"Oh, Dr. Culver, look, there's a whole mess of them," shouts American University graduate student Laura Casper, who is standing on a hillside between the George Washington Memorial Parkway and the Potomac River. She has found tiny crustaceans called amphipods and isopods in a seep of water oozing down the hill.
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| This amphipod, named Stygobromus pizzini, lives in springs in the Potomac River Gorge. (Brent Steury/NPS) |
Amphipods and isopods are related to crabs and lobsters, but are much smaller; most are less than an inch long. Amphipods resemble shrimp with laterally flattened bodies, while isopods are dorsally ventrally flattened. Both live in a variety of freshwater and marine habitats, including subterranean caves and streams, deep wells, and lake sediments. Those living deep underground are virtually colorless and eyeless. Occasionally, they surface when water is pumped aboveground, especially at natural springs and seeps, where they can be found clinging to the undersides of wet leaves or other plant material.
About 200 species of subterranean amphipods and isopods live in North America. Of those, six are found in the Potomac River Gorge and Rock Creek Park. In North America, only the Edwards Aquifer in Texas has more subterranean amphipods and isopods.
David Culver, a professor of biology at American University in Washington, D.C., is fascinated by these bizarre-looking and mysterious animals. He has been studying them since his college days, despite their lack of beauty or power, and there is still much to discover. No one knows, for example, precisely what the amphipods and isopods in the Potomac River Gorge eat, in part because they are hard to study in detail. Culver believes that most eat plant detritus: dissolved organic material and/or fungi and bacteria that feed on dead leaves. The amphipods and isopods are in turn eaten by salamanders and by larger amphipods and isopods when they venture too close to the surface.
Most amphipods and isopods have extremely restricted ranges. Kenk's amphipod (Stygobromus kenki), for example, is known only to exist in Rock Creek Park, while Hay's Spring amphipod (Stygobromus hayi), an endangered species, has only been found in Rock Creek Park and in a spring on National Zoo grounds. Both species live among wet leaves where underground water rises to the surface. During droughts, Culver thinks amphipods and isopods burrow into the ground and go dormant until the rains return.
If undistinguished by their size or beauty or what they eat, amphipods and isopods are useful as barometers of their environment. "They are very sensitive creatures," Culver says. "They live in very stable habitats, so they are very vulnerable to change." That means amphipods and isopods are sentinels for the presence of pollutants in underground water and aboveground seeps.
Still, Culver says, it is remarkable that amphipods and isopods can survive in an urban environment. He worries that new golf courses or storm water drains could change amphipods' and isopods' habitats by draining water from seeps and underground streams. "They are part of the biodiversity of life that should be protected," Culver says.
—Jeffrey P. Cohn
ZooGoer 34(6)
2005. Copyright 2005 Friends of the National Zoo. All
rights reserved.
