Comal Springs Riffle Beetle
Covered Species Information
Authored by Connor Helsel
The Comal Springs riffle beetle (H. comalensis) is a small aquatic beetle found exclusively in the Comal and San Marcos spring systems within the Edwards Aquifer region of Central Texas. The species is listed as an endangered species by the Endangered Species Act due to overconsumption of groundwater, low springflow and changes in water quality. The Edwards Aquifer Habitat Conservation Plan (EAHCP) has sponsored several studies to improve the understanding on key aspects of the Comal Springs riffle beetle including their life cycle, physiological tolerances, diet, and critical habitat. Additionally, the implementation of EAHCP Conservation Measures, such as native riparian and aquatic habitat restoration, aid in the protection of these endangered species.
Comal Springs riffle beetle
Endangered Species Act status
Adults are approximately 2 mm long (BIO-WEST 2016).
Physical description and Life History
H. comalensis are physically similar to other elmids (riffle beetles), however, they are unable to swim nor fly in all stages of its life, unlike other elmid (riffle beetle) species. It is speculated the they move mainly by drifting downstream or crawling. Since Comal Springs riffle beetles lack gills, they respire underwater via a plastron, a mass of setae on the underside that work to trap a thin layer of air and force the diffusion of dissolved oxygen from the surrounding water (Brown 1987).
H. comalensis typically carry 10 eggs with an incubation period of about 21-25 days and undergo seven instar larval stages during its life cycle. Following the last instar, larvae molt into pupae. Pupation occurs for about a month until adulthood. The complete life cycle of the Comal Springs riffle beetle is about two years (BIO-WEST 2017).
Spring regions of Comal and San Marcos Springs (BIO-WEST 2017).
Habitat and Diet
H. comalensis exist within a niche habitat of high quality, low temperature (26℃), streams, and rivers, approximately one meter from spring openings (Cooke 2012).
Diet is thought to be comprised of bacterial biofilms of decomposing leaf- and wood-material, although the precise food source remains unknown (Nowlin et al. 2017).
BIO-WEST, Inc. (2016). Evaluation of the Life History of the Comal Springs Riffle Beetle Egg. Year 1 Report prepared for Edwards Aquifer Authority. 26.
BIO-WEST, Inc. (2017). Comal Springs Riffle Beetle (Heterelmis comalensis): Life History and Captive Propagation Techniques. Final Report prepared for Edwards Aquifer Authority. 36.
Brown, H. P. (1987). Biology of riffle beetles. Annual review of entomology, 32(1), 253-273.
Cooke, M. (2012). Natural history studies on the Comal Springs riffle beetle (Heterelmis comalensis) [master’s thesis]. San Marcos (TX): Texas State University.
Endangered and Threatened Wildlife and Plants; Final Rule To List Three Aquatic Invertebrates in Comal and Hays Counties, TX, as Endangered, 62 Fed. Reg. 242 (December 18, 1997) (to be codified at 50 C.F.R. pt. 17).
Nowlin, W.H., Hahn, D., Nair, P. and Alfano, F. (2017). Evaluation of the trophic status and functional feeding group status of the Comal Springs riffle beetle. EAHCP Project 148-15-HCP.