The Amblyopsidae are a fish

family Family (from ) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictabili ...

commonly referred to as cavefish, blindfish, or swampfish. They are small freshwater fish found in the dark environments of

cave Caves or caverns are natural voids under the Earth's Planetary surface, surface. Caves often form by the weathering of rock and often extend deep underground. Exogene caves are smaller openings that extend a relatively short distance undergrou ...

s (

underground lake An underground lake or subterranean lake is a lake underneath the surface of the Earth. Most naturally occurring underground lakes are found in areas of karst topography, where limestone or other soluble rock has been weathered away, leaving a ca ...

s, pools,

rivers A river is a natural stream of fresh water that flows on land or inside caves towards another body of water at a lower elevation, such as an ocean, lake, or another river. A river may run dry before reaching the end of its course if it ru ...

and

streams A stream is a continuous body of surface water flowing within the bed and banks of a channel. Depending on its location or certain characteristics, a stream may be referred to by a variety of local or regional names. Long, large stream ...

), springs and

swamp A swamp is a forested wetland.Keddy, P.A. 2010. Wetland Ecology: Principles and Conservation (2nd edition). Cambridge University Press, Cambridge, UK. 497 p. Swamps are considered to be transition zones because both land and water play a role in ...

s in the eastern half of the United States. Like other troglobites, most amblyopsids exhibit adaptations to these dark environments, including the lack of functional eyes and the absence of pigmentation. More than 200 species of

cavefish Cavefish or cave fish is a generic term for fresh and brackish water fish adapted to life in caves and other underground habitats. Related terms are subterranean fish, Troglomorphism, troglomorphic fish, troglobitic fish, stygobitic fish, phreat ...

es are known,Riesch, R.; Tobler, M.; and Plath, M. (2015). ''Extremophile Fishes: Ecology, Evolution, and Physiology of Teleosts in Extreme Environments.'' but only six of these are in the family Amblyopsidae.Romero, A., editor (2001). ''The Biology of Hypogean Fishes.'' Developments in Environmental Biology of Fishes. One of these, '' Forbesichthys agassizii'', spends time both underground and aboveground. A seventh species in this family, '' Chologaster cornuta'', is not a cave-dweller but lives in aboveground swamps.

Description

Amblyopsids are generally small, the northern cavefish (largest species in family) reaching up to in length. The amblyopsids are probably ancient in origin. Adaptations common to many cavefish include reduced susceptibility to light, pigment loss, or reduction in skin scales, as well as development of

chemoreceptor A chemoreceptor, also known as chemosensor, is a specialized sensory receptor which transduces a chemical substance ( endogenous or induced) to generate a biological signal. This signal may be in the form of an action potential, if the chemorece ...

s in the sensory organs of the body surface and the

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 epithelia ...

. Three species exhibit efficient

metabolism Metabolism (, from ''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 cellular processes; the co ...

during swimming, as compared with a group of non-cave fish, and many cavefish species exhibit slender bodies adapted to swimming in fast-flowing waters. The cave-dwellers typically lack pigment and are somewhat translucent. They have a naked, moderately depressed head and an elongated body, covered with small cycloid irregular flakes, with tiny or absent

pelvic fin Pelvic fins or ventral fins are paired fins located on the ventral (belly) surface of fish, and are the lower of the only two sets of paired fins (the other being the laterally positioned pectoral fins). The pelvic fins are homologous to the hi ...

s. The anal opening is so far forward that it is in the throat region. The

premaxilla The premaxilla (or praemaxilla) is one of a pair of small cranial bones at the very tip of the upper jaw of many animals, usually, but not always, bearing teeth. In humans, they are fused with the maxilla. The "premaxilla" of therian mammals h ...

, a bone of the upper jaw, is segmented, and the

vomer The vomer (; ) is one of the unpaired facial bones of the skull. It is located in the midsagittal line, and articulates with the sphenoid, the ethmoid, the left and right palatine bones, and the left and right maxillary bones. The vomer forms ...

has no

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, tear ...

. They have no ventral fins. The lateral line is incomplete, but well developed in some species. Its spine has between 27 and 35

vertebrae Each vertebra (: vertebrae) is an irregular bone with a complex structure composed of bone and some hyaline cartilage, that make up the vertebral column or spine, of vertebrates. The proportions of the vertebrae differ according to their spinal ...

. Externally, they resemble killifishes in many respects, although their internal anatomy more closely resembles the trout-perches, with which they are currently classified.

