Bivalves (scallops, mussels and clams) are molluscs with shells consisting of two parts (or valves) and there are currently 30,000 species known worldwide. At least 370 of these species are living in the seas off the coast of Britain. Bivalves have a large variety of habitats: burying in soft sediments (mud/sand/gravels) boring into hard sediments (rocks), attaching to rocks by byssus threads or free-living. Because of this they come in a large variety of forms.
The majority of bivalves are filter feeders, pumping large quantities of water through their bodies; this makes them good bioindicators to monitor the health of the surrounding environment. Deep beneath the sea floor there are large reservoirs of oil and natural gas, but it is only relatively recently that methane has been discovered to seep from the surface of the sea bed. These areas are known as 'gas seeps' and certain bivalves have evolved specifically to take advantage of this unique environment.