 22.jpg, Test of an '' Echinus esculentus'', a regular sea urchin File:Echinodiscus2.jpg, Test of an '' Echinodiscus tenuissimus'', an irregular sea urchin (" sand dollar") File:Phyllacanthus imperialis test.JPG, Test of a '' Phyllacanthus imperialis'', a cidaroid sea urchin. These are characterised by their big tubercles, bearing large radiola. File:Sea urchin shell - pattern (6658690371).jpg, Close-up of the test showing an ambulacral groove with its two rows of pore-pairs, between two interambulacra areas (green). The tubercles are non-perforated. File:Sea Urchin Shell detail.jpg, Close-up of a cidaroid sea urchin apical disc: the 5 holes are the gonopores, and the central one is the anus ("periproct"). The biggest genital plate is the madreporite.


Feeding and digestion

The mouth lies in the centre of the oral surface in regular urchins, or towards one end in irregular urchins. It is surrounded by lips of softer tissue, with numerous small, embedded bony pieces. This area, called the peristome, also includes five pairs of modified tube feet and, in many species, five pairs of gills. The jaw apparatus consists of five strong arrow-shaped plates known as pyramids, the ventral surface of each of which has a toothband with a hard tooth pointing towards the centre of the mouth. Specialised muscles control the protrusion of the apparatus and the action of the teeth, and the animal can grasp, scrape, pull and tear. The structure of the mouth and teeth have been found to be so efficient at grasping and grinding that similar structures have been tested for use in real-world applications. On the upper surface of the test at the aboral pole is a membrane, the periproct, which surrounds the
anus The anus (Latin, 'ring' or 'circle') is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to control the expulsion of feces, the residual semi-solid waste that remains after food digestion, which, ...
. The periproct contains a variable number of hard plates, five of which, the genital plates, contain the gonopores, and one is modified to contain the madreporite, which is used to balance the water vascular system. The mouth of most sea urchins is made up of five calcium carbonate teeth or plates, with a fleshy, tongue-like structure within. The entire chewing organ is known as Aristotle's lantern from
Aristotle Aristotle (; grc-gre, Ἀριστοτέλης ''Aristotélēs'', ; 384–322 BC) was a Greek philosopher and polymath during the Classical period in Ancient Greece. Taught by Plato, he was the founder of the Peripatetic school of ...
's description in his ''
History of Animals ''History of Animals'' ( grc-gre, Τῶν περὶ τὰ ζῷα ἱστοριῶν, ''Ton peri ta zoia historion'', "Inquiries on Animals"; la, Historia Animalium, "History of Animals") is one of the major Aristotle's biology, texts on biolo ...
'' (translated by D'Arcy Thompson): However, this has recently been proven to be a mistranslation. Aristotle's lantern is actually referring to the whole shape of sea urchins, which look like the ancient lamps of Aristotle's time. Heart urchins are unusual in not having a lantern. Instead, the mouth is surrounded by
cilia The cilium, plural cilia (), is a membrane-bound organelle found on most types of eukaryotic cell, and certain microorganisms known as ciliates. Cilia are absent in bacteria and archaea. The cilium has the shape of a slender threadlike proje ...
that pull strings of mucus containing food particles towards a series of grooves around the mouth. The lantern, where present, surrounds both the mouth cavity and the
pharynx The pharynx (plural: pharynges) is the part of the throat behind the mouth and nasal cavity, and above the oesophagus and trachea (the tubes going down to the stomach and the lungs). It is found in vertebrates and invertebrates, though its st ...
. At the top of the lantern, the pharynx opens into the esophagus, which runs back down the outside of the lantern, to join the small
intestine The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system that leads from the mouth to the anus. The GI tract contains all the major organs of the digestive system, in humans an ...
and a single caecum. The small intestine runs in a full circle around the inside of the test, before joining the large intestine, which completes another circuit in the opposite direction. From the large intestine, a
rectum The rectum is the final straight portion of the large intestine in humans and some other mammals, and the gut in others. The adult human rectum is about long, and begins at the rectosigmoid junction (the end of the sigmoid colon) at the l ...
ascends towards the anus. Despite the names, the small and large intestines of sea urchins are in no way homologous to the similarly named structures in vertebrates. Digestion occurs in the intestine, with the caecum producing further digestive
enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products ...
s. An additional tube, called the siphon, runs beside much of the intestine, opening into it at both ends. It may be involved in resorption of water from food.


Circulation and respiration

The water vascular system leads downwards from the madreporite through the slender stone canal to the ring canal, which encircles the oesophagus. Radial canals lead from here through each ambulacral area to terminate in a small tentacle that passes through the ambulacral plate near the aboral pole. Lateral canals lead from these radial canals, ending in ampullae. From here, two tubes pass through a pair of pores on the plate to terminate in the tube feet. Sea urchins possess a hemal system with a complex network of vessels in the mesenteries around the gut, but little is known of the functioning of this system. However, the main circulatory fluid fills the general body cavity, or
coelom The coelom (or celom) is the main body cavity in most animals and is positioned inside the body to surround and contain the digestive tract and other organs. In some animals, it is lined with mesothelium. In other animals, such as molluscs, ...
. This coelomic fluid contains phagocytic coelomocytes, which move through the vascular and hemal systems and are involved in internal transport and gas exchange. The coelomocytes are an essential part of
blood clotting Coagulation, also known as clotting, is the process by which blood changes from a liquid to a gel, forming a blood clot. It potentially results in hemostasis, the cessation of blood loss from a damaged vessel, followed by repair. The mechan ...
, but also collect waste products and actively remove them from the body through the gills and tube feet. Most sea urchins possess five pairs of external gills attached to the peristomial membrane around their mouths. These thin-walled projections of the body cavity are the main organs of respiration in those urchins that possess them. Fluid can be pumped through the gills' interiors by muscles associated with the lantern, but this does not provide a continuous flow, and occurs only when the animal is low in oxygen. Tube feet can also act as respiratory organs, and are the primary sites of gas exchange in heart urchins and sand dollars, both of which lack gills. The inside of each tube foot is divided by a septum which reduces diffusion between the incoming and outgoing streams of fluid.


