The Opalescent Nudibranch is found along the west coast of North America, from Kodiak Island, Alaska, south to Baja California, Mexico, and in the NW Pacific (Japan and south Korea) (Sea Slug Forum, 2011
The defensive repertoire of nudibranchs is unmatched in any other taxon of animals. One line of defense in the nudibranch Hermissenda crassicornis, shared by other aeolid nudibranchs, is the presence of ready-to-discharge nematocysts or stinging cells in special sacs at the tips of the dorsal appendages, or cerata. The nematocysts are derived from hydroid and anemone prey that are eaten and digested. The nematocysts pass through the aeolid's gut and are eventually sequestered undischarged in the ceratal tips. If the aeolid is touched or otherwise irritated, the sacs contract and forcibly expel their contents, including the nematocysts. The nematocysts discharge on expulsion. Although not much is known about how the defense works in nature, there is no question that the nematocysts function in the way that they are supposed to, with toxins being released. The cerata are often colorfully demarcated from the rest of the body, with the tips being highlighted. This is thought to be a type of warning or aposematic coloration, likened to the colours of a wasp, toxic butterfly, or poisonous frog. Potential predators learn to avoid the "prey" after a painful or sickening experience.
To learn more about secondarily-acquired nematocysts in aeolids see A SNAIL'S ODYSSEY.
Note Author: Tom Carefoot, Department of Zoology, University of British Columbia