The rare giant mussel, Pinna nobilis, is found only in the Mediterranean Sea. It is one of the largest bivalves in the world, growing to 120 cm long. The mussels can live for as long as 20 years. It sticks up out of the sea bed so is easily seen when diving, once you know what to look for.
Pinna nobilis has been assessed by the European Union as being in need of special protection (EC Habitats Directive). This means that it is illegal to kill or disturb the species. However, studies have found that in Greece many individuals are killed by fishing. The mussels were poached exclusively by free-divers and fishing mortality was practically zero at depths below 9 m, where the visibility was very bad. Because of this large individuals were restricted to deeper areas.
In another study, this time off Italy, divers found that the giant mussels were managing to hold their own, in spite of all the difficulties of a degraded and heavily polluted environment and the damages of illegal fishing.
Depending on area, the mussels live between about depths of 3 and 60 m. In the Italian study divers didn’t find any below 16 m. In the Adriatic Sea, they are found down to 30 m. They live singly, not in large groups like the mussels we are used to. At best the Italian study found only one every 1.4 hectares (3.5 acres).
The giant bivalve lives on soft bottoms: sand, seagrass meadows and mud. If you find what looks like a small specimen growing on a rock it is probably not P. nobilis but the more common P. rudis.