Three Aquatic Endangered Invertebrate Species
in Comal and Hays Counties, TX

Return to Texas Entomology

[Federal Register: December 18, 1997 (Volume 62, Number 243)] 

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DEPARTMENT OF THE INTERIOR

Fish and Wildlife Service

Final Rule To List Three Aquatic Invertebrates in Comal and Hays Counties, TX, as Endangered

AGENCY: Fish and Wildlife Service, Interior.

ACTION: Final rule.
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SUMMARY: The Fish and Wildlife Service (Service) determines three aquatic invertebrate species known only from Comal and Hays counties, Texas, to be endangered species under the Endangered Species Act of 1973, as amended (Act). The invertebrates to be listed are Peck's cave amphipod (Stygobromus pecki), Comal Springs riffle beetle (Heterelmis comalensis), and Comal Springs dryopid beetle (Stygoparnus comalensis). The primary threat to these species is a decrease in water quantity and quality as a result of water withdrawal and other human activities throughout the San Antonio segment of the Edwards Aquifer. This action implements Federal protection provided by the Act for these three invertebrates.

EFFECTIVE DATE: January 20, 1998.

ADDRESSES: The complete file for this rule is available for inspection, by appointment, during normal business hours at the Ecological Services Field Office, U.S. Fish and Wildlife Service, 10711 Burnet Road, Suite 200, Austin, Texas 78758.

FOR FURTHER INFORMATION CONTACT: Ruth Stanford, Ecologist (512/490-0057; facsimile (512/490-0974).

SUPPLEMENTARY INFORMATION:

Background

    The Service designates Peck's cave amphipod (Stygobromus pecki), Comal Springs riffle beetle (Heterelmis comalensis), and Comal Springs dryopid beetle (Stygoparnus comalensis) as endangered under the authority of the Act (16 U.S.C. 1531 et seq.). These three aquatic invertebrate species are restricted in distribution to spring sites in Comal and Hays counties, Texas, and in the case of Peck's cave amphipod and Comal Springs dryopid beetle, the associated aquifer. Peck's cave amphipod is known from Comal Springs and Hueco Springs, both in Comal County. The Comal Springs riffle beetle is known from Comal Springs and San Marcos Springs (Hays County). The Comal Springs dryopid beetle is known from Comal Springs and Fern Bank Springs (Hays County).

   The water flowing out of each of these spring orifices comes from the Edwards Aquifer (Balcones Fault Zone--San Antonio Region), which extends from Hays County west to Kinney County. Comal Springs are located in Landa Park, which is owned and operated by the City of New Braunfels, and on private property adjacent to Landa Park. Hueco Springs and Fern Bank Springs are located on private property. The San Marcos Springs are located on the property of Southwest Texas State University.

    Peck's cave amphipod is a subterranean, aquatic crustacean in the family Crangonyctidae. The Comal Springs riffle beetle is an aquatic, surface-dwelling species in the family Elmidae. The Comal Springs dryopid beetle is the only known subterranean member of the beetle family Dryopidae. Elmid and dryopid beetles live primarily in flowing, uncontaminated waters.

   The first recorded specimen of the amphipod Stygobromus (=Stygonectes) pecki (Holsinger 1967) was collected by Peck at Comal Springs in June 1964. Reddell collected a second specimen at the same place in May 1965. In 1967, Holsinger named the species Stygonectes pecki, in Peck's honor, selecting the 1965 specimen as the type specimen. Later he included all the nominal Stygonectes species in the synonymy of the large genus Stygobromus. The Service has used "cave amphipod" as a generic common name for members of this genus, and this name was simply transliterated as "Peck's cave amphipod" without reference to a particular cave.

   Over 300 specimens of Peck's cave amphipod have been collected since its description. Most specimens were netted from crevices in rock and gravel near the three largest orifices of Comal Springs on the west side of Landa Park in Comal County, Texas (Arsuffi 1993, Barr 1993). Barr collected one specimen from a fourth Comal spring run on private property adjacent to Landa Park and one specimen from Hueco Springs, about 7 kilometers (km) (4 miles (mi)) north of Comal Springs (Barr 1993). Despite extensive collecting efforts, no specimens have been found in other areas of the Edwards Aquifer.

