Moon Snail (Euspira lewisii)
One of the best-known invertebrates in the sandy intertidal zone is the Moon Snail, both because it is a large (up to 13 cm in diameter) species and because its egg masses are especially notable. This light brown species is close to spherical, with most of the shell taken up by the first whorl. The aperture is large, with a large, horny operculum on the foot that closes the shell.
The foot and mantle have hollow sinuses into which water can be pumped to expand them greatly out and over the shell, forming a plow-like structure with which they push forward while moving through the sand surface. When disturbed, the snail retracts this structure quickly by squirting the contained water forcefully outward.
Moon Snails are voracious predators on the clams that share their habitat. They find a clam, presumably by chemoreception, and envelope it in their big foot and often drag it more deeply into the sand. The radula has seven rows of teeth, with which they dig a hole (easily recognizable as made by this species because countersunk) into the clam shell. A gland on the proboscis secretes enzymes and even hydrochloric acid to help accomplish this.
The snail then rasps and sucks out the clam's tissues over a period of a day or so. They take relatively thin-shelled clams up to 5 cm in length and can eat one every four days in the laboratory. They also eat other snails and, in some circumstances, herring eggs. They are big enough not to have many predators, but Sunflower Stars will attack them, the snail sometimes able to repel one by using its radula to rasp the tube feet of the sea star.
Moon Snails move out into deeper water in winter, then come back toward the shore in summer, when they are breeding. The sexes are separate, and it can be seen from mating pairs that females are slightly larger and thinner-shelled than males. The eggs are laid in characteristic large (to 15 cm diameter) sand collars, sandwiched between two layers of sand cemented together by mucous secretions. The collar, somewhat like a flattened clerical collar with a big opening in the center, contains great numbers of eggs that hatch into veliger larvae within the sand.
As the sand disintegrates over a period of weeks, the larvae are released into the water column. The larvae move into deeper water and feed as herbivores on diatoms and Sea Lettuce for a while, then switch to carnivory as they grow.
Moon Snails were eaten by Native Americans, but as they feed on clams, they may accumulate the poisons that cause paralytic shellfish poisoning, so they should be avoided.