Within Cuatro Ciénegas, an interior basin containing hundreds of spring-fed pools in a desert environment, about half of the 20 species of fish, and 23 of the 34 species of freshwater mollusks are endemic. Some of these fish and mollusk species are restricted to individual pools of only a few square meters in size. Others have not changed appreciably from ancestral forms and are important relict species.
Species here include the aquatic box turtle
), the endemic black softshell turtle (Trionyx ater
), pond slider
), plain-bellied water snake
), and Cuatro Ciénegas cichlid
Three endangered endemic fishes are Cuatro Ciénegas shiner (Notropis xanthicara
), Cuatro Ciénegas darter
), and Sardinilla
). Other endemic fishes are Cuatro Ciénegas gambusia
), Robust gambusia (G. marshi
), Yellowfin gambusia (G. alvarezi
), Conchos pupfish (Cyprinodon eximius
), Comanche springs pupfish (C. elegans
), Bighead pupfish (C. pachycephalus
), Devil's River minnow
), and Roundnose minnow (D. episcopa
Tiny fish called Pecos gambusia give birth to live offspring rather than laying eggs. They also feed on mosquito larvae, helping to keep the mosquito population in check.
Among the endemic invertebrates, which are found predominantly in spring-fed habitats, are a number of aquatic snails and amphipods such as the Pecos assiminea
), Phantom Spring cochliopa (Cochliopa texana
), Phantom Lake tryonia (Tryonia cheatumi
), Phantom Spring amphipod
), San Solomon amphipod (G. sp.
), and Noel's amphipod (G. desperatus