Noble Pen Shell - "Pinna Nobilis"

During one of the scuba diving we met the Pinna nobilis, common name the noble pen shell or fan mussel, is a large species of Mediterranean clam, a  marine bivalve mollusc in the family Pinnidae, the pen shells. It reaches up to 100 cm of shell length. The bivalve shell is usually 30–50 cm long, but can reach 100 cm. Its shape differs depending on the region it inhabits. It attaches itself to rocks using a strong byssus.  These keratin fibres that the animal secretes by its byssus gland can be up to 6 cm long. The inside of the shell is lined with brilliant mother-of-pearl. This species is endemic to the Mediterranean Sea, where it lives offshore at depths ranging between 0.5 and 40 m. It could be found buried beneath soft-sediment areas. nacchera pinna nobilis

Like all pen shells, it is relatively fragile to pollution and shell damage. It attaches itself to rocks using a strong byssus composed 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 6 cm. As with other members of its genus, Pinna nobilis hosts symbiotic crustaceans which live inside its shell; in this case it is the shrimp Pontonia pinnophylax and the pea crab Nepinnotheres pinnotheres. 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.

By 2019, mortality spots had been detected in Greece, Croatia, Turkey, Tunisia, France and Morocco. In the Trieste area, considerable efforts have also been made to conserve the deposits since 2020. In the past, Pinna nobilis faced 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.

(extract from Wikipedia)

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