| Cavefishes | |
|---|---|
| Northern cavefish, Amblyopsis spelaea | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Actinopterygii |
| Order: | Percopsiformes |
| Family: | Amblyopsidae |
| Genera | |
|
Amblyopsis |
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The fish family Amblyopsidae, are commonly referred to as cavefish, blindfish, or swampfish. They are small freshwater fish found in the dark environments of swamps, caves, and some deep lakes. All members of this family exhibit adaptations to these dark environments, including the lack of functional eyes and, in some species, the lack of pigmentation.
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Similar darkness-adaptive traits can be seen in other fish families, in separate isolated environments, around the world. This is a manifestation of the phenomenon known as convergent evolution, and has been observed in Characidae, Cyprinidae, Gobiidae, Ostariophysi, Poeciliidae, Synbranchidae and to the freshwater and marine fish group Bythitidae.[1]There are more than 80 known varieties of cavefish.
Although the cave habitat generally offers a poor food supply, the advantages of the environment include extremely stable conditions, few competitors and few predators.[1] Some adaptations common to many of the cavefish include reduced susceptibility to light, pigment loss, or reduction in the skin scales[1], as well as development of chemoreceptors in the sensory organs of the body surface and the lateral line. Three species of cave dwelling fishes have exhibited a more efficient metabolism during swimming, as compared with a group of non-cave fish[1], and many cavefish species living exhibit slender bodies adapted to swimming in fast-flowing waters.[1]
Since the cave environment is dark, there is no plant life performing photosynthesis, and organic food is mainly introduced from the outside world by other organisms. Chronic food shortage leads to a decrease in population density, which has been estimated for Amblyopsidae to be only about 0.005 to 0.15 animals per square meter.[1] The duration of the reproductive cycle of Amblyopsidae is very low. Breeding only once a year, occuping about 10% of the population, 40-60 have reached sexual maturity.[1] The eggs are large and are protected for 4-5 months over the gills of the female cavity, this is considered the longest fish protection period. Also the period growth is slower and reach sexual maturity after hatching, having a long life.[1]
Cave fish have a limited range and the cave habitat is vulnerable to changes in the environment and is threatened by water pollution and population of the exotic species in recent years.[1] The Amblyopsidae family is not an exception, the World Conservation Union (IUCN) has placed four of the cave fish on the Red List of threatened species.[1] Alabama cave fish, Speoplatyrhinus poulsoni, which live only in certain caves in Alabama, are listed as Critically Endangered.[2], the highest risk class.
They are probably ancient in origin. All are small and typical of the fresh waters of the southern regions of North America, significantly cave-dwelling fishes of Kentucky cave system called the Mammoth caves. Externally, they resemble killifishes in many respects, although their internal anatomy more closely resembles the trout-perches, with which they are currently classified.[3] Some are deep in the swamps, and others in the lakes and streams in caves gander. These last are those with rudimentary eyes, like so many other fauna in the media devoid of light, and have the body apigmentado and translucent. The family includes a few genera and many species, quite similar.
The fish of this family are called amblyopsis by the fact that they have, at least in part, rudimentary eyes, hidden under the skin. Therefore they are blind or can only tell the difference between light and darkness. They have the naked head moderately depressed and a elongated body, covered with little cycloid irregular flakes, with tiny or absent pectoral fins and the anal opening, placed so far forward that is in the throat region. The premaxilla, a bone of the upper jaw, is segmented, the vomer is having no teeth. They have no ventral fins, only Amblyopsis spelaea has small pelvic fins. Differently to the trout-perchs, they have not adipose fin. The lateral line is incomplete but well developed in some species. Its skeleton is having between 27-35 vertebrae.
Although some species have tiny, vestigial but functional eyes, like the springfish and the ricefish, three species have no eyes at all. Blindfishes do, however, have rows of sensory papillae on their skin, which they use to help navigate their lightless environment. The majority of cavefish have little to no pigment in their skin. Cavefishes can only be found in caves that have streams running into them; a cave with no inlets, such as Blanchard Springs Caverns in Arkansas, will not contain cavefishes. One species, the swampfish Chologaster cornuta, inhabits above-ground swamps, rather than caves.[3] A rare feature of this family is the forward placement of its cloaca, under the head, anterior to the pelvic fins.[3] This placement allows the females to place their eggs more precisely. This feature is present also in others species of the Percopsiformes order, like Aphredoderidae.
They are found mostly in caves and tunnels flooded regions of the U.S., feeding on shrimp, gammarus and arachnids falling into the water, using the vibrations of it to orient themselves to prey. The blindfish species are blind and depigmented. Despite the lack of eyes it does respond to light and moves away from it, scotophilia. Blindfish are generally small, ranging up to 11 centimetres (4.3 in) in length. Most do not have pelvic fins.
The Typhlichthys subterraneus is mostly lentic, but can also be found in pools of streams near water tables. This blindfish feeds mostly on aquatic arthropods, such as amphipods and isopods. However, their metabolic rates are depressed in order to survive food shortages.[4] The reproductive capabilities of this species is considerably low, with fewer than 50 eggs per female. This provides restrictions on its capabilities for recovering from an even minor population decline. When young are produced, they brood in the female's gill chamber. Sexual maturity requires approximately two years, and the life span is approximately four years.[5]
The Forbesichthys agassizii is dark brown dorsally and fades to a creamy brown towards the pelvis that feed primarily on micro crustaceans such as amphipods, like Gammarus troglophilus, but also prey on midge larvae and annelid worms.
The Amblyopsis rosae populations lives in caves that have a relatively large source of nutrients, such as bat guano or blown leaf litter. Water quality in caves that contain cavefish is usually high. Ozark cavefish are able to tolerate the extremely low oxygen content of ground water found in caves. Cavefish tend to occur in flowing cave streams as opposed to quiet pools. The ozark cave fish can receive nutrients with out photosynthesis or chemosynthesis, by the tree roots above the cave. The roots are full of nutrients and water. The roots are spreading photosynthesis the cave and organisms like Ozark cavefish and other species are able to feed off the roots.
Although most Amblyopsidae do not have pelvic fins, Amblyopsis spelaea has small ones with up to six rays. Like others blindfish troglodytes or abyssal they are predator just only resulting from the lack of edible vegetation, inhabiting subterranean water or caves which have consolidated mud-rock substrates in shoals and silt-sand substrates in pools but is more often found in caves with uniform silt-sand substrates. Has low reproduction rate and broods eggs in gill cavity for up to 4-5 months. Can live for 2 years without food because of his low metabolic rate.
Many of the species are listed as threatened species in the United States and the IUCN lists the species as vulnerable, by the very restricted distribution area, and their adaptation to the shortage with a very low reproductive rate in a range of highly specialized and isolated habitat of oxigenated streams not prone to changes and competition with other species.
A new research has suggested that cavefish have unusual body clock, that does not respond to light. The still functional body clock, or circadian rhythm, is of duration 47 hours.[6][7]
The 'Percopsiformes' is a small order that contains just nine species, grouped into three families:[8], Percopsidae (trout-perches), Aphredoderidae (pirate perch) and Amblyopsidae. The family Amblyopsidae includes six species in five genera:[9]
Family Amblyopsidae