Madison Cave Isopod

Description and biology

Isopods are tiny, shrimplike crustaceans that have flattened bodies and no carapace (pronounced KAR–a–pace) or shell. The Madison Cave isopod measures 0.47 inch (1.19 centimeters) long and 0.16 inch (0.41 centimeter) wide.

It has no eyes and is colorless. Its diet consists of decaying organic matter such as leaf litter, small twigs, wood particles, and insect remains.

Biologists (people who study living organisms) have been unable to observe the reproductive habits of this isopod.

Habitat and current distribution

The Madison Cave isopod is found only in caves and fissures (long narrow cracks or openings) in the Shenandoah Valley in northwestern Virginia. It inhabits two deep subterranean (underground) pools in Madison Cave and one in nearby Stegers Fissure.

The pools seep into the South River, a tributary of the South Fork Shenandoah River. Biologists recently discovered new populations at four nearby locations, thereby extending the isopod’s range.

Madison Cave isopods prefer to inhabit freshwater pools that have clay banks.

History and conservation measures

Madison Cave has a significant place in American history. Thomas Jefferson mapped the cave, the first instance of cave mapping in the United States. George Washington’s signature also appears on one of the walls in the cave.

The first Madison Cave isopod specimen was not collected until 1958; biologists did not name the species until 1964. It is the only species of its kind found in North America north of Texas.

The Madison Cave isopod currently faces many threats. Because a single groundwater system feeds the caves and connects them to South River, the cave pools can become quickly contaminated with pollution.

Mercury has been discovered in South River. Conservationists (people protecting the natural world) worry that herbicides and pesticides, which run off into the river from nearby farms, could easily reach toxic (poisonous) levels.

Madison Cave has also been damaged by humans. Many people like to explore caves (an activity called spelunking) for recreation. Others have entered the cave to collect bat guano or feces, which is used to produce saltpeter (potassium nitrate), a component of gunpowder.

As people have walked along the banks, they have knocked clay into the pools, destroying the isopod’s habitat by increasing the amount of silt (mineral particles) in the water. Garbage has also accumulated in the cave as more and more humans have come and gone.

In 1981, a gate was put up over the entrance to Madison Cave. Only scientists and educators seeking to study the Madison Cave isopod and other species in its habitat are now allowed access to the cave.