NEW BRAUNFELS, Texas -- Destroying an invading horde of giant, nocturnal snails may sound like the plot of a bad science-fiction movie, but it could be San Antonio's best hope to increase its access to water from the Edwards aquifer.
The giant ramshorn snails are eating machines native to Central and South America.
They devour the eggs and habitat of the fountain darter, a tiny endangered fish at the center of a Sierra Club lawsuit to limit unrestricted pumping from the aquifer.
To maintain a safe environment for the fountain darter, federal officials say, water must flow out of the springs at a minimum rate of 200 cubic feet per second. That figure indirectly dictates how much water the city and other aquifer users can pump, especially in times of drought.
Snails throw a wrench into the works.
More snails mean fewer fountain darters. Fewer fountain darters mean the court could require pumping cutbacks to increase spring flow and thus increase the survival of the fountain darter.
But a report by federal scientists says that if the snails are controlled and their effect on the fountain darter neutralized, the minimum flow rate could be cut 25 percent to 150 cubic feet per second.
A later report suggests that for short periods, the minimum allowable flow rate could be lowered to 60 cubic feet per second if both the snail and spring flow could be controlled.
That means more water for San Antonio in times of low rainfall and high consumption. The Edwards aquifer is San Antonio's sole source of water.
"It could be San Antonio's cheapest water," said Rick Illgner, general manager of the Edwards Underground Water District, which has spent $25,000 to study the snails and budgeted another $15,000 to determine how to get rid of them.
Mr. Illgner estimated that successful management of the snails would yield an extra 3,000 acre-feet of water every month if a spring flow of 150 cubic feet per second is allowed, and as much as 8,400 acre feet a month if the level were allowed to drop to 60 cubic feet per second.
An acre-foot would cover a football field with a foot of water.
The snails probably were dumped into the Comal and San Marcos rivers more than a decade ago by soft-hearted aquarium hobbyists who grew tired of the gastropods.
The snails, striped with alternating bands of yellow or beige and dark brown, can grow larger than a silver dollar and weigh more than half an ounce.
The snails' habits have made them outlaws in Texas. Possessing the ramshorn is a class B misdemeanor, punishable by 180 days in jail and a $2,000 fine, said Bob Howells, a Texas Parks and
Wildlife biologist.
The ramshorn snails first showed up in the Comal Springs in 1984, when four shells were discovered on the banks of state-owned Landa Lake, which is where Comal Springs are located in New Braunfels.
But it wasn't until 1989, when the Comal Springs flow averaged )) below 200 cubic feet a second, that the ramshorn attracted much attention.
Facing no natural predators or diseases in the springs, the snails took full advantage of the slower spring currents, which enabled them to more easily climb aquatic vegetation, which they eat.
The snail population exploded. At night, throngs of snails could be seen feeding.
Although the ramshorns also have invaded San Marcos springs, dTC home to four other rare and endangered species, they have not (( had the same destructive effect because currents move there much faster, inhibiting their population growth.
Although snail numbers have stabilized as spring flow rates have rebounded over the last three years, some biologists fear that their population could explode again if spring flows drop sharply.