The Opalescent Nudibranch ("noo-di-brank") is a beautiful mollusk without a shell. Related to snails and other animals with shells, nudibranchs are also kin to the slugs in your garden. That's why the many species of nudibranchs are sometimes called "sea slugs." On the west coast the most common of the larger ones is the Opalescent Nudibranch. Usually no more than an inch and a half long, this creature can compress to half that or stretch to double its length.

Many nudibranchs are very colorful. The Opalescent Nudibranch varies, but its body is usually yellowish-green with orange lines on top and electric blue stripes running down its sides. Many fringe-like projections cover the creature's back, each one tipped with white above an orange band. These fringes, called "cerata," take in oxygen and also contain part of the animal's digestive system.

Though they look delicate, Opalescent Nudibranchs are aggressive and will eat anything small enough for them to take on, including others of their own species. When this creature attacks an anemone (another kind of marine animal), it bites some of the tentacles that make anemones look like flowers. These tentacles have stinging cells to ward off attackers, but the nudibranch seems unaffected by them. In fact, it swallows the stinging cells which then travel through its body to end up at the tips of the nudibranch's cerata. There they become part of its own defense. It may be that the nudibranch's bright colors warn predators like fish away from the painful effects of those stingers.

When you're at the beach, look carefully into tidepools, among eelgrass blades, or even on mudflats, and you may spot this beautiful mollusk without a shell.