Phyllidia coelestis, a sea slug

© Jurgen Freund/Aurora Photos

Phyllidia coelestis, a sea slug

This sea slug, usually less than 2.5 inches long, crawls along the sea floor of the Pacific and Indian Oceans at depths of nearly 100 feet. The strange color and markings have a purpose: It’s a warning to predators. Aposematic coloring, as this is called, usually indicates that an animal is either poisonous or doesn’t taste good. It’s an easy way to avoid becoming prey. Honestly, we aren’t hungry. We just like that this sea slug’s markings make it look like a rubber monster mask, cast off after a long night of Halloween fun.