Opalescent Nudibranch     Phidiana (Hermissenda) crassicornis



This common, one 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 shell-less snails.  One of the more beautiful nudibranchs in our area, it is characterized by the orange-tipped fleshy appendages on it's back called cerata.  Like the bold yellow color of a yellow jacket wasp which warns of its sting, this nudibranch's brilliant colors signal that this species too can sting.  Hidden in the cerata are the stinging cells from it's common food - the hydroid.  The eaten stinging cells are not digested, but instead moved to the tips by way of the brown digestive gland which can be seen extending into the cerata.  The head carries several protrusions covered with sense organs.


Along with hydroids, the nudibranch will also consume other molluscs, sea pens, eggs and pieces of fish and other scraps.  They are quite aggressive toward members of its own species with battles often leading to cannibalism.


The life span is very short, probably not exceeding 4 months.  But with year round reproduction and a short generation time of 2.5 months keeps this species going.  They are hermaphrodites with an individual having both sperm and eggs, but cross fertilization is usually needed.  Upon hatching the larvae become members of the plankton and drift about until settling.


This species is found from southeast Alaska to southern Baja California, Mexico where it inhabits eelgrass beds, rocky intertidal areas and under marina floats.


An interesting aside: the name in parentheses in the scientific name of this animal is the old generic name which was used up until very recently when it was changed by nudibranch specialists.