The bivalve shell is usually long, but can reach. Its shape differs depending on the region it inhabits. Like all pen shells, it is relatively fragile to pollution and shell damage. It attaches itself to rocks using a strong byssuscomposed of many silk-like threads which used to be made into cloth. The animal secretes these fibres from its byssus gland; they consist of keratin and other proteins and may be as long as. The inside of the shell is lined with brilliant mother-of-pearl. As with other members of its genus, Pinna nobilis hosts symbiotic shrimp which live inside its shell. It is believed that when it sees a threat, the shrimp warns the host, perhaps by retracting its claws or even by pinching. The clam then closes shut. It has been demonstrated that the shrimp has a similar filter-feeding diet to its host and the relationship is likely mutualistic.
Distribution
This species is endemic to the Mediterranean Sea, where it lives offshore at depths ranging between. It could be found buried beneath soft-sediment areas.
Human relevance
This species is the origin of sea silk, which was made from the byssus of the animal.
Threats
In 2016, there was an outbreak of a disease that caused the mortality of 99% of population in Spain. The cause of the disease is a newly discovered pathogen, Haplosporidium pinnae and it is posing a serious threat to the survival of the species. By 2019, mortality spots have been detected in Greece, Croatia, Turkey, Tunisia, France and Morocco. In the past, Pinna nobilis has been threatened with extinction, due in part to fishing, incidental killing by trawling and anchoring, and the decline in seagrass fields; pollution kills eggs, larvae, and adult mussels. Such threats, however, have been very localised and have not led to such a widespread and rapid population decline. The pathogen, which is still present in the environment, will make recovery a challenge, so continuing declines are expected. The percentage of population size reduction over the last ten years is over 80%. In December 2019, Pinna Nobilis has entered the IUCN Red List as critically endangered. The noble pen shell has been listed as an endangered species in the Mediterranean Sea. The European Council Habitats Directive 92/43/EEC, on conservation of natural habitats and the wildfauna and flora, proclaims that P. nobilis is strictly protected – all forms of deliberate capture or killing of fan mussel specimens are prohibited by law. As part of the Costa Concordiadisaster recovery effort in Italy in 2012, a group of about 200 Pinna nobilis was relocated to a nearby area due to the threat posed by subsequent engineering work.
