Noble pen shell

The bivalve shell is usually 30–50 cm (1.0–1.6 ft) long, but can reach 120 cm (4 ft). 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 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 (2.4 in). 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.