Texas hornshell Popenaias popeii
ESA status: candidate for listing

This species has been awaiting Endangered Species Act listing for
Years Days Hours Mins Secs
Texas hornshell pc Joel Lusk USFWS

Mussels are a species commonly associated with marine environments, but the most imperiled mussels in the world inhabit freshwater systems, including desert streams. In the southwestern U.S., freshwater mussel imperilment is signaling the dire conditions of the region’s rivers and streams. The Southwest was once home to a multitude of freshwater mussels—more than 52 species were identified in Texas, and eight were known in New Mexico. Now, many southwestern streams no longer support any mussel species.

The Texas hornshell is found in shallow, slow-running water, tucked under travertine shelves and in between boulders where soft sediment gathers. This Texas-sized mussel grows longer than 2.3 inches. The hornshell is a filter feeder, siphoning microorganisms, organic matter and inorganic material from the water. Like other freshwater mussels, female hornshells release larvae in a sticky mucous mass or string that attach to the gills, fins, or head of a host fish. The larvae (called glochidia) encyst and feed off the fish’s body fluids. The glochidia appear to metamorphosize into juvenile mussels in 6-10 days, after which they release from their host into the stream’s benthic community. Adult hornshells may live to 20 years.

The Texas hornshell is the last remaining native mussel in New Mexico: all of the seven other mussel species in the state have been extirpated. The hornshell was confirmed to exist along an eight-mile segment of the Black River (a tributary of the Pecos River) in 1996, where it had not previously been seen since the 1930s. In Texas, 48 dead hornshells have been found in the Rio Grande in Big Bend National Park in surveys starting in 2005. Two live hornshells were found in the Devil’s River and one was found within the Rio Grande Wild and Scenic River downstream of Big Bend in 2008. An extant population is believed to occur in the Rio Grande near Laredo, and dead shells were found in the Llano River in Llano County and South Concho River in Tom Green County (both tributaries of the Colorado River) in 2004.

The U.S. Fish and Wildlife Service designated the Texas hornshell as a candidate for listing under the Endangered Species Act in 1989. The agency identified a battery of threats to the species, including alteration of stream habitat from land uses; water pollution; water diversion and groundwater pumping; contamination from oil and gas operations; siltation and sedimentation; and a lack of legal protections.

Freshwater mussels require perennial river flow, adequate water quality, and suitable substrates. The widespread loss of mussels and habitat in the Southwest is evidence of our mismanagement of these riverine environments. Protecting the Texas hornshell under the Endangered Species Act may assist in preventing this species from becoming the next mussel to vanish from Southwestern waterways and can help bring some of the region’s rivers and streams back to life.

photo credit: Joel Lusk, U.S. Fish & Wildlife Service