Nervous system and senses

The nervous system of sea urchins has a relatively simple layout. With no true brain, the neural center is a large nerve ring encircling the mouth just inside the lantern. From the nerve ring, five nerves radiate underneath the radial canals of the water vascular system, and branch into numerous finer nerves to innervate the tube feet, spines, and pedicellariae. Sea urchins are sensitive to touch, light, and chemicals. There are numerous sensitive cells in the epithelium, especially in the spines, pedicellaria and tube feet, and around the mouth. Although they do not have eyes or eye spots (except for diadematids, which can follow a threat with their spines), the entire body of most regular sea urchins might function as a compound eye. In general, sea urchins are negatively attracted to light, and seek to hide themselves in crevices or under objects. Most species, apart from pencil urchins, have statocysts in globular organs called spheridia. These are stalked structures and are located within the ambulacral areas; their function is to help in gravitational orientation.


Life history


Reproduction

Sea urchins are
dioecious Dioecy (; ; adj. dioecious , ) is a characteristic of a species, meaning that it has distinct individual organisms (unisexual) that produce male or female gametes, either directly (in animals) or indirectly (in seed plants). Dioecious reproducti ...
, having separate male and female sexes, although no distinguishing features are visible externally. In addition to their role in reproduction, the
gonad A gonad, sex gland, or reproductive gland is a mixed gland that produces the gametes and sex hormones of an organism. Female reproductive cells are egg cells, and male reproductive cells are sperm. The male gonad, the testicle, produces sp ...
s are also nutrient storing organs, and are made up of two main type of cells:
germ cell Germ or germs may refer to: Science * Germ (microorganism), an informal word for a pathogen * Germ cell, cell that gives rise to the gametes of an organism that reproduces sexually * Germ layer, a primary layer of cells that forms during embr ...
s, and
somatic cell A somatic cell (from Ancient Greek σῶμα ''sôma'', meaning "body"), or vegetal cell, is any biological cell forming the body of a multicellular organism other than a gamete, germ cell, gametocyte or undifferentiated stem cell. Such cells com ...
s called nutritive phagocytes. Regular sea urchins have five gonads, lying underneath the interambulacral regions of the test, while the irregular forms mostly have four, with the hindmost gonad being absent; heart urchins have three or two. Each gonad has a single duct rising from the upper pole to open at a gonopore lying in one of the genital plates surrounding the anus. Some burrowing sand dollars have an elongated papilla that enables the liberation of gametes above the surface of the sediment. The gonads are lined with muscles underneath the peritoneum, and these allow the animal to squeeze its
gamete A gamete (; , ultimately ) is a haploid cell that fuses with another haploid cell during fertilization in organisms that reproduce sexually. Gametes are an organism's reproductive cells, also referred to as sex cells. In species that produce ...
s through the duct and into the surrounding sea water, where
fertilization Fertilisation or fertilization (see spelling differences), also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes to give rise to a new individual organism or offspring and initiate its development. Pro ...
takes place.


Development

During early development, the sea urchin
embryo An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...
undergoes 10 cycles of
cell division Cell division is the process by which a parent cell divides into two daughter cells. Cell division usually occurs as part of a larger cell cycle in which the cell grows and replicates its chromosome(s) before dividing. In eukaryotes, there ...
, resulting in a single
epithelial Epithelium or epithelial tissue is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. It is a thin, continuous, protective layer of compactly packed cells with a little intercellu ...
layer enveloping the
blastocoel The blastocoel (), also spelled blastocoele and blastocele, and also called cleavage cavity, or segmentation cavity is a fluid-filled or yolk-filled cavity that forms in the blastula during very early embryonic development. At this stage in mammal ...
. The embryo then begins
gastrulation Gastrulation is the stage in the early embryonic development of most animals, during which the blastula (a single-layered hollow sphere of cells), or in mammals the blastocyst is reorganized into a multilayered structure known as the gastrula. ...
, a multipart process which dramatically rearranges its structure by invagination to produce the three
germ layer A germ layer is a primary layer of cells that forms during embryonic development. The three germ layers in vertebrates are particularly pronounced; however, all eumetazoans (animals that are sister taxa to the sponges) produce two or three p ...
s, involving an epithelial-mesenchymal transition; primary
mesenchyme Mesenchyme () is a type of loosely organized animal embryonic connective tissue of undifferentiated cells that give rise to most tissues, such as skin, blood or bone. The interactions between mesenchyme and epithelium help to form nearly every ...
cells move into the blastocoel and become
mesoderm The mesoderm is the middle layer of the three germ layers that develops during gastrulation in the very early development of the embryo of most animals. The outer layer is the ectoderm, and the inner layer is the endoderm.Langman's Medical Emb ...
. It has been suggested that
epithelial polarity Epithelial polarity is one example of the cell polarity that is a fundamental feature of many types of cells. Epithelial cells feature distinct 'apical', 'lateral' and 'basal' plasma membrane domains. Epithelial cells connect to one another via ...
together with planar cell polarity might be sufficient to drive gastrulation in sea urchins. An unusual feature of sea urchin development is the replacement of the larva's
bilateral symmetry Symmetry in biology refers to the symmetry observed in organisms, including plants, animals, fungi, and bacteria. External symmetry can be easily seen by just looking at an organism. For example, take the face of a human being which has a ...
by the adult's broadly fivefold symmetry. During cleavage, mesoderm and small micromeres are specified. At the end of gastrulation, cells of these two types form
coelom The coelom (or celom) is the main body cavity in most animals and is positioned inside the body to surround and contain the digestive tract and other organs. In some animals, it is lined with mesothelium. In other animals, such as molluscs, ...
ic pouches. In the larval stages, the adult rudiment grows from the left coelomic pouch; after metamorphosis, that rudiment grows to become the adult. The animal-vegetal axis is established before the egg is fertilized. The oral-aboral axis is specified early in cleavage, and the left-right axis appears at the late gastrula stage.


