NATURE NOTES:
The Texas Hornshell Mussel: An Endangered “Engineer” of an Endangered River System

By Andrew Stuart

It’s a member of one of the most endangered groups of animals in the United States, and its home is one of the most imperiled river systems in the world.

The Texas hornshell mussel is native to the Rio Grande and its tributaries. But like that river system, it’s at the brink. In February 2018, the mussel received federal protection under the Endangered Species Act.

Brownish in color, the hornshell is nondescript. But it has a fascinating life history. And its fate is tied to one of this arid land’s most precious resources: its rivers and streams.

Freshwater mussels are hard-pressed across the Southwest. The Texas hornshell is now the only native mussel found in New Mexico – once home to eight species. It’s among 15 threatened mussels in Texas.

In fact, by 1996, the hornshell was known only in a single stream – Black River, near the Guadalupe Mountains in New Mexico. In the early 2000s, it was found in the Rio Grande near Laredo.

Dr. Charles Randklev heads a Texas A&M lab focused on freshwater mussels. He and his team have been on the hornshell hunt since 2010.

“A lot of the survey work is done by getting into rivers and streams and physically looking for them,” Randklev said, “either using snorkel or scuba, or, if it’s more turbid rivers like we have in West Texas, it’s kind of grubbing around trying to feel for them.”

 

Searching collections at the Smithsonian and other museums, the team established the hornshell’s historic distribution. The mussel persists in less than 20 percent of its former range. It’s found mostly in a few “strongholds,” including the Lower Canyons of the Rio Grande, the Devils River and Lower Pecos.

The hornshell’s plight is encapsulated on the Pecos. The mussel was abundant there in the 1960s. Not so today.

“We surveyed a bunch of sites, and found a lot of old, old shell,” Randklev said. “I couldn’t date it, but it wasn’t recent, and we only found three live individuals.”

The Pecos has been dammed, diverted for agriculture and industry. As flows fell, salinity and water temperature increased – likely causes for the hornshell’s decline. Human activity has similarly impacted the Rio Grande and its other tributaries.

One key feature sets the hornshell’s strongholds apart: the presence of springs. On the Devils, the Lower Canyons and the Lower Pecos, springs provide the water quantity and quality mussels need.

Historically, hornshells and other mussels were “engineers” of aquatic ecosystems. As “filter feeders,” they cleaned rivers. Their waste supports aquatic insects, which support fish in turn. They’re food for muskrats, raccoons, birds.

And, as essentially immobile creatures, they evolved a truly unique method for reproducing.

Male hornshells release sperm into the water column. A female takes in the drifting sperm – and broods larvae, or glochidia. Then, she releases the glochidia – in a thick “mucus web.”

“It looks like a giant blob of snot,” Randklev said. “It will float through the water column and ensnare its host fish. It’s pretty crazy.”

The process is thought to depend on three fish: the red shiner, river carpsucker and gray redhorse. The carpsucker and redhorse likely consume the “blobs” as they feed – glochidia pass through the fish and “encist” in its gills. Researchers think the red shiner is ensnared by the mucus web, which attaches to its head.

In a parasitic relationship, the glochidia feed on their host. It doesn’t seriously harm the fish. After several weeks, the juvenile mussels drop loose. Only a fraction survive to adulthood. Those that make it can grow to 6 inches, and live for decades.

The Rio Grande and its tributaries once supported diverse aquatic creatures. Many of those creatures have vanished. The World Wildlife Fund identifies the Rio Grande as among the world’s 10 most at-risk rivers.

Endangered-species listings often raise landowner concerns about impacts to property rights. Randklev said that, as much as the mussel itself, the hornshell’s listing is a way of shining a light on an ecosystem.

“Because that’s really what we’re talking about here,” Randklev said. “I mean, I love mussels to death, I think they’re great, but at the end of the day they’re an indicator and a tool to help preserve these aquatic ecosystems.”

Protecting the mussel will mean protecting what remains of clean, flowing water in this parched land.