Molluscs

The molluscs are a large and diverse group of animals characterised by a muscular foot, a head and one or more shells. They include the octopi and squid, cockles and mussels, winkles, limpets and abalone. The ‘lower molluscs’ such as cockles and mussels are sedentary and feed by filtering seawater over their gills to extract food, but the ‘higher molluscs’ such as octopi and squid have highly developed nervous systems, complicated eyes and tentacles and are sophisticated hunters.

• The Common Limpet (Patella vulgata) can best be described as a tiny cow with one large foot, living under a crash helmet. Limpets are a common sight on rocky shores where their cone-shaped shells can be seen at most levels of the tide. The single muscular foot acts as a sucker to secure the animal to the rock. The shell protects it against drying out and against predators such as birds and crabs, while the mouth contains a tough rasping tongue called a ‘radula’ which scrapes seaweeds and tiny encrusting animals off the rocks for the limpet to eat.

• Edible Periwinkles (Littorina littorea) are even more widespread on the shore than limpets and extend down into deeper water. They have dark brown coiled shells and can be found under seaweed, as well as in rock pools and crevices. The mouth of the shell can be closed by a horny plate called an ‘operculum’ which means that, even if the Periwinkle is knocked from its rock, it can still protect itself from drying out or even from attack, until it rights itself again.

• ‘Coat-of-Mail Shells or ‘Chitons’ (Lepidochitona cinerea) resemble small limpets with a segmented shell made up of interlocking plates, like that of an armadillo. Like armadillos, they can roll into a ball to protect themselves if they happen to be knocked off their rock. They are found around low water mark on rocky shores and feed on seaweeds, which they rasp off the rock.

• Dogwhelks (Nucella lapillus) feed by boring a tiny hole in another shellfish, such as a mussel or limpet, and digesting that unfortunate prey by squirting their own digestive juices into it. Their shells are usually white, but may have brown bands. They are found below the middle shore on rocky beaches and lay eggs in tiny cases that resemble grains of rice.

• The Common or Blue Mussel (Mytilus edulis) is a two-shelled (bivalve) mollusc that lives on hard substrates such as rock, on the legs of piers and the moorings of boats or buoys. It attaches itself to its substrate by opening its shell, putting out a muscular food and placing a dab of natural “superglue” down, which it draws back before it hardens to form a tough byssus thread. Mussels are filter feeders and need to be covered with water to breathe, feed or release their eggs and sperm. For this reason they are found on the middle and lower shore of rocky beaches, but are also farmed intensively on ropes along with West of Ireland, or in large commercial mussel beds along the East Coast.

• Cockles (Cerastoderma edule) are burrowing bivalve molluscs that live in sand or mud from the middle to the subtidal shore. They live close enough to the surface of the sand or mud to push their siphons out into the water so that they can breathe and filter out food. Cockles form valuable fisheries around the coast of Ireland , particularly on the East Coast.

• The European or Edible Oyster (Ostrea edulis) is cultivated around the coast of Ireland in large oyster beds in sheltered coves and inlets. The most famous of these oyster beds are at Tralee, Clarinbridge, Cork Harbour and Clew Bay , although smaller oyster farms occur all around the country and cultivate not only this species, but also the faster growing Pacific Oyster (Crassostrea gigas) in plastic net bags and trays.

• The Great Scallop (Pectan maximus) is a large bivalve mollusc that lives below the subtidal zone on Irish beaches and can grow up to 15 cm in diameter. Its two shells can be opened and closed rapidly like a pair of false teeth, ejecting jets of water and allowing the Scallop to escape any dangerous predators by ‘flying’ up off the bottom. Scallops also form valuable fisheries around the Irish coast.