Naked Molluscs of the Sea
A. Hyatt Verrill
As their name implies, the naked molluscs include those genera of mollusca in which the shell is absent or rudimentary.
The group includes the Nudibranchs, naked Tectibranchs, and Cephalopods, although the latter are not properly included in the group. The form, colors, habits, etc., of the various genera are very variable, and even different species of the same genus would hardly be recognized as related. The northern waters are not very prolific in species, but the various genera are well represented. On the New England coast Doris, Aplysia, Aeolis, Elysia, as well as the squids (Loligo), and in deeper water the Octopus occur.
The Doris is readily distinguishable by its ovate body, branched or plumose gills and single pair of tentacles. Aplysia includes those curious creatures the "Sea Cats" or Sea Hares, and are distinguished by the broad flaps or "fins" on the back, two pairs of tentacles, rudimentary shell and partially concealed gills, as well as by their habit of emitting a colored fluid or "ink," similar to that of the Cephalopods. Aeolis is a pretty genus with body covered with papillae, usually of considerable length. In tropical and neotropical waters these creatures are very abundant and the colors,—sometimes protective, sometimes warning,—are often striking and brilliant.
The Aplysias are the largest of the true naked molluscs and often attain a length of 12 or 14 inches. They are slow-moving, somewhat sluggish, creatures, crawling deliberately about on the algae and browsing, much in the manner of ruminants; in fact the similarity is so pronounced that "sea-cow" would be far more appropriate as a name. They swim readily and gracefully by their large body-flaps and eject large quantities of purplish or reddish "ink" when disturbed. They are usually greenish or grayish in color, marked with darker shades, but some species are entirely brown or black. The eggs are laid in long strings attached to algae or stones and usually more or less coiled. They are considered poisonous by ignorant people, and as far as known no other animals will eat them. They are very abundant in the Bermudas, and frequently several hundred individuals may be seen at one time. The species of the Doris group,—belonging to several genera,—are small, seldom reaching a greater size than three inches in length and are usually brightly colored. Purple, green, orange, blue, pink and in fact nearly every conceivable tint and shade is worn by these pretty creatures. The color, however, is generally protective.
Chromodoris roseopicta is gray, marked with white, black, and coral-red and exactly matches the small red sponges and gray bathing sponges on which it lives. Chromodoris zebra is brilliant blue and golden-orange, arranged in longitudinal stripes, but when beneath the surface of the exceedingly blue Bermuda water is hardly noticeable. The eggs of the Doris family are laid in broad gelatinous ribbons coiled and attached to dead corals, sponges or stones.
The Aeolis group are usually semi-transparent, bluish or yellowish in tint, with papilla tipped and banded with red, blue or orange. They live on algae and are small in size. They are delicate and beautiful things but very difficult to preserve in good shape.
Pleurobranchus and Pleurobranchopsis are oval, rounded forms, the former possessing a concealed shell and free gill at side; the latter having gill attached along its entire length and lacking a shell. Pleurobranchopsis, as far as known, contains but one species, P. aurantiaca, of Bermuda,—a bright, orange-colored species, two or three inches long. It lives under stones and among corals and is exactly the same color as numerous sponges associated with it.
To my mind, however, the most interesting individuals of the entire group of naked molluscs are the Elysias,—small species, in general form resembling the Aplysias, but differing in having no external gill, one pair of tentacles, no siphon-tube and no shell. They are lively, often brightly colored animals and usually gregarious, especially during the breeding season.
Elysia ornata,—which until the present year was only known from a poor figure, —was found by the writer among green algae at Bermuda. When first discovered only two specimens were found, and were considered a great prize. A few days later I again visited the locality and was agreeably surprised to find them so exceedingly abundant that over 400 were collected in less than half an hour. At this time they were engaged in depositing their eggs,—a very interesting process,— on the algae and stones just below low water mark. The eggs were first laid in a long, slender string, attached by one end to a suitable support. The owner then passed slowly along, guiding the string of eggs with his mouth and foot, coiling it neatly and fastening it securely down as he did so. The color of this species is olive-green, speckled with black and white, with the edge of mantle and tips of rhinophores broadly ornamented with a band of orange and black. Three days after they were observed breeding they had become scarce, and a few weeks later had entirely disappeared.
Two specimens of another (undescribed) species were obtained about the same time, and these, for brilliancy of coloration, eclipsed all other species I have ever seen. The body was bright wine-color, spotted with creamy-yellow and pale-green, while mantle and tentacles were decorated with bands of azure-blue, vermillion, yellow and green. Despite this gaudy coloring, the little fellows were scarcely visible among the mass of algae, sponges, hydroids, etc., on the under side of the stone where found. Many naked molluscs are specialized for particular habits and localities. Syllia, for example,—which lives on the sargassum of the Gulf Stream,—matches in its mottled orange, olive and brown the floating gulf weed, and to still further-simulate its surroundings, possesses long, irregular shaped projections or filaments on the back.
Dolabella, on the other hand, although closely related to Aplysia, has the "fins" rudimentary, folded close to the back, and like the rest of the body, covered with branching, tree-shaped appendages or papillae. These and the dull brown or olive-green color, render it almost invisible upon the slimy, algae-coated rocks and reefs which are its habitat.
From these examples it will be seen how far the shell-less molluscs of the sea surpass in variety and beauty those of the land, which are known as slugs.