Activists were expected to rally Feb. 3 in San Francisco to protest the construction of a U.S. military base in Japan that they say threatens the Okinawa dugong, an endangered marine mammal.

The protest was expected to take place outside the Ninth Circuit Court of Appeals, where a hearing is set for a lawsuit challenging the air base development in Okinawa.

The Turtle Island Restoration Network and The Center for Biological Diversity are suing the Department of Defense, maintaining that the base’s construction will pave over and fill in hundreds of acres of coral and seagrass habitat crucial to the Okinawa dugong.

Scientists estimate there are fewer than 50 Okinawa dugong left, said Stepph Sharpe, development manager at Turtle Island Restoration Network.

Dugongs, related to manatees, are long-lived and can grow to nearly 1,000 pounds.