   Like all members of the exclusively subterranean genus Stygobromus, this species is eyeless and unpigmented, indicating that its primary habitat is a zone of permanent darkness in the underground aquifer feeding the springs. Above ground, individuals are easy prey for predators, but they usually take shelter in the rock and gravel crevices and may succeed in reentering the spring orifice. Barr (1993) got most specimens in drift nets at spring orifices and found them less often as she moved downstream, supporting the notion that they may be easy prey and do not likely survive for long outside the aquifer.

   The Comal Springs riffle beetle is a small, aquatic beetle known from Comal Springs and San Marcos Springs. It was first collected by Bosse in 1976 and was described in 1988 by Bosse et al. The closest relative of H. comalensis appears to be H. glabra, a species that occurs farther to the west in the Big Bend region (Bosse et al. 1988).

   Adult Comal Springs riffle beetles are about 2 millimeters (mm) (1/8 inch (in)) long, with females slightly larger than males. Unlike the other two organisms listed here, the Comal Springs riffle beetle is not a subterranean species. It occurs in the gravel substrate and shallow riffles in spring runs. Some riffle beetle species can fly (Brown 1987), but the hind wings of H. comalensis are short and almost certainly non-functional, making the species incapable of this mode of dispersal (Bosse et al. 1988).

   Larvae have been collected with adults in the gravel substrate of the spring headwaters and not on submerged wood as is typical of most Heterelmis species (Brown and Barr 1988). Usual water depth in occupied habitat is 2 to 10 centimeters (cm)(1 to 4 in) although the beetle may also occur in slightly deeper areas within the spring runs. Populations are reported to reach their greatest densities from February to April (Bosse et al. 1988). The Comal Springs riffle beetle has been collected from spring runs 1, 2, and 3 at Comal Springs in Landa Park (springs j, k, and l in Brune 1981) and a single specimen was collected from San Marcos Springs 32 km (20 mi) to the northeast.

   The Comal Springs dryopid beetle is a recently discovered species. It was first collected in 1987 and described as a new genus and species in 1992 by Barr (California State University) and Spangler (National Museum of Natural History, Smithsonian Institution). Adult Comal Springs dryopid beetles are about 3.0-3.7 mm (1/8 inch) long. They have vestigial (non-functional) eyes, are weakly pigmented, translucent, and thin-skinned. This species is the first subterranean aquatic member of its family to be discovered (Brown and Barr 1988; Barr, in litt. 1990; Barr and Spangler 1992).

   Collection records for the Comal Springs dryopid beetle are primarily from spring run 2 at Comal Springs, but they have also been collected from runs 3 and 4 at Comal and from Fern Bank Springs about 32 km (20 mi) to the northeast in Hays County. Collections have been from April through August. Most of the specimens have been taken from drift nets or from inside the spring orifices. Although the larvae of the Comal Springs dryopid beetle have been collected in drift nets positioned over the spring openings, they are presumed to be associated with air-filled voids inside the spring orifices since all other known dryopid beetle larvae are terrestrial. Unlike Peck's cave amphipod, the Comal Springs dryopid beetle does not swim, and it may have a smaller range within the aquifer.

   The exact depth and subterranean extent of the ranges of the two subterranean species (Comal Springs dryopid beetle and Peck's cave amphipod) are not precisely known because of a lack of methodologies available for studying karst aquifer systems and the organisms that inhabit such systems. Presumably an interconnected area, the subterranean portion of this habitat, provides for feeding, growth, survival, and reproduction of the Comal Springs dryopid beetle and Peck's cave amphipod. However, no specimens of these species have appeared in collections from 22 artesian and pumped wells flowing from the Edwards Aquifer (Barr 1993) suggesting that these species may be confined to small areas surrounding the spring openings and are not distributed throughout the aquifer. Barr (1993) also surveyed nine springs in Bexar, Comal, and Hays counties considered most likely to provide habitat for endemic invertebrates and found Stygoparnus comalensis only at Comal and Fern Bank springs and Stygobromus pecki only at Comal and Hueco springs.

