Mussels, often overlooked in river ecosystems, play a crucial role in maintaining water quality by filtering out pollutants and sediments.
Justin Grimm, Brazos River Authority's (BRA) environmental programs coordinator, explains, "Mussels are at the same time a liver and a canary. They have the ability to filter out a huge amount of suspended particles, bacteria, and algae from the water column, and at the same time, their reaction to water quality can inform us as to the overall health of the system. If you start losing mussels, you know something is amiss."
Mussels filter our water, helping to maintain water quality and clarity, which benefits other aquatic organisms. Their presence in riverbeds and lakebeds helps to stabilize the substrate they live in, reducing erosion and providing habitat for other species.
So why should we care? A diverse and abundant mussel population usually indicates a healthy environment, which means good fishing, clearer water for recreation and good water quality.
The BRA's Environmental Services Department regularly monitors different fish and wildlife species, including freshwater mussels, as a way of tracking the health of the Brazos River Basin ecosystem.
All freshwater mussels are relatively inactive and capable of moving only small distances. Their primary mode of moving or colonizing new areas is accomplished by the movement of fish hosts or by high-flow events that scour adult mussels from their current location and move them downstream.
Mussels are ecological powerhouses, providing essential services that promote biodiversity and healthy aquatic habitats. There are more than 50 mussel species in Texas.
The Brazos River Basin is home to 22 different freshwater mussel species, three of which are listed by either the state or federal government as threatened or endangered: the Brazos Heelsplitter (Potamilus streckersoni), the Texas Fawnsfoot (Truncilla macrodon), and the Balcones Spike (Fusconaia iheringi).
Each mussel found in the Brazos River Basin plays an important role in our waterways, making their protection essential to every living thing within the basin. Two are currently listed as Federally Endangered and one is categorized as a state-listed threatened species.
In 1973, categories were established by the U.S. Congress through the Endangered Species Act to protect plants, animals and insects, allowing them and their ecosystems time to recover. The federal law created three main categories where a species in danger may be placed: threatened, endangered, and candidate.
Brazos Heelsplitter
In a 2019 study, the Brazos Heelsplitter was found to be a unique species found only in the Brazos River Basin.
"The Brazos Heelsplitter has mostly been found in sluggish pools and backwater areas on sand/silt/clay mixtures," said Grimm. "They also presumably use the freshwater drum as a host fish. They are usually a lot larger than the other two species. Oval-shaped and usually a deep purplish brown color on the outside and a pearly deep pink/purple in the interior."
Recent surveys find them predominantly in the mainstem of the Brazos River downstream of the city of Waco and venturing near the confluences of the Little River, Navasota River, and Yegua Creek. The Brazos Heelsplitter is commonly found in soft substrates of sand, silt, gravel, and mud near banks, backwaters, and pools.
The Brazos Heelsplitter is a state-listed threatened species.
Texas Fawnsfoot
The Texas Fawnsfoot is most commonly observed in riffles within streams and rivers but has been identified in a variety of habitats.
"The Texas Fawnsfoot presumably use Freshwater Drum as a host fish and tend to settle out in areas where a freshwater drum would browse for food," Grimm said. "Juveniles are often buried thoroughly in fine gravel riffles and runs that contain a little clay base. They often get swept away downstream in high flows, and adults can persist in backwater and bank areas where they have another chance to infect drum fish. The shape, rays, and chevron patterns are very distinct from other mussels in the Brazos."
Fawnsfoot has been found in multiple locations within the Brazos River Basin, including the Clear Fork of the Brazos River, the Brazos River between Possum Kingdom Lake and Lake Granbury, the Brazos River below Waco, as well as the Navasota and Little Rivers.
The Texas Fawnsfoot is currently listed as Federally Threatened.
Balcones Spike
"The Balcones Spike utilizes minnows that hang out in riffles and shallow runs, and we find the mussels in the same area," Grimm said. "The younger ones are mostly in sandy pockets in riffles, and the older ones tend to wash out a little bit into large gravel/small cobble dominate runs downstream. They can look a lot like the more common Tampico Pearlymussel, but the Balcones Spike has a more abrupt posterior slope and a less curved ventral margin. The sister species False Spike often has faint pustules and ridges, but they are usually missing on the Brazos Basin Balcones Spike."
Within the Brazos River Basin, the Balcones Spike is currently known to reside in the Little River, San Gabriel River, and Brushy Creek.
The Balcones Spike is currently listed as Federally Endangered.
Ongoing Research
A significant amount of research is done on freshwater mussels. Agencies across the state are researching them, including the U.S. Fish and Wildlife Service (USFWS), the Texas Comptroller of Public Accounts, the Texas Parks and Wildlife Department (TWDP), and the BRA.
Several universities also are researching freshwater mussels. These studies include surveys to identify new populations, genetic investigations, and tolerance studies.
Recently, Smithsonian Collections Digitization announced its exciting collaboration with the Department of Invertebrate Zoology at the Smithsonian National Museum of Natural History to digitize over 45,000 specimens in their historic collection of freshwater mussels. Over 120,000 images will be added to the Smithsonian's digital collections, which will help researchers better understand the distribution of the species over time and support these at-risk animals thriving into the future. You can watch the video here.
The TPWD also maintains the Texas Mussel Watch Program, where members of the public can submit their observations of mussels to help gain a better understanding of the distribution and status of mussels. This site covers all of Texas.
In August 2020, the BRA submitted a Candidate Conservation Agreement with Assurances to the USFWS for consideration, in which the BRA commits to performing voluntary conservation activities aimed at reducing threats to the Texas Fawnsfoot and Balcones Spike. Such actions are therefore beneficial to Brazos Heelsplitter occupying much of the same range in the Brazos basin. This agreement was executed in mid-2021.
Mussel populations are good indicators of the overall health of aquatic ecosystems. When mussel populations are at risk, it indicates problems for other fish, wildlife species, and even people.
Grimm says, "All of these species are protected by federal and/or state law, so it is best to leave them alone if possible."