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Strayer (1994) noted that the Edwards aquifer in the SCTX study unit is one of a few regions in the world where a large diversity of subterranean species are found. Longley (1981) reported that the Edwards aquifer might be the most diverse subterranean biological community on earth. The list of species associated with the aquifer is large and is expected to grow as more faunal studies are completed. Both vertebrate (salamanders and two species of blind catfish) and invertebrate troglobitic (restricted to ground-water habitats) species have been found within this aquifer. Ninety-one species or subspecies, including 44 troglobitic species, have been identified as endemic to the aquatic ecosystems (including springs) associated with the Edwards aquifer (Barr and Spangler, 1992; Bowles and Arsuffi, 1993; Longley, 1986). Table 7 lists the known endemic troglobitic species of the Edwards aquifer.

Longley (1986) reported that the biological invasion of the Edwards aquifer probably began during the deposition of the Edwards Limestone more than 100 million years ago. Alternate periods of submergence and exposure of the region by the shallow Cretaceous sea allowed early formation of caverns. At least 10 crustaceans found in the Edwards aquifer are related to typically marine species and likely evolved from the marine environment that last covered the area during the late Cretaceous or early Tertiary (65 to 70 million years ago) (Holsinger and Longley, 1980).

About 12 to 17 million years ago, during the Miocene, a period of extensive faulting in south-central Texas began that resulted in the subsequent formation of the Balcones fault zone, changing the movement patterns of ground water within the Edwards (Longley, 1986). This faulting created new springs and points of surface-water entry (recharge) into the ground water (Barker and Ardis, 1996), providing many new entry locations for the freshwater species of south-central Texas. During this time, further dissolution of the limestone increased cavernous porosity in the limestone; this increased formation of caverns in the limestone, and the linkage between caverns created new habitat and distribution patterns for the area ground-water species. Langecker and Longley (1993) believe extensive cave development during the Miocene could have allowed colonization of the ancestors of the two species of blind catfish presently in the Edwards aquifer. Langecker and Longley (1993) conclude that the morphological adaptations of these fish, including degree of eye reduction, are evidence that these fish are among the oldest known cave fish.

Invasions of an aquifer by freshwater organisms are most likely during periods of environmental stress (Holsinger, 1988). Longley (1986) indicated that the onset of the ice age 3 million years ago had a major influence on the biology of the Edwards aquifer. During extremely cold periods, the ground water maintains constant temperature and offers a refuge for aquatic organisms. Severe droughts in the region also could have contributed to the invasion of the Edwards; in particular, the ground-water salamanders of Central Texas could have migrated into cave streams when their surface habitats dried up (Sweet, 1982).

Because of the lack of light within the aquifer, food derived from photosynthesis is not available to the ground-water communities. In the area near San Marcos Springs, where the aquifer habitat is near the recharge zone, organic debris washed in from the surface is the source of energy to the primary consumers, chiefly amphipods, shrimp, and snails (Browning, 1977; Longley, 1981). Here the blind salamanders are at the top of the food chain; captive specimens have been observed feeding on a variety of aquifer invertebrates (Longley, 1981). Another likely predator is a blind species of the predaceous diving beetle family.

In deeper parts of the aquifer (1,300 to 2,000 ft) near San Antonio, the organic matter brought in from distant recharge areas would not be sufficient to support the aquifer biota. Instead, it has been theorized that fossil organic matter supports fungi and bacteria, which in turn support the invertebrates and blind catfish in this region of the Edwards aquifer (Longley, 1981). The food source might also be similar to that discovered at Movile Cave in Romania, which is isolated from terrestrial inputs. The energy source for the diverse community of organisms in Movile Cave appears to be hydrogen sulfide-fixing bacteria (Sarbu and others, 1996). The top of the food chain in the deeper parts of the Edwards aquifer appears to be occupied by the widemouth blindcat, Satan eurystomus, whose stomach contents include crustacean skeletons (Langecker and Longley, 1993). The mouthparts and stomach contents of the other catfish in the aquifer, the toothless blindcat (Trogloglanis pattersoni), indicate that it forages on organic matter coating the cave walls.

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