This bivalve mollusc measures around 5-9 cm and filter-feeds on plankton and organic micro-particles. It is very common throughout the Mediterranean, but since it is extensively farmed, it is even more widely distributed; indeed it can be said to have colonized a large proportion of the coastline of the entire planet: it can be found along the Atlantic coast of the Iberian peninsula and in South Africa, China, Australia and New Zealand, as well as the west coast of the United States. In its natural state, it forms large, extremely dense colonies along the foreshore (the area of shoreline that is exposed at low tide). It is able to withstand large variations in salinity as well as rather prolonged periods out of the water, the latter thanks to the water the animal can store inside its hermetically sealed shell. Mussels are composed of two symmetric valves forming an elongated shell that is pointed at one end and splayed at the other and can be blue, black or brown. It emits a special filament that becomes extremely tough on contact with water, with which the animal anchors itself to rocks or any other kind of solid object. The sexes are separate and easily distinguishable when the shell is opened: males are yellowish-ochre in colour and females red. Fertilization is external, in other words sperm and eggs are discharged directly into the water. This species is farmed commercially on a large scale.