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Opalescent nudibranch

Opalescent nudibranch

On Exhibit

Animal Facts

  • Scientific Name

    Hermissenda crassicornis

  • Animal Type


  • Diet

    mainly hydroids; also small sea anemones, bryozoans

  • Size

    to 3 inches (80 mm) long

  • Relatives

    snails, chitons, oysters, mussels; Phylum: Mollusca; Order: Nudibranchia

  • Habitat

    Rocky Shores

  • Range

    Alaska to Baja California from intertidal to 120 feet (37 m)

Natural History

Opalescent nudibranchs are one of the prettiest and most colorful species of nudibranchs. Though their colors vary, they always have bright orange areas on their backs and blue lines along each side. Cerrata (fingerlike projections) on their backs are brownish yellow, with white and gold tips.

These "sea slugs" eat hydroids and anemones, which are armed with nematocysts (stinging capsules). These nematocysts don't harm the nudibranch; in fact, the animal transfers some of its prey's unfired nematocysts to the tips of its own cerrata, where they become part of the nudibranch's defense system. Some experts believe that nudibranchs' gaudy colors warn predators of these potent weapons.


Nudibranchs are often found on rocky shores, where pollution can be heaviest. You can help keep ocean waters clean by properly disposing of motor oils, paints, paint solvents and other harmful materials.

Cool Facts

Opalescent nudibranchs are aggressive fighters. When two of them meet head-to-head, they're likely to lunge into a biting battle. If one meets the tail of another and gets the first bite, it usually wins the battle and consumes the loser. 

Because opalescent nudibranchs live less than one year, they have to grow and reproduce quickly—they can't lose time looking for a mate. A meeting between two or more can be a mutual mating session, since these creatures are hermaphroditic (they have both male and female sexual organs). Later, each lays an egg string in narrow coils that looks like tiny sausage links. They attach their eggs to eelgrass and algae.

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