WASHINGTON (AP) _ The blueish, shrimp-like creature is only about a half-inch long. It has no commercial value and plays no known role in nature outside its home in the cold water of a few caves in southwestern Illinois.

Yet the Illinois cave amphipod is at the center of a debate between farmers and scientists whether it should be considered an endangered species and worthy of federal protection.

The elusive crustacean's fate _ and what changes its human neighbors might be forced to make to accommodate it _ hinges on the answers to several questions.

For one, are the amphipod's numbers actually dwindling? If so, what is most to blame, agricultural chemical runoff or residential pollution from the growing St. Louis suburbs?

Those against conferring the new status say there's little reason to care.

However, those in favor say the rapidly growing human population has an enormous stake in the matter. If the water is bad enough to kill the cave amphipod, they say, it can't be good for people, either.

``We have serious groundwater contamination problems,'' said Joan Bade of the Monroe-Randolph Bi-County Health Department. ``Ensuring the amphipod's survival by protecting its habitat will also protect the health of people in the area.''

National conservation groups say the case is a perfect example of how the 24-year-old Endangered Species Act is supposed to work: Protecting species because of the implications for people, even though the value of the creatures themselves is unclear.

The Illinois cave amphipod, or Gammarus acherondytes, has been a candidate for the federal endangered species list since 1991. The regional office of the U.S. Fish and Wildlife Service formally proposed adding it in July.

If it makes the list _ a process that could take a year _ it would be the first species in Illinois to be added since the Hine's emerald dragonfly in January 1995. The national program, with 1,107 plants and animals, admits an average of about 50 new species a year.

The crustacean already is on the state's endangered list, but that affords the creature protection only from direct harm _ not from indirect injury caused by changes to its habitat.

The cave amphipod was once found in six caves in a 10-mile radius of Waterloo, Ill., but is now known to live only in three. Scientists believe it has been eliminated from two caves and are unable to search one more because its entrance has been sealed.

``Long-term survival of the species is doubtful,'' said Lawrence Page, a scientist with the Illinois Natural History Survey who was asked by the government to review its proposal.

That conclusion has been questioned since the Fish and Wildlife Service published its proposal.

``Maybe if you were looking in all the caves you would have found them,'' Catherine Hoffman, a farmer from Waterloo, wrote the agency as part of a letter-writing campaign organized by local Farm Bureau chapters,

Hoffman and others said the government's own proposal indicates that more research is needed.

``There are few data on which to base population, productivity or trend estimates for this species,'' the official proposal said. ``Survey data do not, and probably cannot, show a decline in numbers of this small subterranean invertebrate, but they do demonstrate a decline in its range and number of extant populations.''

Scientists fear the cave amphipod is in danger of extinction in part because its habitat is so small that one flood or other event could wipe it out. But the biggest threat is water pollution _ a problem made worse by the terrain of its home, an area in Monroe and St. Clair counties known as the Sinkhole Plain.

The so-called karst terrain is dotted with depressions where water has eroded limestone formations. These sinkholes provide a path for waterborne pollution _ from agricultural chemicals, livestock facilities, septic systems and dumping _ into the underground water system.

Thomas Aley, who heads the Ozark Underground Laboratory in Protem, Mo., and studies the impact of land use on groundwater quality, believes development of the area into a St. Louis suburb poses a greater threat than agricultural use.

Aley said establishing grassy or wooded buffers around sinkholes would eliminate the most dangerous effects of pesticide runoff. But solving the problem of more intensive land uses like suburban housing will be trickier, he said.

The government would not make plans for reducing contamination in the cave streams unless the cave amphipod goes on the Endangered Species List. Some options being discussed include further regulating agricultural chemical use, limiting access to the caves and restricting construction of septic systems.

Farm groups have asked that any action come with an assurance that use of fertilizers and pesticides will not be banned in the area that feeds into the underground streams.

Cave enthusiasts have weighed in as well. They pleaded with the agency not to close down Illinois Caverns, the cave that provides the broadest access to the public.

At the insistence of the farm groups and Rep. Jerry Costello, D-Ill., the Fish and Wildlife Service extended its original comment period, which ended Sept. 26, to Dec. 8. A final decision on the listing should be made by next summer.

Steven Taylor, a scientist with the Center for Biodiversity at the Illinois Natural History Survey in Champaign who was involved in finding cave amphipod samples, asked the agency to tread as lightly as possible on farmers if it goes forward.

``That agriculture and the amphipod coexist is a good sign, suggesting that the farmers' worries need not be too great,'' he said. ``Only minor changes in land use practices and education could make large differences in water quality.''