Scientists have discovered a new species of tiny pea crab within a large date mussel collected in the Solomon Islands.

The crab has been named Serenotheres janus after Janus, the Roman two-faced god because of a large plate that covers its upper carapace, giving it the illusion of being two-faced.

S. janus is the second known species within the pea crab genus Serenotheres, members of which parasitize rock-boring mussels of the subfamily Lithophaginae, the researchers write.

Scientists have discovered a new species of tiny pea crab within a large date mussel collected in the Solomon Islands.

Pea crabs — about the size of peas — are parasitic, living inside oysters, mussels, clams and other shellfish. They feed off the food filtered by the hosts, and rely on them for safety.

The newly described pea crab, too, is a parasite, and lives inside the golden date mussel (Leiosolenus obesus), researchers report in a study published last month in the journal ZooKeys.

Discoverers Peter Ng of the National University of Singapore, and Christopher Meyer of the U.S. National Museum of Natural History, Smithsonian Institution, have named the new species Serenotheres janus after Janus, the Roman two-faced god because the crab has a large, overhanging plate covering its upper carapace, giving it an illusion of being two-faced.

The new crab is the second known species within the pea crab genus Serenotheres that parasitize rock-boring mussels of the subfamily Lithophaginae, the researchers write. The genus was previously represented only by S. besutensis, which was reported from an unidentified species of mussel collected in live coral from an island off Peninsular Malaysia. The new crab species S. janus has a broader carapace than its relative and is cream-yellow in color.

Both crabs have an additional large plate covering the upper side of their shell. But the purpose of the plate is still unknown, researchers say.

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