Life cycle and development

In most cases, the female's eggs float freely in the sea, but some species hold onto them with their spines, affording them a greater degree of protection. The unfertilized egg meets with the free-floating sperm released by males, and develops into a free-swimming
blastula Blastulation is the stage in early animal embryonic development that produces the blastula. In mammalian development the blastula develops into the blastocyst with a differentiated inner cell mass and an outer trophectoderm. The blastula (fro ...
embryo in as few as 12 hours. Initially a simple ball of cells, the blastula soon transforms into a cone-shaped echinopluteus larva. In most species, this larva has 12 elongated arms lined with bands of cilia that capture food particles and transport them to the mouth. In a few species, the blastula contains supplies of nutrient
yolk Among animals which produce eggs, the yolk (; also known as the vitellus) is the nutrient-bearing portion of the egg whose primary function is to supply food for the development of the embryo. Some types of egg contain no yolk, for example ...
and lacks arms, since it has no need to feed. Several months are needed for the larva to complete its development, the change into the adult form beginning with the formation of test plates in a juvenile rudiment which develops on the left side of the larva, its axis being perpendicular to that of the larva. Soon, the larva sinks to the bottom and
metamorphoses The ''Metamorphoses'' ( la, Metamorphōsēs, from grc, μεταμορφώσεις: "Transformations") is a Latin narrative poem from 8 CE by the Roman poet Ovid. It is considered his '' magnum opus''. The poem chronicles the history of the ...
into a juvenile urchin in as little as one hour. In some species, adults reach their maximum size in about five years. The purple urchin becomes sexually mature in two years and may live for twenty.


Ecology


Trophic level

Sea urchins feed mainly on
algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic eukaryotic organisms. It is a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from unicellular micr ...
, so they are primarily
herbivore A herbivore is an animal anatomically and physiologically adapted to eating plant material, for example foliage or marine algae, for the main component of its diet. As a result of their plant diet, herbivorous animals typically have mouthpar ...
s, but can feed on sea cucumbers and a wide range of invertebrates, such as
mussel Mussel () is the common name used for members of several families of bivalve molluscs, from saltwater and freshwater habitats. These groups have in common a shell whose outline is elongated and asymmetrical compared with other edible clams, which ...
s, polychaetes,
sponge Sponges, the members of the phylum Porifera (; meaning 'pore bearer'), are a basal animal clade as a sister of the diploblasts. They are multicellular organisms that have bodies full of pores and channels allowing water to circulate throu ...
s, brittle stars, and crinoids, making them omnivores, consumers at a range of
trophic level The trophic level of an organism is the position it occupies in a food web. A food chain is a succession of organisms that eat other organisms and may, in turn, be eaten themselves. The trophic level of an organism is the number of steps it ...
s.


Predators, parasites, and diseases

Mass mortality of sea urchins was first reported in the 1970s, but diseases in sea urchins had been little studied before the advent of aquaculture. In 1981, bacterial "spotting disease" caused almost complete mortality in juvenile ''
Pseudocentrotus depressus ''Pseudocentrotus depressus'', commonly known as the pink sea urchin, is a species of sea urchin, one of only two species in the genus ''Pseudocentrotus''. It was first described in 1864 by the American marine zoologist Alexander Agassiz as '' ...
'' and '' Hemicentrotus pulcherrimus'', both cultivated in Japan; the disease recurred in succeeding years. It was divided into a cool-water "spring" disease and a hot-water "summer" form. Another condition,
bald sea urchin disease Bald sea urchin disease is a bacterial disease known to affect several species of sea urchins in the Mediterranean Sea, North Atlantic and along the California coastline. Research suggests two pathogens are responsible for the disease, '' Listone ...
, causes loss of spines and skin lesions and is believed to be bacterial in origin. Adult sea urchins are usually well protected against most predators by their strong and sharp spines, which can be venomous in some species. The small urchin clingfish lives among the spines of urchins such as '' Diadema''; juveniles feed on the pedicellariae and sphaeridia, adult males choose the tube feet and adult females move away to feed on shrimp eggs and molluscs. Sea urchins are one of the favourite foods of many
lobster Lobsters are a family (Nephropidae, synonym Homaridae) of marine crustaceans. They have long bodies with muscular tails and live in crevices or burrows on the sea floor. Three of their five pairs of legs have claws, including the first pair, ...
s,
crab Crabs are decapod crustaceans of the infraorder Brachyura, which typically have a very short projecting "tail" (abdomen) ( el, βραχύς , translit=brachys = short, / = tail), usually hidden entirely under the thorax. They live in all th ...
s,
triggerfish Triggerfish are about 40 species of often brightly colored fish of the family Balistidae. Often marked by lines and spots, they inhabit tropical and subtropical oceans throughout the world, with the greatest species richness in the Indo-Pacif ...
, California sheephead,
sea otter The sea otter (''Enhydra lutris'') is a marine mammal native to the coasts of the northern and eastern North Pacific Ocean. Adult sea otters typically weigh between , making them the heaviest members of the weasel family, but among the smal ...
and wolf eels (which specialise in sea urchins). All these animals carry particular adaptations (teeth, pincers, claws) and a strength that allow them to overcome the excellent protective features of sea urchins. Left unchecked by predators, urchins devastate their environments, creating what biologists call an urchin barren, devoid of macroalgae and associated
fauna Fauna is all of the animal life present in a particular region or time. The corresponding term for plants is '' flora'', and for fungi, it is '' funga''. Flora, fauna, funga and other forms of life are collectively referred to as '' biota''. ...
. Sea urchins graze on the lower stems of kelp, causing the kelp to drift away and die. Loss of the habitat and nutrients provided by kelp forests leads to profound cascade effects on the marine ecosystem. Sea otters have re-entered
British Columbia British Columbia (commonly abbreviated as BC) is the westernmost province of Canada, situated between the Pacific Ocean and the Rocky Mountains. It has a diverse geography, with rugged landscapes that include rocky coastlines, sandy beaches, for ...
, dramatically improving coastal ecosystem health. File:Wolf eel eating a sea urchin.jpg,
Wolf eel The wolf eel (''Anarrhichthys ocellatus'') is a species of marine ray-finned fish belonging to the family Anarhichadidae, the wolf fishes. It is found in the North Pacific Ocean. Despite its common name and resemblance, it is not a true eel. It ...
, a highly specialized predator of sea urchins File:Sea otter with sea urchin.jpg, A
sea otter The sea otter (''Enhydra lutris'') is a marine mammal native to the coasts of the northern and eastern North Pacific Ocean. Adult sea otters typically weigh between , making them the heaviest members of the weasel family, but among the smal ...
feeding on a
purple sea urchin ''Strongylocentrotus purpuratus'', the purple sea urchin, lives along the eastern edge of the Pacific Ocean extending from Ensenada, Mexico, to British Columbia, Canada. This sea urchin species is deep purple in color, and lives in lower int ...
. File:Carpilius convexus is consuming Heterocentrotus trigonarius in Hawaii.jpg, A crab ('' Carpilius convexus'') attacking a slate pencil sea urchin ('' Heterocentrotus mamillatus'') File:Saddle Wrasse are feeding on sea urchin in Kona.jpg, A
wrasse The wrasses are a family, Labridae, of marine fish, many of which are brightly colored. The family is large and diverse, with over 600 species in 81 genera, which are divided into 9 subgroups or tribes. They are typically small, most of them le ...
finishing the remains of a damaged ''
Tripneustes gratilla ''Tripneustes gratilla'', the collector urchin, is a species of sea urchin. Collector urchins are found at depths of in the waters of the Indo-Pacific, Hawaii, the Red Sea, and The Bahamas. They can reach in size. Description Collector urchin ...
''


