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Along U.S. Highway 281 headed north to the Comal County line, drivers can see shopping centers and housing developments that look indistinguishable from suburbia elsewhere in the country. But hundreds of feet below the highway, inside the Edwards Aquifer and some local caves, new species of salamanders, crustaceans, and other creatures are being discovered.
Animals that live inside the Edwards aquifer include two species of blind catfishes. A recent report also documents the discovery of several species of crustaceans (shrimp-like creatures) in the aquifer.
“There are some really cool things, really close to home,” said Andy Gluesenkamp, the director of conservation at the San Antonio Zoo who has studied four new local species of cave salamanders, including one from the Edwards Aquifer that was found in a well in a bowling alley parking lot in New Braunfels.
Gluesenkamp is writing a paper about these previously unknown species that describes their morphology, behavior, habitat, and other characteristics.
Caves do strange things to the animals that live there. Having adapted differently from their river ancestors, Mexican cavefish (Astyanax tetras) have lost their color and become pale white, and are eyeless because eyes are useless in the pitch-black caves. They rarely sleep, are fat and have poor blood sugar control, and they have low metabolic rates that allow them to survive for years without food. Despite all this, Mexican cavefish live for years.
Evolutionary biologists have been puzzling over how and why these fish got that way. The answer could be found in the caves of the Texas Hill Country.
In 1908, surface-dwelling Astyanax tetras – the same species as the Mexican
fish – were collected from the Rio Grande and introduced into the San Pedro Springs to stock as bait fish. They were likely introduced to the Guadalupe River sometime in the 1950s.
The Astyanax tetras now live in Honey Creek, a branch of the Guadalupe River basin. Honey Creek waters flow out of the longest known cave in Texas, near Guadalupe River State Park in Comal County. Some of the introduced tetras thrived in the surface creek, while others became cave-dwelling.
In a recent study, Gluesenkamp and his colleague Suzanne McGaugh at the University of Minnesota showed that the surface fish and cave-dwelling fish in the Honey Creek ecosystem have intriguing differences in their behavior, morphology, and physiology, even though they’ve been separated for only about 70 years. These changes hint at the beginning of the process by which the Mexican cavefish, which have lived in the caves for thousands of years, adapted.
Because of their interesting features, the fish have become a boon to biomedical scientists who study a variety of human diseases, McGaugh explains.
“Mexican surface and cavefish counterparts of the fish in the Honey Creek ecosystem have quickly become models for understanding eye development, heart regeneration, type 2 diabetes, and sleep disorders,” writes McGaugh, who grew up in Angleton, in Southeast Texas.
Gluesenkamp also is studying pairs of surface and cave-dwelling salamanders, which show many similar features as the fish. The cave salamanders are white and blind, too. They have large, flat heads and spindly, long legs.
When the salamanders are underground and food is scarce, they not only burn fat and muscle, but even resorb up to 8 percent of their bone, shrinking in body size, Gluesenkamp said. When the rains come and bring food, the salamanders grow again. Throughout their life, they can shrink, grow, and shrink repeatedly.
Ultimately, Gluesenkamp and his collaborators would like to understand the genetic basis for these adaptations, which could have relevance for understanding and treating osteoporosis in humans.
But they are facing time pressures because the habitat of these species is at risk. The waters of Honey Creek are threatened by potential plans to build a sewage treatment plant, which would discharge up to an average of 500,000 gallons per day of treated wastewater into the creek. The habitats for these animals could be destroyed before the researchers can find answers to big scientific questions that really can’t be answered anywhere else.
“I’m stunned at what a pristine riparian ecosystem Honey Creek is and have been so filled with pride that my home state had the forethought to preserve the area,” wrote McGaugh in an email. “This area is so sensitive that it is only open to public through limited, guided tours. It is difficult to understand, then, why the state would allow release of 1/2 million gallons of waste water per day into such a delicate space.”
“This is extremely unique site for research on how quickly animals can adapt to new environments,” she added.
She warns that endocrine disruptors, antidepressants, antibiotics, and other contaminants found in treated wastewater could negatively impact the species reliant on Honey Creek for survival.
“We are losing species at an incredible rate,” Gluesenkamp said.
Losing the animals in Honey Creek and the connected cave would destroy not only a valuable ecosystem, but also eliminate the opportunity to make scientific discoveries and gain insight into the biology of cave dwelling animals that can’t be made elsewhere. Losing these species would be “like going to the library and burning all the books you haven’t read yet,” Gluesenkamp said.
The Texas Commission on Environmental Quality will hold a public meeting about the plans to build the sewage treatment plant on Aug. 20 at 7 p.m. at the Rahe Bulverde Elementary School in Bulverde. Members of the public can submit comments and ask questions about the proposed project.