   Although these species are fully aquatic and two of the three require flowing water for respiration, the absolute low water limits for survival are not known. They survived the drought of the middle 1950's, which resulted in cessation of flow at Comal Springs from June 13 through November 3, 1956. Hueco Springs is documented to have gone dry in the past (Brune 1981, Barr 1993) and, although no information is available for Fern Bank Springs, given its higher elevation, it has probably gone dry as well (Glenn Longley, Edwards Aquifer Research and Data Center, personal communication, 1993). San Marcos Springs has not gone dry in recorded history.

   These invertebrates were not extirpated by the only recorded temporary cessation of spring flow. However, given that they are fully aquatic and that no water was present in the springs for a period of several months, they were probably negatively impacted. These species are not likely adapted to surviving long periods of drying (up to several years in duration) that may occur in the absence of a water management plan for the Edwards Aquifer that accommodates the needs of these invertebrates. Stagnation of water may be a limiting condition, particularly for the Comal Springs dryopid beetle and Peck's cave amphipod.

   Stagnation of water and/or drying within the spring runs and the photic (lighted) zone of the spring orifices would probably be limiting for the Comal Springs riffle beetle because natural water flow is considered important to the respiration and therefore survival of this invertebrate species. Elmid and dryopid beetles have a mass of tiny, hydrophobic (unwettable) hairs on their underside where they maintain a thin bubble of air through which gas exchange occurs (Chapman 1982). This method of respiration loses its effectiveness as the level of dissolved oxygen in the water decreases. A number of aquatic insects that use dissolved oxygen rely on flowing water to obtain oxygen.

References Cited

Arsuffi, Thomas L. 1993. Status of the Comal Springs Riffle Beetle (Heterelmis comalensis Bosse, Tuff, and Brown), Peck's Cave Amphipod (Stygobromus pecki Holsinger), and the Comal Springs Dryopid Beetle (Stygoparnus comalensis Barr and Spangler). Prepared for the U.S. Fish and Wildlife Service. 25 pp.

Barr, C.B. 1993. Survey for two Edwards Aquifer invertebrates: Comal Springs dryopid beetle Stygoparnus comalensis Barr and Spangler (Coleoptera: Dryopidae) and Peck's cave amphipod Stygobromus pecki Holsinger (Amphipoda: Crangonyctidae). Prepared for U.S. Fish and Wildlife Service. 70 pp.

Barr, C.B., and P.J. Spangler. 1992. A new genus and species of stygobiontic dryopid beetle, Stygoparnus comalensis (Coleoptera: Dryopidae), from Comal Springs, Texas. Proc. Biol. Soc. Wash. 105(1):40-54.

Bosse, L.S., D.W. Tuff, and H.P. Brown. 1988. A new species of Heterelmis from Texas (Coleoptera: Elmidae). Southwestern Naturalist 33(2):199-203.

Brown, H.P. 1987. Biology of Riffle Beetles. Annual Review of Entomology. 32:253-73

Brown, H.P., and C.B. Barr. 1988. First report of stygobiontic (subterranean) riffle beetles in North America. Program abstract for April 22, 1988, meeting of Southwestern Association of Naturalists. 5 pp.

Brune, G. 1981. Springs of Texas, Volume 1. Branch-Smith Inc., Ft. Worth, Texas. 

Chapman, R.F., 1982. The Insects: Structure and Function. Harvard University Press, Cambridge, MA. 919 pp.

Holsinger, J.R. 1967. Systematics, speciation, and distribution of the subterranean amphipod genus Stygonectes (Gammaridae). Bull. U.S. Nat. Mus. 259:1-176.

Longley, G. 1991. Status and trends of the Edwards (Balcones Fault Zone) Aquifer in the San Antonio Region. pp. 4-18 In: Proceedings of South Texas Irrigation Conference. Guy Fipps, ed. 146 pp.