Madison Cave Isopod

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Madison Cave Isopod

Antrolana lira

ListedOctober 4, 1982
FamilyCirolanidae (Cave isopod)
DescriptionWhite, blind, shrimplike crustacean.
HabitatSubterranean freshwater pools.
ThreatsHabitat destruction, groundwater pollution.


The only member of the genus Antrolana, the Madison Cave isopod, A. lira, is a white, shrimp-like crustacean lacking eyes. This cave-adapted (troglobitic) isopod grows up to 0.4 in (1.02 cm) in length, making it among the longer, and the largest, subterranean isopods in the eastern United States. Females are slightly larger than males. The body is flattened, compact, and approximately three times longer than it is broad.


The Madison Cave isopod consumes organic matter, such as leaf litter or dead insects, that is washed into its aquatic habitat by surface runoff. Females carrying eggs have never been found, but juveniles have been located, showing that reproduction is occurring. It is believed that females may hide in the leaf litter in the bottom of the fissures or in the inaccessible channels which feed the pools.


The Madison Cave isopod inhabits freshwater, subterranean pools, which are fed primarily by an aquifer. Little is known of the physical and chemical conditions of its habitat. The temperature of the water ranges from 51.8-57.2°F (11-14°C), typical of groundwater for the latitude, and the water is saturated with calcium carbonate, a condition also typical of groundwater in areas of limestone. The level of the karst aquifers can fluctuate for tens of meters at some sites. The extent of the recharge zone of the aquifer at any site is unknown.

Biologists have observed a slow seepage of water from Madison Cave pools into the nearby South River, a tributary of the South Fork Shenandoah River. Water levels in the cave vary somewhat with the river flow. For instance, two habitat pools found in Madison Cave have decidedly different depths: One is about 35 ft (11 m) deep, the other 75 ft (23 m) deep. Another habitat pool, in Stegers Fissure, is quite deep, at 100 ft (30.4 m).

Madison Caveuntil 1990 thought to be the species' only habitatwas the first cave ever mapped in the United States, and the mapper was Thomas Jefferson (president of the United States, 1801-1809). George Washington (president of the United States, 1789-1797) also visited the cave and left his signature on the cave wall.


The Madison Cave isopod is endemic to Madison Cave and the nearby Stegers Fissure (Augusta County), Virginia, as well as Front Royal Caverns and Linville Quarry Cave No. 3. This isopod is the only member of the Cirolanidae family in the eastern United States. Other members are found in Texas, Mexico, and the Caribbean.

Only a few of these isopods have ever been collected, and the size of the population is unknown. The isopod is difficult to study and collect, and is known only from areas where fissures descend to the groundwater table, thus allowing access to the surface of underground lakes, or deep karst aquifers.

Until 1990, the species was known only from two sites, Madison Cave and Stegers Fissure (near the cave); since June 1990, the isopod has been collected from five additional sites. Although specimens from all seven sites are morphologically identical, they probably represent more than one but less than seven genetic populations. Population size appears to be extremely small at five of the species' seven occurrence sites.


As early as 1812 deposits of bat guano in Madison Cave were mined for saltpeter (potassium nitrate) for use in the manufacture of gunpowder. Over the years visitors and spelunkers have left behind an accumulation of trash and contributed to siltation of the pools by trampling the steep clay talus banks. The entrance to the cave has now been secured against unauthorized entry, and the cave's private owner has developed a conservation plan, which is designed to satisfy parties interested in the cave's history, as well as those interested in the welfare of the Madison Cave isopod.

The aquatic resources of the isopod's habitat face the serious threat of mercury contamination. High levels of mercury have been measured in the South River, discharged by the now-defunct E. I. du Pont de Nemours and Company factory upstream at Waynesboro. Although no mercury has yet been found in the cave pools, it is feared that the ground-water could be contaminated during prolonged periods of high water. Even low levels of contamination would jeopardize the surviving population of the Madison Cave isopod.

Conservation and Recovery

Madison Saltpeter Cave is protected through cooperation between the property owner and cave conservation organizations. The cave entrance is protected by a steel gate, and all visits to the cave must be approved for scientific or educational purposes.

In 1995, a draft Recovery Plan was announced, developed to protect the isopod's population from potential threats to the quality of its deep karst aquifer habitat, thereby enabling the removal of the species from the federal list of endangered and threatened wildlife and plants. According to the plan, delisting may be considered when: (1) populations of Madison Cave isopod and groundwater quality at Front Royal Caverns, Linville Quarry Cave No. 3, and Madison Saltpeter Cave/Stegers Fissure are shown to be stable over a 10-year monitoring period; (2) the recharge zone of the deep karst aquifer at each of these population sites is protected from all significant contamination sources; and (3) sufficient population sites are protected to maintain the genetic diversity of the species.

Recovery activities designed to achieve these objectives include: (1) determining the number of genetic populations, (2) searching for additional populations, (3) identifying potential sources and entry points of contamination of the deep karst aquifer habitat, (4) protecting known populations and habitats from a watershed perspective, (5) collecting baseline ecological data for management and recovery, and (6) implementing a program to monitor recovery progress and future needs. Contingent on vigorous implementation of all recovery tasks, the plan reports that full recovery is anticipated by 2005.


Regional Office of Endangered Species
U.S. Fish and Wildlife Service
300 Westgate Center Dr.
Hadley, Massachusetts 01035


Bowman, T. E. 1964. " Antrolana lira, A New Genus and Species of Troglobitic Cirolanid Isopod from Madison Cave, Virginia." International Journal of Speleology 1(1-2):229-236.

Holsinger, J. R. 1979. "Freshwater and TerrestrialIsopod Crustaceans (Order Isopoda)." Proceedings of the Endangered and Threatened Plants and Animals of Virginia Conference 1978. Virginia Department of Fish and Game, Richmond.

U.S. Fish and Wildlife Service. 1996. "Recovery Plan for the Madison Cave Isopod." USFWS, Newton, MA.

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Madison Cave Isopod

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