Anti-predator defences

The spines, long and sharp in some species, protect the urchin from
predator Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill t ...
s. Some tropical sea urchins like Diadematidae, Echinothuriidae and
Toxopneustidae Toxopneustidae is a family of globular sea urchins in the class Echinoidea. Characteristics All Camarodonts have imperforate tubercles and compound ambulacral plates. In addition, the Toxopneustids are characterised by the peristome, or opening ...
have venomous spines. Other creatures also make use of these defences; crabs, shrimps and other organisms shelter among the spines, and often adopt the colouring of their host. Some crabs in the
Dorippidae Dorippidae is a small family of crab Crabs are decapod crustaceans of the infraorder Brachyura, which typically have a very short projecting "tail" (abdomen) ( el, βραχύς , translit=brachys = short, / = tail), usually hidden entire ...
family carry sea urchins, starfish, sharp shells or other protective objects in their claws. Pedicellaria are a good means of defense against ectoparasites, but not a panacea as some of them actually feed on it. The hemal system defends against endoparasites.


Range and habitat

Sea urchins are established in most seabed habitats from the intertidal downwards, at an extremely wide range of depths.. Some species, such as '' Cidaris abyssicola'', can live at depths of several kilometres. Many genera are found in only the
abyssal zone The abyssal zone or abyssopelagic zone is a layer of the pelagic zone of the ocean. "Abyss" derives from the Greek word , meaning bottomless. At depths of , this zone remains in perpetual darkness. It covers 83% of the total area of the ocean an ...
, including many cidaroids, most of the genera in the Echinothuriidae family, and the "cactus urchins" '' Dermechinus''. One of the deepest-living families is the
Pourtalesiidae The pourtalesiids (Pourtalesiidae) are a family of irregular sea urchins that live in the deep sea. They are secondarily bilateral-symmetrical and like other representatives of the taxon Holasteroida they lack the lantern of Aristotle, which is ...
,. strange bottle-shaped irregular sea urchins that live in only the hadal zone and have been collected as deep as 6850 metres beneath the surface in the Sunda Trench.. Nevertheless, this makes sea urchin the class of echinoderms living the least deep, compared to brittle stars,
starfish Starfish or sea stars are star-shaped echinoderms belonging to the class Asteroidea (). Common usage frequently finds these names being also applied to ophiuroids, which are correctly referred to as brittle stars or basket stars. Starfish a ...
and crinoids that remain abundant below and
sea cucumber Sea cucumbers are echinoderms from the class Holothuroidea (). They are marine animals with a leathery skin and an elongated body containing a single, branched gonad. Sea cucumbers are found on the sea floor worldwide. The number of holothuri ...
s which have been recorded from . Population densities vary by habitat, with more dense populations in barren areas as compared to kelp stands. Even in these barren areas, greatest densities are found in shallow water. Populations are generally found in deeper water if wave action is present. Densities decrease in winter when storms cause them to seek protection in cracks and around larger underwater structures. The shingle urchin (''Colobocentrotus atratus''), which lives on exposed shorelines, is particularly resistant to wave action. It is one of the few sea urchin that can survive many hours out of water. Sea urchins can be found in all climates, from warm seas to polar oceans. The larvae of the polar sea urchin ''
Sterechinus neumayeri ''Sterechinus neumayeri'', the Antarctic sea urchin, is a species of sea urchin in the family Echinidae. It is found living on the seabed in the waters around Antarctica. It has been used as a model organism in the fields of reproductive biology ...
'' have been found to use energy in metabolic processes twenty-five times more efficiently than do most other organisms.Antarctic Sea Urchin Shows Amazing Energy-Efficiency in Nature's Deep Freeze 15 March 2001
University of Delaware. Retrieved 22 March 2018
Despite their presence in nearly all the marine ecosystems, most species are found on temperate and tropical coasts, between the surface and some tens of meters deep, close to
photosynthetic Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored in ...
food sources. File:Sea urchins in california tide pools.jpg, Purple sea urchins at low tide in
California California is a state in the Western United States, located along the Pacific Coast. With nearly 39.2million residents across a total area of approximately , it is the most populous U.S. state and the 3rd largest by area. It is also the m ...
. They dig a cavity in the rock to hide from predators during the day. File:Expl1825 - Flickr - NOAA Photo Library.jpg, ''
Dermechinus horridus ''Dermechinus'' is a genus of sea urchin in the family Echinidae found in deep water in the southern Indian, Pacific and Atlantic Oceans. It is monotypic, with ''Dermechinus horridus'', sometimes called the cactus urchin, being the only species ...
'', an abyssal species, at thousands of meters deep File:Underwater mcmurdo sound.jpg, Antarctic sea urchin (''
Sterechinus neumayeri ''Sterechinus neumayeri'', the Antarctic sea urchin, is a species of sea urchin in the family Echinidae. It is found living on the seabed in the waters around Antarctica. It has been used as a model organism in the fields of reproductive biology ...
'') inhabits frozen seas. File:Colobocentrotus atratus Shingle urchin.jpg, The shape of the shingle urchin allows it to stay on wave-beaten cliffs.


