"Snails without Shells"
Many of us are familiar with snails that have a shell--the ordinary garden snail, moon snails, the black Turban snails, tiny and abundant periwinkles high in the intertidal zone, the pretty tropical black or pink Murex, and wavy-top or turbinid shell. Many of us have also seen, either at underwater film festivals or while diving, sea slugs and/or nudibranchs. These latter mollusks, or soft-bodied animals, are essentially snails that through evolution have either greatly reduced the size of the shell or eliminated the shell entirely in the adult stage of the life cycle.
Although the nudibranchs are most spectacular in their vivid colors, there are a couple of other nearly shell-less snails which are common and readily visible to divers. One is the California Sea Hare, Aplysia californica, which is often misplaced in the genus Tethys. It is a large sea slug, often reaching lengths of nearly 20 inches; the body is brown, or mottled brown, or sometimes almost jet black. It discharges a purplish ink when disturbed. The sea hare is lacking an external shell, but it does have a thin, internal remnant of a shell. The possession of long, ear-like tentacles, which are used to detect odors and chemicals in the water, gives the common name to this sluggish, common mollusk.
The other sea slug commonly seen by many of us sea lovers is the so-called striped sea slug, Navanax inermis. This soft animal is brown, with paler brown bars and bright yellow and iridescent blue spots along the sides of the body. It grows up to 12 .5inches long and is considerably sleeker and less bloated in appearance than the Sea Hare. Unlike the Sea Hare, Navanax is found in the quieter waters of the tidepools and especially abundant in the eel-grass covered mudflats of shallow bays and channels. The interesting feature of this slug is that it is carnivorous on other sea slugs (in fact, it even eats other individuals of its own species, being cannibalistic)! Preliminary work done at Scripps Institution of Oceanography indicates that each Navanax individual secretes a substance and deposits it along its trail on the sand or mud that repels other Navanax individuals. The evolutionary significance of this chemical repellent is not quite clearly worked out yet, but undoubtedly this permits the effective utilization of prey within a given area without the competition or danger afforded by having another hunter in the same area.
Most underwater photographers in southern California rapidly focus on the slow-moving, often graceful and elegant, distinctively-colored nudibranchs (pronounced nude-ee-brank) as subjects for a slide
There are two general groups of nudibranchs: the aeolid group and the dorid animals. These can be differentiated by the pattern of distribution of the cerata. The aeolid ones have these "gills" scattered as a fringe along the sides of the animal, or scattered in several groups along the back, or spread evenly throughout the back; the dorid type has these structures gathered together in a tree-like cluster on the posterior or rear part of the back. There are distinctive family groupings within each type of nudibranch, but for our purposes of identification, the categories of nudibranch--aeolid or dorid type--genus and species or common name should usually suffice.
Nudibranchs are most often found in the quiet waters of tidepools, or at deeper depths. A favorite observation trick for the hardy nudibranch-chaser is that of getting up at 4 a.m. and hitting the tidepools at a minus tide before dawn, especially in the spring months. Of course, many of us divers have taken the easy way out and get up at 9 a.m., go on a dive to Scripps Canyon in the middle of the day, and find these gorgeous creatures in abundance along the vertical walls. These often brightly-colored animals are found creeping along the various attached algal plants on the bottom, feeding on the hydroids (anemone-like creatures) on the blades of the plants, or on sponges encrusting rocky areas. Occasionally, one sees a nudibranch in the water column, swimming by violent convulsive and jerking motions; this is most commonly observed with the abundant purple and orange nudibranch, Flabellina iodinea, commonly called the Spanish Dancer because of this behavior. All nudibranchs are apparently carnivorous, mostly feeding on the hydroids and sponges already mentioned. Thus, in collections made for his scientific categorizations of nudibranchs at Scripps, James Lance takes special care to collect the hydroids or sponges in the immediate area that the nudibranch was found. Jim is the acknowledged world-authority on nudibranchs, especially with the Californian forms. He has an extensive slide collection of over 105 species, and can answer virtually any question one might pose on nudibranchs. He can be contacted at Scripps Institution of Oceanography; undoubtedly, he will invite you along on one of his insane early (pre-dawn) trips to the local tidepools.
The aeolid nudibranchs have been detected with stinging cells in the tips of the cerata. Experiments have demonstrated that these stinging cells are derived from the hydroid food animals as they are present when these hydroids are part of the diet and absent when the hydroids are withheld from the diet. It is obvious that the feeding of these nudibranchs somehow keeps the stinging cells of the hydroids from discharging and these cells pass through the digestive system and migrate out to the ends of the gill-like extensions along the back of the animal. These stinging cells then form a protective mechanism for the nudibranch. What a marvelous evolutionary maneuver to provide protection and food at the same time for these otherwise defenseless creatures!
The conspicuous colors of many of the species actually allow the animals to harmonize well with the colorful background of sea anemones, hydroids, gorgonians, sponges, and plants. Further, although work in this aspect is not definitive, it is thought that many of the species of nudibranchs are distasteful or exude an offensive odor which provides the protection from being eaten by fishes and other predators. Along this line, there is an old published record of a Professor Herdman who experimented with various species and found that several of the larger species were left alone by fishes. He thus decided to test their palatability and ate a live specimen of a conspicuous species, Ancula cristata; he reported that "...the taste was pleasant, distinctly like that of an oyster..." Interviews with hundreds of divers and other seafood and seashore lovers offer no other opinion--no one else seems to have tasted a nudibranch!
Lest you fear that this article is going to end without identifying some of the more common and conspicuous nudibranchs other than those already discussed, be assured that your fear has proven to be real. The verbal and written description of these colorful and camera-ready models of sea animals cannot do justice to them; rather, should you have a particular species you need identified, your best bet is to contact Jim Lance at Scripps or to find a color plate of the animal in Behrend's 1980 paperback called Pacific Coast Nudibranchs. Of course, you can always fall back on the least desirable method--i.e., by contacting me. I would have to go through books or call Jim myself, and I believe that you can gain precious time and mountains more information by working directly with those contacts yourself.
Wouldn't it be exciting some evening, especially those wintry days in San Diego when the heavens pour out their leaky aquariums, to sit around a nice fire and watch slides of 40-50 different species of local nudibranchs? Try the Underwater Photographic Society (272-1120 for more information), and you might be able to entice an artist like Fred Fischer or Dave Slidders to come and present such a show. Perhaps a nice lobster dinner or tender steak as an inducement would insure their presence. Or, better yet, how about your own slide show, presenting it to your favorite person(s) and inviting me for the lobster dinner!!!.
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