A trail advocacy group and two Montgomery County residents filed a federal lawsuit Tuesday alleging that building a light-rail Purple Line would harm two rare shrimp-like creatures, including one that is a federally protected endangered species.

The legal complaint — filed by Friends of the Capital Crescent Trail and Chevy Chase residents John M. Fitzgerald and Christine Real de Azua against four federal agencies — is aimed at finding an alternative route to part of the 16-mile rail line’s proposed alignment. The western four-mile section between Bethesda and Silver Spring would run along what is now a popular wooded jogging and cycling trail adjacent to neighborhoods, including the Town of Chevy Chase.

Sparing the trail, the plaintiffs said, would protect the Rock Creek watershed, including habitat for the Hay’s spring amphipod, which has been listed as endangered since 1982, and the Kenk’s amphipod, which is a candidate for federal listing.

Federal officials have said that the Purple Line’s construction would have no impact on either animal, saying their habitat is too far away or would be unaffected by the watershed’s ground water.

Discussion of a legal challenge has drawn some criticism in the environmental community. Many activists say transit projects’ overall environmental benefits — limiting growth in traffic and sprawl development — typically far outweigh any local impact of their construction.

The Maryland Transit Administration put the $2.37 billion project out to bid in July. State officials have said they hope to begin construction in 2015 and open the line to service in 2020.