Evolution


Fossil history

The earliest echinoid
fossils 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 ...
date to the Middle Ordovician period (''circa'' 465 Mya). There is a rich fossil record, their hard tests made of
calcite Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on the Mohs scale of mineral hardness, based on scratc ...
plates surviving in rocks from every period since then. Spines are present in some well-preserved specimens, but usually only the test remains. Isolated spines are common as fossils. Some
Jurassic The Jurassic ( ) is a geologic period and stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The Jurassic constitutes the middle period of ...
and
Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...
Cidaroida had very heavy, club-shaped spines. Most fossil echinoids from the
Paleozoic The Paleozoic (or Palaeozoic) Era is the earliest of three geologic eras of the Phanerozoic Eon. The name ''Paleozoic'' ( ;) was coined by the British geologist Adam Sedgwick in 1838 by combining the Greek words ''palaiós'' (, "old") and ...
era are incomplete, consisting of isolated spines and small clusters of scattered plates from crushed individuals, mostly in
Devonian The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, wh ...
and
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 ''Carboniferou ...
rocks. The shallow-water
limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms w ...
s from the Ordovician and
Silurian The Silurian ( ) is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the shortest period of the Paleoz ...
periods of
Estonia Estonia, formally the Republic of Estonia, is a country by the Baltic Sea in Northern Europe. It is bordered to the north by the Gulf of Finland across from Finland, to the west by the sea across from Sweden, to the south by Latvia, an ...
are famous for echinoids. Paleozoic echinoids probably inhabited relatively quiet waters. Because of their thin tests, they would certainly not have survived in the wave-battered coastal waters inhabited by many modern echinoids. Echinoids declined to near extinction at the end of the Paleozoic era, with just six species known from the
Permian The Permian ( ) is a geologic period and stratigraphic system which spans 47 million years from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.9 Mya. It is the last period of the Paleo ...
period. Only two lineages survived this period's massive extinction and into the
Triassic 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 per ...
: the genus ''Miocidaris'', which gave rise to modern cidaroida (pencil urchins), and the ancestor that gave rise to the euechinoids. By the upper Triassic, their numbers increased again. Cidaroids have changed very little since the Late
Triassic 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 per ...
, and are the only Paleozoic echinoid group to have survived. The euechinoids diversified into new lineages in the
Jurassic The Jurassic ( ) is a geologic period and stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The Jurassic constitutes the middle period of ...
and
Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...
periods, and from them emerged the first irregular echinoids (the
Atelostomata The Atelostomata are a type of sea urchins. They are distinguished from other sea urchins by their irregular shape and the absence of a feeding lantern. The group includes the well known heart urchins, as well as some less familiar and extinct ...
) during the early Jurassic. Some echinoids, such as ''
Micraster ''Micraster'' is an extinct genus of echinoids from the Late Cretaceous to the early Eocene. Its remains have been found in Africa, Antarctica, Europe, and North America. Micraster was an infaunal echinoid living in a burrow below the sedi ...
'' in the chalk of the Cretaceous period, serve as zone or
index Index (or its plural form indices) may refer to: Arts, entertainment, and media Fictional entities * Index (''A Certain Magical Index''), a character in the light novel series ''A Certain Magical Index'' * The Index, an item on a Halo megastru ...
fossils. Because they are abundant and evolved rapidly, they enable geologists to date the surrounding rocks. In the
Paleogene The Paleogene ( ; also spelled Palaeogene or Palæogene; informally Lower Tertiary or Early Tertiary) is a geologic period and system that spans 43 million years from the end of the Cretaceous Period million years ago ( Mya) to the beginning o ...
and
Neogene The Neogene ( ), informally Upper Tertiary or Late Tertiary, is a geologic period and system that spans 20.45 million years from the end of the Paleogene Period million years ago ( Mya) to the beginning of the present Quaternary Period Mya. ...
periods (''circa'' 66 to 1.8 Mya), sand dollars (Clypeasteroida) arose. Their distinctive, flattened tests and tiny spines were adapted to life on or under loose sand in shallow water, and they are abundant as fossils in southern European limestones and sandstones. File:Archaeocidaris brownwoodensis MHNT.jpg, ''
Archaeocidaris ''Archaeocidaris'' is an extinct genus of echinoid that lived from the Late Devonian to the Late Permian. Its remains have been found in Africa, Europe, and North America. Sources * ''Fossils'' (Smithsonian Handbooks) by David Ward (Page 176 ...
brownwoodensis'', Cidaroida,
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 ''Carboniferou ...
, c. 300 mya File:Miocidaris coaeva MHNT.PAL.2006.94.jpg, '' Miocidaris coaeva'', Cidaroida, Middle
Triassic 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 per ...
, c. 240 mya File:Clypeus plotti, echinoid, Middle Jurassic, Gloucester, Gloucestershire, England - Houston Museum of Natural Science - DSC01800.JPG, '' Clypeus plotti'',
Irregularia Irregularia is an extant infraclass of sea urchins that first appeared in the Lower Jurassic. Description and characteristics These particular sea urchins are distinguished from other sea urchins by their irregular shape: the anus and often e ...
, Middle
Jurassic The Jurassic ( ) is a geologic period and stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The Jurassic constitutes the middle period of ...
, c. 162 mya File:Fossil Echinoid Echinocorys.jpg, ''
Echinocorys ''Echinocorys'' is an extinct genus of echinoids that lived from the Late Cretaceous to the Paleocene. The genus belongs to the Holasteridae family. Its remains have been found in Asia, Europe, Australia (Oceania) and North America. Sources * ...
'',
Holasteroida Holasteroida is an order of irregular sea urchins. Characteristics These irregular sea urchins are characterized by a particularly marked bilateral symmetry, including for the apical system, which is highly elongated. In some contemporary aby ...
, Upper
Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...
, c. 80 mya File:Echinolampas ovalis M Eocene Civrac-en-Médoc France.JPG, ''
Echinolampas ''Echinolampas'' is a genus of echinoderms belonging to the family Echinolampadidae. The genus has cosmopolitan distribution In biogeography, cosmopolitan distribution is the term for the range of a taxon that extends across all or most of ...
ovalis'',
Cassiduloida Cassiduloida is an order of sea urchins. The group was extremely diverse with many families and species during the Mesozoic, but today, only seven extant species remain. A 2019 phylogenetic systematics study by Souto et al. presented a revised ...
, Middle
Eocene The Eocene ( ) Epoch is a geological epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period in the modern Cenozoic Era. The name ''Eocene'' comes from the Ancient Greek (''ēṓs'', ...
, c. 40 mya File:Clypeaster portentosus.jpg, ''
Clypeaster ''Clypeaster'', common name "cake urchins" or "sea biscuits", is a genus of echinoderms belonging to the family Clypeasteridae. Etymology The genus name ''Clypeaster'' is derived from the Latin “clypeus” (meaning ''round shield'') and “a ...
portentosus'', Clypeasteroida,
Miocene The Miocene ( ) is the first geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and means "less recen ...
, c. 10 mya


