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Sea snails and shells

Gibbula umbilicalis, a small topshell, that lives on the rocky shores of the European Atlantic coast. Here it crawls on bladderwrack (Fucus vesiculosus).

The sea is the basic home of all molluscs. Like the rest of the group the snails also evolved from the sea and have developed terrestrial life but with time. In English generally all sea-living molluscs with a shell are called shells, some of which are snails, other mussels. On this page it is therefore necessary to speak of snails to distinguish them from the other mollusc groups.

In the sea places rich in food are particularly inhabited by snails. Such places especially are the tidal region (litoral region), coral reefs and similar habitats, as well as the ocean floor. Some snails have adapted specially to life in the tidal region. They have shells with thick walls or a very strong foot, by which they can cling to the rock surface, so that it is barely possible to separate them from it without harming the shell. Some animals have specialized in doing so, for instance the walrus uses its long teeth to break of limpets from the rocks.

The high sea (pelagial region) is also inhabited by snails. Janthina the violet snail, lives on the lower side of a foam raft it produces itself and waits for nutritious things to come by. Janthina by the way is a predator - it can also feed on the Portuguese manowar, a large jellyfish the sting of which is extremely painful for humans, but cannot help the jellyfish against the snail.

Among opisthobranch snails there are many sea slug groups, the members of which are also capable of free swimming. Often they have developed fin-like structures by means of which the can move like a ray with undulating movements.

Sea slugs that have completely lost their mantle and therefore alsothe mantle cavity, are the nudibranchs. The breathe by dorsal prolongings the bright colours f which make the nudibranchs a likely photo object to many divers. Those prolongings however not only serve as breathing organ but also for defense. Many nudibranchs feed on jellyfish and not only are not harmed by their nettle capsules (that sting humans so painfully), but even aquire these cells for their own use. The nettle capsules are covered in mucus, so they cannot sting and then are stored inside the dorsal prolongings of the sea slug. An animal trying to attack such an armed nudibranch will get the nettle capsules and leave the nudibranch alone.

Especially in the coral reefs, where there are many snail-eating fish and other animals, there are also many cone shell species, that defend themselves with venomous darts, that basically had been developed as a hunting weapon.

Even in the sea snails usually move by their foot. Some snails such as the queen conch (Strombus gigas) use their lid (Operculum), that all prosobranch snail have on the tailtip of their foot. They dig the lid's end into the ocean floor and then jerk themselves forward. That way they jump over the sea floor. The conch in some languages (i.e. in German) are also called fencer snails. That is because the conch also uses the sharp lid for its defense. When attacked, it sways its tailtip around and tries to hit the enemy with the sharp edge of the lid.

Sea snails usually perform an internal fertilization and thus are obliged to mate. The male transfers a spermatophore to the female, that contains sperm cells. The female afterwards lays a number of eggs. Because of the internal fertilization those eggs can be kept in egg capsules to protect them. Well known are the egg capsules of the whelk, a sea snail living near the coast of European shores. Most sea snails have separate sexes, though among the opisthobranch snails there are also many hermaphroditic species.

Sea snails' shells because of their hardness and unbreakability are often used by other animals as a second home. For instance the hermit crab uses shells of periwinkles in early age, as well as of whelks in older age.