Texas hornshell (Popenaias popeii) | ESA status: endangered

Texas hornshell habitat

Mussels are a species commonly associated with marine environments, but the most imperiled mussels in the world inhabit freshwater systems, including desert 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 the last remaining native mussel in New Mexico: all of the seven other mussel species in the state have been extirpated.

The Texas hornshell is found in shallow, slow-running water, tucked under travertine shelves and in-between boulders where soft sediment gathers. In 1996, surveys confirmed its presence along an eight-mile segment of the Black River (a tributary of the Pecos River), where it had not been seen since the 1930s. In Texas, surveys starting in 2005 found 48 dead hornshells in the Rio Grande in Big Bend National Park. 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. There may be a population in the Rio Grande near Laredo, and 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.

Texas hornshell life cycle

Truly a Texas-sized mussel, the hornshell grows longer than 2.3 inches. It 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 attaches 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 metamorphose into juvenile mussels in six to 10 days, after which they release from their host into the stream’s benthic community. Adult hornshells may live up to 20 years.

What are the threats to the Texas hornshell?

Freshwater mussels require perennial river flow, adequate water quality, and suitable substrates. A battery of threats endanger the hornshell, including alteration of stream habitat from land uses; water pollution; water diversion and groundwater pumping; contamination from oil and gas operations; and siltation and sedimentation.

How WildEarth Guardians is protecting the Texas hornshell

The U.S. Fish and Wildlife Service first found that the Texas hornshell warranted ESA protections in 1989, but the agency declined to actually list it, instead placing it on the “candidate” list. After many years of advocacy from Guardians and our allies, the Service listed the mussel as endangered.