Phylogeny


External

Echinoids are
deuterostome Deuterostomia (; in Greek) are animals typically characterized by their anus forming before their mouth during embryonic development. The group's sister clade is Protostomia, animals whose digestive tract development is more varied. Some ...
animals, like the
chordate A chordate () is an animal of the phylum Chordata (). All chordates possess, at some point during their larval or adult stages, five synapomorphies, or primary physical characteristics, that distinguish them from all the other taxa. These fi ...
s. A 2014 analysis of 219 genes from all classes of echinoderms gives the following
phylogenetic tree A phylogenetic tree (also phylogeny or evolutionary tree Felsenstein J. (2004). ''Inferring Phylogenies'' Sinauer Associates: Sunderland, MA.) is a branching diagram or a tree showing the evolutionary relationships among various biological spec ...
. Approximate dates of branching of major clades are shown in millions of years ago (mya).


Internal

The phylogeny of the sea urchins is as follows: The phylogenetic study from 2022 presents a different topology of the
Euechinoidea The subclass Euechinoidea includes almost all living species of sea urchin, and fossil forms going back as far as the Triassic. Taxonomy List of orders according to World Register of Marine Species : * Infra-classis '' Acroechinoidea'' ** ...
phylogenetic tree.
Irregularia Irregularia is an extant infraclass of sea urchins that first appeared in the Lower Jurassic. Description and characteristics These particular sea urchins are distinguished from other sea urchins by their irregular shape: the anus and often e ...
are sister group of Echinacea (including Salenioida) forming a common clade Carinacea, basal groups
Aspidodiadematoida The Aspidodiadematidae are a family of sea urchins. Characteristics All the members of the two extant genera, ''Aspidodiadema'' and ''Plesiodiadema'', are found in tropical seas at bathyal and abyssal depths, often on the submarine slopes of i ...
, Diadematoida,
Echinothurioida The Echinothurioida are an order of sea urchins in the class Echinoidea. Echinothurioids are distinguished from other sea urchins by the combination of a flexible test and hollow spines. The membrane around the mouth contains only simple plates ...
, Micropygoida, and Pedinoida are comprised in a common basal clade Aulodonta.


Relation to humans


Injuries

Sea urchin injuries are puncture wounds inflicted by the animal's brittle, fragile spines. These are a common source of injury to ocean swimmers, especially along coastal surfaces where coral with stationary sea urchins are present. Their stings vary in severity depending on the species. Their spines can be venomous or cause infection. Granuloma and staining of the skin from the natural dye inside the sea urchin can also occur. Breathing problems may indicate a serious reaction to toxins in the sea urchin. They inflict a painful wound when they penetrate human skin, but are not themselves dangerous if fully removed promptly; if left in the skin, further problems may occur.