Eyes

The name of the family, Amblyopsidae, refers to their eyes (compare amblyopia). Most in this family are either blind or can only detect the difference between light and darkness. The true cave-dwellers have only rudimentary eyes, like so many other fauna that live in the dark. Although some cave-dwellers have tiny,

vestigial Vestigiality is the retention, during the process of evolution, of genetically determined structures or attributes that have lost some or all of the ancestral function in a given species. Assessment of the vestigiality must generally rely on co ...

but functional eyes, others, such as the '' Amblyopsis'' and '' Typhlichthys'' have no eyes at all. Blindfish do, however, have rows of sensory papillae on their skin, which they use to help navigate. Similar darkness-adaptive traits can be seen in many fish families where members live underground and is known as

convergent evolution Convergent evolution is the independent evolution of similar features in species of different periods or epochs in time. Convergent evolution creates analogous structures that have similar form or function but were not present in the last comm ...

.Rantin B., and M.E. Bichuette (2013). ''Phototactic behaviour of subterranean Copionodontinae Pinna, 1992 catfishes (Siluriformes, Trichomycteridae) from Chapada Diamantina, central Bahia, northeastern Brazil.'' International Journal of Speleology 41(1): 57–63

Distribution

All members of this family are small and typical of the fresh waters of the eastern and southern regions of the United States. Some live deep in the swamps, and others in the lakes and streams or in caves, significantly the

Kentucky Kentucky (, ), officially the Commonwealth of Kentucky, is a landlocked U.S. state, state in the Southeastern United States, Southeastern region of the United States. It borders Illinois, Indiana, and Ohio to the north, West Virginia to the ...

cave system called the Mammoth caves. Cavefish can only be found in caves that have streams running into them; a cave with no inlets does not contain cavefish.

Ecology

Although the cave habitat generally offers a poor food supply, the advantages of the environment include extremely stable conditions, few competitors, and few predators. Since the cave environment is dark, no plant life is performing

photosynthesis Photosynthesis ( ) is a system of biological processes by which photosynthetic organisms, such as most plants, algae, and cyanobacteria, convert light energy, typically from sunlight, into the chemical energy necessary to fuel their metabo ...

, and food is mainly introduced from the outside world by other organisms. Limited food leads to low population density, which has been estimated for Amblyopsidae to be only about 0.005 to 0.150 animals per square meter. Cave habitats are vulnerable to changes in the environment such as water pollution and exotic species. The Alabama cavefish (''Speoplatyrhinus poulsoni''), which live only in the Key Cave in Alabama, is listed as

Critically Endangered An IUCN Red List critically endangered (CR or sometimes CE) species is one that has been categorized by the International Union for Conservation of Nature as facing an extremely high risk of extinction in the wild. As of December 2023, of t ...

by the

IUCN The International Union for Conservation of Nature (IUCN) is an international organization working in the field of nature conservation and sustainable use of natural resources. Founded in 1948, IUCN has become the global authority on the status ...

, the highest risk class.

Life history and behavior

Cavefish breed only once a year, occupying about 10% of the population. Members of the family Amblyopsidae lay eggs. Uniquely among fish, the genus ''Amblyopsis'' brood their eggs in the

gill A gill () is a respiration organ, respiratory organ that many aquatic ecosystem, aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow r ...

chambers (somewhat like mouthbrooders). Formerly it was incorrectly speculated that a similar brooding behavior existed in other members of this family, as well as the pirate perch (''Aphredoderus sayanus''). Cavefish protect their eggs for the longest period of any fish. A rare feature of this family is the forward placement of its

cloaca A cloaca ( ), : cloacae ( or ), or vent, is the rear orifice that serves as the only opening for the digestive (rectum), reproductive, and urinary tracts (if present) of many vertebrate animals. All amphibians, reptiles, birds, cartilagin ...

, under the head, anterior to the pelvic fins. This placement allows the females to place their eggs more precisely, and is present also in other species of the

Percopsiformes The Percopsiformes are a small order of freshwater teleost fishes measuring less than 20 cm in length, comprising the trout-perch and its allies. It contains just ten extant taxon, extant species, grouped into seven genus, genera and three ...

order, such as the

Aphredoderidae The Percopsiformes are a small order of freshwater teleost fishes measuring less than 20 cm in length, comprising the trout-perch and its allies. It contains just ten extant taxon, extant species, grouped into seven genus, genera and three ...

. They feed on shrimp,

gammarus ''Gammarus'' is an amphipod crustacean genus in the family Gammaridae. It contains more than 200 described species, making it one of the most species-rich genera of crustaceans. Different species have different optimal conditions, particularly ...

, and arachnids that fall into the water, using vibrations and current changes to seek out their prey.

References

External links

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{{Authority control Cave fish Euteleostei families Taxa named by Charles Lucien Bonaparte