High Quality Content by WIKIPEDIA articles! Vargula hilgendorfii, sometimes called the sea-firefly, one of three bioluminescent species known in Japan as umi-hotaru, is a species of ostracod crustacean. It is the only member of genus Vargula to inhabit Japanese waters; all other members of its genus inhabit the Gulf of Mexico, the Caribbean Sea, and waters off the coast of California.V. hilgendorfii was formerly more common, but its numbers have recently fallen significantly.