Science

Sea urchins are traditional model organisms in
developmental biology Developmental biology is the study of the process by which animals and plants grow and develop. Developmental biology also encompasses the biology of regeneration, asexual reproduction, metamorphosis, and the growth and differentiation of st ...
. This use originated in the 1800s, when their embryonic development became easily viewed by microscopy. The transparency of the urchin's eggs enabled them to be used to observe that
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, ...
cells actually fertilize ova. They continue to be used for embryonic studies, as
prenatal development Prenatal development () includes the development of the embryo and of the fetus during a viviparous animal's gestation. Prenatal development starts with fertilization, in the germinal stage of embryonic development, and continues in fetal deve ...
continues to seek testing for fatal diseases. Sea urchins are being used in longevity studies for comparison between the young and old of the species, particularly for their ability to regenerate tissue as needed. Scientists at the
University of St Andrews (Aien aristeuein) , motto_lang = grc , mottoeng = Ever to ExcelorEver to be the Best , established = , type = Public research university Ancient university , endowment ...
have discovered a genetic sequence, the '2A' region, in sea urchins previously thought to have belonged only to viruses that afflict humans like foot-and-mouth disease virus. More recently,
Eric H. Davidson Eric Harris Davidson (April 13, 1937 – September 1, 2015) was an American developmental biologist at the California Institute of Technology. Davidson was best known for his pioneering work on the role of gene regulation in evolution, on embryo ...
and
Roy John Britten Roy John Britten (1 October 1919 – 21 January 2012) was an American molecular biologist known for his discovery of repeated DNA sequences in the genomes of eukaryotic organisms, and later on the evolution of the genome. Early life and educati ...
argued for the use of urchins as a model organism due to their easy availability, high fecundity, and long lifespan. Beyond
embryology Embryology (from Greek ἔμβρυον, ''embryon'', "the unborn, embryo"; and -λογία, '' -logia'') is the branch of animal biology that studies the prenatal development of gametes (sex cells), fertilization, and development of embr ...
, urchins provide an opportunity to research cis-regulatory elements. Oceanography has taken an interest in monitoring the health of urchins and their populations as a way to assess overall
ocean acidification Ocean acidification is the reduction in the pH value of the Earth’s ocean. Between 1751 and 2021, the average pH value of the ocean surface has decreased from approximately 8.25 to 8.14. The root cause of ocean acidification is carbon dioxid ...
, temperatures, and ecological impacts. The organism's evolutionary placement and unique embryology with five-fold symmetry were the major arguments in the proposal to seek the sequencing of its
genome In the fields of molecular biology and genetics, a genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding ...
. Importantly, urchins act as the closest living relative to chordates and thus are of interest for the light they can shed on the evolution of
vertebrate Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, with ...
s. The genome of '' Strongylocentrotus purpuratus'' was completed in 2006 and established homology between sea urchin and vertebrate
immune system The immune system is a network of biological processes that protects an organism from diseases. It detects and responds to a wide variety of pathogens, from viruses to parasitic worms, as well as cancer cells and objects such as wood splinte ...
-related genes. Sea urchins code for at least 222
Toll-like receptor Toll-like receptors (TLRs) are a class of proteins that play a key role in the innate immune system. They are single-pass membrane-spanning receptors usually expressed on sentinel cells such as macrophages and dendritic cells, that recognize ...
genes and over 200 genes related to the
Nod-like-receptor The nucleotide-binding oligomerization domain-like receptors, or NOD-like receptors (NLRs) (also known as nucleotide-binding leucine-rich repeat receptors), are intracellular sensors of pathogen-associated molecular patterns (PAMPs) that enter the ...
family found in vertebrates. This increases its usefulness as a valuable model organism for studying the
evolution 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. Variation ...
of innate immunity. The sequencing also revealed that while some genes were thought to be limited to vertebrates, there were also innovations that have previously never been seen outside the chordate classification, such as immune transcription factors
PU.1 Transcription factor PU.1 is a protein that in humans is encoded by the ''SPI1'' gene. Function This gene encodes an ETS-domain transcription factor that activates gene expression during myeloid and B-lymphoid cell development. The nuclear pr ...
and
SPIB Transcription factor Spi-B is a protein that in humans is encoded by the ''SPIB'' gene. Function SPI1 (MIM 165170) and SPIB are members of a subfamily of ETS (see ETS1; MIM 164720) transcription factors. ETS proteins share a conserved ETS do ...
.


