A rare shrimplike creature found in Rock Creek Park — believed to be its only location in the world — could end up taking center stage in a fight against a proposed light-rail Purple Line in the Maryland suburbs.

The tiny Hay’s Spring amphipod has been listed as a federally protected endangered species since 1982 and has been spotted in Rock Creek Park in the District, downstream of where Purple Line trains would cross the park in Montgomery County.

The recently released final study of a Purple Line’s environmental impact, some activists say, erred in omitting the colorless crustacean as an endangered species that must be avoided or protected.

John M. Fitzgerald, a Chevy Chase resident and lawyer, said he and other environmentalists are contemplating a lawsuit to require the Maryland Transit Administration to consider a light-rail line’s impact on the animal. The little-known crustacean is significant because it signals good water quality, he said.

“When these creatures begin to disappear, that means the clean springs that feed Rock Creek are going,” Fitzgerald said. “When they’re gone, it means we’re losing the best sources of the purest water we’ve got.”

Map of the proposed Purple Line

Maryland transit officials said neither the Maryland Department of Natural Resources nor the U.S. Fish and Wildlife Service mentioned any endangered species along the Purple Line’s proposed 16-mile route between Montgomery and Prince George’s counties.

In a February 2010 letter, the Department of Natural Resources said there were “no state or federal records for rare, threatened or endangered species” within the Purple Line project boundaries.

The letter added: “This statement should not be interpreted, however, as meaning that rare, threatened or endangered species are not in fact present. If appropriate habitat is available, certain species could be present without documentation because adequate surveys have not been conducted.”

Jonathan McKnight, a biologist for the Department of Natural Resources, said the agency did not find any Hay’s Spring amphipods in a late-1990s search of 35 springs in lower Montgomery’s Rock Creek watershed. The study did turn up Kenk’s amphipods, another tiny crustacean listed as a “candidate” for federal endangered-species listing, but that spring would be too far from the Purple Line alignment to be harmed, he said.

“We didn’t find anything in that area that would be affected by the project,” McKnight said. “We’re confident this was a good reconnaissance.”

David Culver, an American University environmental science professor, said he knows little about a Purple Line’s proposed route, and he is not involved in the fight to stop it. He said he has never heard of a Hay’s Spring amphipod sighting in Montgomery, but he said it is “extremely likely” the creatures are there.

Culver said he found the crustaceans in five to six places in the District’s part of Rock Creek Park about 10 years ago, when he did a study for the National Park Service. Because the park is a continuous habitat, Culver said, “If you look hard, you’d probably find them. . . . It’s not like there’s a wall between D.C. and Montgomery County.”

Rock Creek Park is thought to be the only place where the creature has been spotted, he said. The crustacean, which is about one-quarter to a half-inch long, can be difficult to see, he said. It is usually found on the backside of leaves or burrowed a few inches under the muck in “seeps” where groundwater pools.

“They should look for them,” Culver said. “There’s no reason not to look. The only reason not to look is if you don’t want to find anything.”

Purple Line trains would cross Rock Creek Park on a new bridge that would replace a trestle east of Jones Mill Road. Trains would follow the four-mile, wooded Georgetown Branch Trail that would be rebuilt between Bethesda and Silver Spring.

Henry Kay, the state’s executive director for transit development, said a Purple Line would have relatively little effect on streams, wildlife and other natural resources because trains would run mostly along local roads in heavily developed areas.

The portion with the most impact, he said, would be the several miles along the trail, where much of the 48 acres of trees affected by the project would be cut. Montgomery converted that land into an interim cycling and running trail after it bought it in the late 1980s to preserve it for a trolley line.

“They could be somewhere in Rock Creek Park,” Kay said of the Hay’s Spring amphipod. “But we count on the agencies that are legal stewards of them to give us the information.”

If an endangered species were found in the Purple Line’s impact area, Kay said, it would require “pretty significant mitigation” to build the transitway.

A Purple Line would have 21 stations between Bethesda and New Carrollton. Trains would connect Maryland’s spokes of the Metrorail system with MARC commuter and Amtrak rail stations. Transit planners say it would provide faster east-west public transit than buses and focus new development around stations in older inner-Beltway communities.

Kay said the state expects the Federal Transit Administration to approve the final environmental impact statement in December. Lawsuits could be filed within the following 150 days, he said.

If highly competitive federal funding is secured to help build the $2.2 billion line, construction is scheduled to begin in mid-2015, and the line would open in 2020.

Purple Line advocates say the localized impact of a transitway’s construction pales in comparison with its overall environmental benefits — providing cleaner air by reducing the need to drive and limiting sprawl by focusing growth around transit stations.

Wayne Phyillaier, a longtime Purple Line advocate, pointed to the Sierra Club recently naming the Purple Line one of the country’s top 25 transportation projects.

Phyillaier said the bridge carrying trains over Rock Creek would cause little disturbance to the stream valley below. But he said he is concerned Purple Line opponents will use an endangered species lawsuit to delay the project.

“I find it hard to believe,” Phyillaier said, “that opponents are suddenly concerned about this very obscure species.”