The opalescent nudibranch, Hermissenda crassicornis, is commonly found in the rocky shores of California. It eats hydroids and anemones, which have nematocysts or stinging cells. Instead of being digested, some of these nematocysts are transferred to the tips of the nudibranch’s cerrata (the orange fingerlike projections on its back), where they become part of the nudibranch’s defense system.
Opalescent nudibranchs are aggressive fighters. When two of them meet, they’ll lunge at each other and try to bite the other’s tail! The one who gets the first bite usually wins and eats the loser.
They’re only aggressive toward members of their own species, though. In the lab, I’m keeping one of these guys in the same tank as my sea hares, and a fight hasn’t broken out yet… But I’ll keep you posted.