As food

The gonads of both male and female sea urchins, usually called sea urchin roe or corals, are culinary delicacies in many parts of the world, especially Japan. In Japan, sea urchin is known as , and its roe can retail for as much as ¥40,000 ($360) per kilogram; it is served raw as ''
sashimi is a Japanese delicacy consisting of fresh raw fish or meat sliced into thin pieces and often eaten with soy sauce. Origin The word ''sashimi'' means "pierced body", i.e. " 刺身" = ''sashimi'', where 刺 し = ''sashi'' (pierced, stu ...
'' or in '' sushi'', with
soy sauce Soy sauce (also called simply soy in American English and soya sauce in British English) is a liquid condiment of Chinese origin, traditionally made from a fermented paste of soybeans, roasted grain, brine, and ''Aspergillus oryzae'' or '' As ...
and '' wasabi''. Japan imports large quantities from the United States,
South Korea South Korea, officially the Republic of Korea (ROK), is a country in East Asia, constituting the southern part of the Korea, Korean Peninsula and sharing a Korean Demilitarized Zone, land border with North Korea. Its western border is formed ...
, and other producers. Japan consumes 50,000 tons annually, amounting to over 80% of global production. Japanese demand for sea urchins has raised concerns about overfishing. In Mediterranean cuisines, ''Paracentrotus lividus'' is often eaten raw, or with lemon, and known as ''ricci'' on Italian menus where it is sometimes used in pasta sauces. It can also flavour omelettes, scrambled eggs,
fish soup Fish soup is a food made by combining fish or seafood with vegetables and stock, juice, water, or another liquid. Hot soups are additionally characterized by boiling solid ingredients in liquids in a pot until the flavors are extracted, form ...
,
mayonnaise Mayonnaise (; ), colloquially referred to as "mayo" , is a thick, cold, and creamy sauce or dressing commonly used on sandwiches, hamburgers, composed salads, and French fries. It also forms the base for various other sauces, such as tarta ...
,
béchamel sauce Bechamel sauce ( ) is a sauce traditionally made from a white roux (butter and flour in a 1:1 mixture by weight) and milk. Bechamel may also be referred to as besciamella (Italy), besamel (Greece), or white sauce (U.S.). French, Italian and Gree ...
for tartlets, the ''boullie'' for a soufflé, or
Hollandaise sauce Hollandaise sauce ( or ; ), also called Dutch sauce, is a mixture of egg yolk, melted butter, and lemon juice (or a white wine or vinegar reduction). It is usually seasoned with salt, and either white pepper or cayenne pepper. It is well kn ...
to make a fish sauce. In Chilean cuisine, it is served raw with lemon, onions, and olive oil. Though the edible '' Strongylocentrotus droebachiensis'' is found in the North Atlantic, it is not widely eaten. However, sea urchins (called ''uutuk'' in Alutiiq) are commonly eaten by the Alaska Native population around Kodiak Island. It is commonly exported, mostly to
Japan Japan ( ja, 日本, or , and formally , ''Nihonkoku'') is an island country in East Asia. It is situated in the northwest Pacific Ocean, and is bordered on the west by the Sea of Japan, while extending from the Sea of Okhotsk in the n ...
. In the West Indies, slate pencil urchins are eaten.Davidson, Alan (2014) '' Oxford Companion to Food''.
Oxford University Press Oxford University Press (OUP) is the university press of the University of Oxford. It is the largest university press in the world, and its printing history dates back to the 1480s. Having been officially granted the legal right to print book ...
, 3rd edition. pp. 730–731.
On the Pacific Coast of North America, ''
Strongylocentrotus franciscanus The red sea urchin (''Mesocentrotus franciscanus'') is a sea urchin found in the northeastern Pacific Ocean from Alaska to Baja California. It lives in shallow waters from the low-tide line to greater than deep, and is typically found on rocky ...
'' was praised by Euell Gibbons; '' Strongylocentrotus purpuratus'' is also eaten. Native Americans in California are also known to eat sea urchins. The coast of Southern California is known as a source of high quality ''uni'', with divers picking sea urchin from kelp beds in depths as deep as 24 m/80 ft. As of 2013, the state was limiting the practice to 300 sea urchin diver licenses. In New Zealand, '' Evechinus chloroticus'', known as ''kina'' in
Māori Māori or Maori can refer to: Relating to the Māori people * Māori people of New Zealand, or members of that group * Māori language, the language of the Māori people of New Zealand * Māori culture * Cook Islanders, the Māori people of the Co ...
, is a delicacy, traditionally eaten raw. Though New Zealand fishermen would like to export them to Japan, their quality is too variable. File:Uniryori.jpg, Sea urchin (''uni'') as ''
sashimi is a Japanese delicacy consisting of fresh raw fish or meat sliced into thin pieces and often eaten with soy sauce. Origin The word ''sashimi'' means "pierced body", i.e. " 刺身" = ''sashimi'', where 刺 し = ''sashi'' (pierced, stu ...
'' with a dab of '' wasabi'' File:Uni-ikura-don7030188ウニイクラ丼.jpg, Japanese ''uni-ikura don'', sea urchin egg and salmon egg ''
donburi is a Japanese "rice-bowl dish" consisting of fish, meat, vegetables or other ingredients simmered together and served over rice. ''Donburi'' meals are usually served in oversized rice bowls which are also called ''donburi''. If one needs to ...
'' File:Ricci di mare2.jpg, Open sea urchins in Sicily


Aquaria

Some species of sea urchins, such as the slate pencil urchin (''
Eucidaris tribuloides ''Eucidaris tribuloides'', the slate pencil urchin, is a species of cidaroid sea urchins that inhabits littoral regions of the Atlantic Ocean. As a member of the basal echinoid order Cidaroida, its morphological, developmental and molecular g ...
''), are commonly sold in aquarium stores. Some species are effective at controlling filamentous algae, and they make good additions to an
invertebrate Invertebrates are a paraphyletic group of animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This is a grouping including all animals apart from the chorda ...
tank.


Folklore

A folk tradition in Denmark and southern England imagined sea urchin fossils to be thunderbolts, able to ward off harm by lightning or by witchcraft, as an apotropaic symbol. Another version supposed they were petrified eggs of snakes, able to protect against heart and liver disease, poisons, and injury in battle, and accordingly they were carried as
amulet An amulet, also known as a good luck charm or phylactery, is an object believed to confer protection upon its possessor. The word "amulet" comes from the Latin word amuletum, which Pliny's ''Natural History'' describes as "an object that protect ...
s. These were, according to the legend, created by magic from foam made by the snakes at midsummer.


Explanatory notes


References


External links

*
The sea urchin genome project

Sea Urchin Harvesters Association – California
Also, (604) 524-0322.
The Echinoid Directory
from the Natural History Museum.
Virtual Urchin
at Stanford



at UCMP Berkeley {{Authority control Animal developmental biology Animal models Articles containing video clips Commercial echinoderms Edible shellfish Extant Ordovician first appearances Japanese seafood Korean seafood Late Ordovician first appearances Negligibly senescent organisms Seafood in Native American cuisine