The work of recent Texas A&M University graduate Xenia Rangaswami provides critical insight into the thermal and salinity tolerances of two subpopulations of the endangered Texas hornshell mussel found in the Devils River in Texas and the Black River in New Mexico.

“Mussels are one of the most imperiled group of species, and they are also excellent bioindicators,” said Rangaswami, who earned a master's degree in rangeland, wildlife and fisheries management. “An abundance of mussels can indicate really good ecosystem health, but when they disappear, they’re kind of the canary in the coal mine for an ecosystem.”
 
According to Charles Randklev, Ph.D., principal investigator with the Texas A&M AgriLife freshwater mussel research team, of the 300 mussel species known to have occurred in the U.S., 12% are thought to be extinct and 23% are considered threatened or endangered.

This is largely due to habitat loss, declines in water quality, changes in stream flow, as well as impoundments.

“The Devils River in south-central Texas harbors the endangered Texas hornshell, and there is concern that water withdrawals from the underlying aquifer may be negatively impacting this species,” Rangaswami said.

After collecting larval and juvenile specimens in the field, Rangaswami exposed them to different water temperatures to determine the upper thermal limits that resulted in 5% and 50% mortality.

She had a similar methodology for the salinity portion of her research. Instead of having water baths at varying temperatures, Rangaswami calculated the mortality rates of mussels at different salinity concentrations.

Additionally, Rangaswami reviewed the upper thermal tolerances of the species’ presumed host fish, which are necessary for mussel reproduction.

She explained that during the larval stage of mussel development, known as glochidia, the tiny mussels attach themselves to the gills of fish where they live and develop before detaching.

“We assessed how salinity affected the ability of larvae to attach to fish gills, as well its effect on the metamorphosis from the glochidia to the juvenile stage,” Rangaswami said.  

Data loggers measuring temperature and salinity levels were also deployed in the Devils River throughout her research. This allowed Rangaswami to compare the thresholds established in the lab with actual levels occurring in the river.

“We were looking for exceedances above the thermal or salinity tolerances and also tied them back to the stream flow conditions in the river,” Rangaswami said. “If stream flow is low, that might exacerbate high water temperature and concentrate the salinity.”

From a practical standpoint, Rangaswami said this information will enable river management authorities to implement management activities to maintain flow levels above the thresholds identified to prevent mussel mortality.

Her research, supported by the U.S. Fish and Wildlife Service and the Texas Parks and Wildlife Department, is a component of the freshwater mussel research program housed in the Texas A&M AgriLife Research and Extension Center at Dallas.