Opalescent nudibranch, (Phidiana [Hermissenda] crassicornis)
This common, 1-inch-long nudibranch (pronounced nu-da-brank), or sea slug, is known by several common names, such as opalescent or thick-horned aeolid. Nudibranchs are snails without shells. One of the more beautiful nudibranchs in our area, it is characterized by the orange-tipped fleshy appendages on its back called cerata.
Like the bold yellow color of a yellow jacket wasp, which warns of its sting, this nudibranchs brilliant colors signal that this species, too, can sting. Hidden in the cerata are the stinging cells from its common food, the hydroid. The eaten stinging cells are not digested; instead, they are moved to the tips by way of the brown digestive gland that can be seen extending into the cerata. The head carries several protrusions covered with sense organs.
Along with hydroids, the nudibranch consumes other molluscs, sea pens, eggs and pieces of fish and other scraps. They are aggressive toward members of their own species, with battles often leading to cannibalism.
The life span is short, probably not exceeding four months. But year-round reproduction and a short generation time of 21/2 months keeps this species going. They have both sperm and eggs, but cross-fertilization usually is needed. Upon hatching, the larvae become part of the plankton and drift about until settling.
This species is found from southeast Alaska to southern Baja California, Mexico, where it inhabits eel grass beds, rocky intertidal areas and the undersides of marina floats.
An interesting aside: the name in parentheses in the scientific name of this animal is the old, generic name. It was used until recently, when it was changed by nudibranch specialists.
David W. Jamison is a Boston Harbor resident and marine biologist.