Invasive snail, parasite threaten Central Texas fish stocks
A specimen of the invasive Asian snail, Melanoides tuberculatus, collected from the Guadalupe River. The snail is the only host for a parasitic flatworm that has the potential to infect many native fish species. PHOTO by DAVID HUFFMAN
A little snail from Asia may be poised to create some very big problems for fish in Central Texas rivers.
The aquatic snail, with a distinctive, conical spiral shell, is highly competitive and tends to displace native snail species, said David Huffman, a parasitologist in the Department of Biology at Texas State University.
Huffman is one of more than 40 scientists throughout the Texas State University System working through the Institute for the Study of Invasive Species in Huntsville to study and develop strategies to deal with invasive species across the state. The bigger concern with this snail, however, comes from the fact that it is the sole host of another invasive species from Asia—a parasitic flatworm that infects the gills of many native fish species.
Endangered fountain darters in the Comal River have suffered greatly since the parasite first appeared there in the 1990s, and minnows and game fish such as bass and catfish are also susceptible, Huffman said. Researchers had taken some measure of solace in the fact that the tropical snail struggles to survive when water temperatures drop to 18 degrees Celsius (64 Fahrenheit) and rapidly dies off when temperatures dip to 15°C (59°F), which effectively contained them—and the parasitic flatworm they host—within the relatively warm, spring-fed waters of the Comal. Until now.
In 2009, Huffman began finding snails thriving in the much colder waters of the Guadalupe River, and by 2011 they’d moved upstream as far as Gruene Crossing and downstream through Lake Dunlap and as far as one mile into Lake McQueeny. In January 2012, Huffman found hundreds of snails seemingly thriving near Dunlap Dam in water that had been between 11-13°C (51-55°F) for weeks—temperatures that should’ve killed the snails within two or three days.
“It hadn’t gotten out into any surface-fed streams and been able to reproduce there because it couldn’t make it through our winters. So we’d been comforted in the past knowing that while it is in our beautiful springs, that at least the endangered species and sport fish and the rest of our surface-fed rivers and reservoirs are safe,” Huffman said. “Suddenly, in New Braunfels, Texas, the snail seems to have busted out of its temperature limitation that we’ve been trusting to prevent it from escaping the springs into surface waters. I don’t see any reason now why it can’t—if it’s transported to other surface waters in Central Texas and southward—just go gangbusters.
“We were naïve in thinking it wasn’t ever going to bust out of this temperature limitation,” he said. “Now that it’s busted out of that, there’s a lot of recreational traffic out at Dunlap. There’s a big boat ramp at I-35, and skiers come in there and they back their trailers in the water. Fishermen are in there and they’re dragging stuff around and pulling up and going into another reservoir, so it’s probably going to get into other reservoirs now.”
Because of the continuous, wild temperature swings at the confluence of the Comal and Guadalupe rivers in New Braunfels, Huffman predicted as far back as 2000 that if the snail were to ever make an evolutionary adaptation to colder temperatures, that’s where it would happen—and that prediction now seems to have been borne out.
The lack of mature fish in the pristine Comal River gives stark testimony to the deadliness of the snail-borne parasite, but the invasion has gone largely unnoticed due to the lack of dramatic fish kills such a virulent parasite ought to cause. Huffman has a theory why that is the case.
“I tell you, I noticed the absence of fish down there in New Braunfels,” he said. “That’s not caused by aluminum cans in the water—it’s the parasite.
“They encyst on the gills of a fish and wait for that fish to get eaten by a bird–in this case, it’s a green heron. Then the green heron poops in the water and the life cycle starts again,” Huffman said. “My theory on the reason why we don’t see fish kills on this, is that the gill parasite slows the fish down so it gets picked off by predators, so we don’t see them.”
Because the flatworm encysts on the gills of fish, it interferes with the fish’s ability to oxygenate the blood. With enough parasites, the effect would be like a person trying to run a race during an asthma attack. For fish, such infections would make them slow and sluggish, easy picking for predators long before the parasites killed them outright. To test his idea, Huffman has invested several thousand dollars out of pocket to design and build testing equipment to measure the swim speed and endurance of fish at various degrees of infection. It’s basic research, but given how little information exists on the impact of the invasives in the Comal and Guadalupe, it is essential to determine just what kind of long-term threat Central Texas waters may be facing.
“This snail is known to support 37 different kinds of flukes and 11 of those are human parasites in some areas of the world,” Huffman said. “Now, we haven’t seen any human parasites from the snail here yet, and the snail’s been here for more than 50 years, but we also hadn’t seen the snail survive in cold water for the past 50 years, either.”
— JAYME BLASCHKE/TEXAS STATE NEWS SERVICE