The Madison Cave isopod, a rare, blind, pigment- less cave dweller, must be the weirdest looking sentinel of water quality in the country.

Found only on 12 sites in certain parts of the Shenandoah Valley and a splinter of West Virginia, the free-swimming crustacean is fully adapted to a light-free life underground and underwater. Scientists are trying to determine how many there are and where else they may live.

The answers may help protect water resources of the Shenandoah Valley.

The isopod "may prove invaluable for water-supply protection and planning," said Wil Orndorff, a geologist with the Virginia's Department of Conservation and Recreation Natural Heritage Program. "It serves as something of a sentinel species-a canary in the flooded coal mine, if you will."

"It's an indicator of good quality water," said John R. Holsinger, a biology professor at Old Dominion University, who spearheaded the successful effort to list the isopod under the Endangered Species Act.

But Holsinger, known as the father of cave biology in Virginia, said there is another reason to study and protect the strange cave dweller.

"It's unique. That's grounds for preserving it,'" he said. "It's part of the biodiversity of the planet. We don't just want people running around. It wouldn't be any fun."

Among its special characteristics is its marine ancestry. "It tells us a lot about the natural history of the valley," Orndorff said. "At one point in the past, it had close contact with marine water."

Millions of years ago, he said, the valley had one giant aquifer that was separated into different blocks when rivers cut out their beds.

By studying the location of the isopods and their genetic makeup, scientists can learn how and where aquifers are connected. It appears now that there are three populations of the isopod with distinct, if small, genetic differences. That suggests that certain Shenandoah Valley aquifers are not connected.

That knowledge is important because the size of an aquifer can determine how best to manage housing developments, agricultural practices or even natural disasters such as droughts.

The isopod "tells us about the plumbing of the valley, more than any hydrological study," Orndorff said. "That is key."

Orndorff is surveying caves and old, hand-dug wells in the Shenandoah Valley to better determine where the isopod lives. On a recent fall day, he straddled a fissure inside a cave near Grottoes, VA, to trap the creatures, which though they only grow to about a half-inch in length, are fierce hunters.

Orndorff lowered a plastic bottle, baited with raw shrimp, into the pool of water at the bottom of the fissure. Within a short time, he had trapped a handful of the creatures.

The isopod appears whitish and translucent at the same time. No remnant of eyes remains in this creature, which has 14 legs and an exoskeleton, and propels itself through water by its tail.

Orndorff said the isopod feeds on small insects and animals that fall into the water. A jumbo shrimp can be eaten by the creatures in about two hours, he said. "They can skeletonize an animal," he said.

Lyle Steger's family owns the property on which the isopod was discovered in 1959. He played in the caves in the area for many years. The creatures have always fascinated him.

"They've always looked to me like roly-poly bugs," he said, using the popular name for a terrestrial isopod to which the Madison isopod is related. The terrestrial isopod is also known as a pillbug or sowbug.

Studying the creatures "gives us a way to protect the thing we most need-clean water," Orndorff said. "And of course it is just neater than hell."