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Among the more extreme aquatic environments imaginable are underground water systems where no light penetrates and where food availability depends on infrequent replenishment from surface regions. However, cave living has advantages, including a scarcity of competitors and predators and a constant, relatively moderate climate. Fishes have evolved independently in caves around the world and, not surprisingly, similar adaptations to cave life have evolved repeatedly despite phylogenetic differences. The darkness, low productivity, and even high atmospheric pressure of cave environments have also led to some surprisingly strong convergences between cave and deepsea fishes.
Caves usually develop in limestone formations (karst) because of the solubility of carbonaceous rock, although caves exist in other rock types such as lava tubes on volcanic slopes. Caves include places where water dives underground and resurfaces after a short distance, or where springs upwell near the surface and are illuminated by dim but daily fluctuating daylight (technically a cavern). The classic cave environment is a continually dark, subterranean system where fluctuations in temperature, oxygen, and energy availability are minimal and where little interchange occurs with other areas. The biota of caves are especially interesting because a continuum of habitats exists between the surface, caverns, and deep caves. We can consequently often identify closely related and even ancestral organisms from which cave populations and species evolved. This allows comparison of cave and surface forms and analysis of the processes and selection pressures that have produced cave adaptations.
Approximately 136 species and 19 families in 10 different orders of teleostean fishes have colonized caves. These unusual fishes – termed variously hypogean, troglobitic, phreatic, and stygobitic – occur in scattered locales at tropical and warm temperate latitudes on all continents except Antarctica and Europe (Proudlove 1997a, 2006; Weber et al. 1998). With the exception of some bythitid cusk-eels and gobies, the families are restricted to fresh water. Most cave fishes are ostariophysans (characins, loaches, minnows, and eight catfish families), which is not surprising given the overwhelming success of this superorder in freshwater habitats. The remaining four families are either paracanthopterygian (ambloypsid cavefishes) or acanthopterygian (poeciliid livebearers, synbranchid swamp eels, and cottid sculpins). Only one family, the amblyopsid cavefishes, consists primarily (four of six species) of cave-dwelling forms. Many are known from only one or a few locations, although sampling difficulties make accurate population estimations difficult. But isolation seems to be commonplace: at least 48 species are known from only their type locality.
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