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Snails, Mussels and their Relatives
Almost all of the shells that you find on Irish beaches belong to one of the most diverse groups of animals on the planet: the molluscs. On Irish shorelines these include the speedy octopus and squid (cephalopods), the slow-moving sea snails (gastropods) and chitons (polyplacophorans), and the sedentary blue mussel and oyster (bivalves). Many of Ireland’s rocky shores are exposed to the wind and waves, which actually suits these hardy animals.
Despite their diversity, all molluscs share a number of defining features: the body has a head and a foot, and is at least partly covered by the mantle, which is like a thick cloak of skin. There is a space between the mantle and the rest of the body called the mantle cavity. In most marine molluscs, gills pump water in and out of this cavity to absorb the oxygen from it. Some molluscs are filterfeeders.In these species, the gills also pull in food. Most molluscs secrete a shell. This shell has three layers: a horny outer layer, a chalky middle layer and a pearly inner layer.
Molluscs have been historically important as a source of food for coastal communities. Irish people still have a taste for mussels and scallops, but we tend to shun the snails, now exporting them to other countries.
Snails, slugs and limpets
Sea slugs and snails are all gastropods, which means ‘belly-footed’ and refers to the large muscular foot on which the animals glide. Gastropod shells range from the dome shape of the common limpet to the pointed spirals of the periwinkles and whelks. Shells provide protection from predators, heat and drying out during low tide. Because sea snails don’t have external shells, it is uncommon to find them out of the water.
Gastropod shells have openings that require added protection. For this reason, limpets use their muscular foot to cling tightly to the rocks. Many of the snails have a hard disc called an operculum that they can shut like a trapdoor to cover the opening of their shells. You may also find periwinkles or whelks congregating in groups in damp, hidden areas of the shore, such as in crevices, under ledges and in seagrass beds. Despite all this protection, gastropods are eaten by birds, crabs, mammals and other molluscs.
Gastropods have a sharp, rasping tongue called a radula.Some, like the edible periwinkle and limpet, graze from the surface of the rocks using a scraping radula. Others, like the dog whelk, use their radula to drill through the shells of their prey.
Bivalves – open and shut
Bivalves, such as the blue mussel, have two shells that are attached at one point and can be opened or shut. They are all filter feeders: they strain food particles from the water as it flows across them. Many bivalves also have two siphons: one to carry water into the gills and one to release the water. Some species, including the tellin, have very long siphons which they use like vacuum cleaners to suck debris directly from the seabed.
Bivalves cannot glide like gastropods, but many burrow into the sand, and some, such as scallops, can even swim. Unlike the burrowing bivalves, mussels attach themselves to rocks using thick hairs called byssus threads. These are the ‘beards’ that we wash off mussels before eating them.
Other Molluscs
Surprisingly, chitons and cephalopods are also members of this family. Chitons are also known as coat-of-mail shells and can be found hugging tightly to the underside of rocks on the shore. Cephalopods include octopus, squid and cuttlefish. While we have these in Ireland, you do not often see them from the shore. The cephalopod you are most likely to encounter is the internal shell of cuttlefish (see page 49).
The delicate shell of a juvenile dog whelk.
Dog whelks lay yellow egg capsules in crevices and under stones.
Dog Whelk
Irish Cuachma chon
Latin Nucella lapillus
Size <4cm
Di...
