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• A huge clam that was on the verge of extinction has made a comeback, with a surge in numbers in waters off Croatia, marine biologists say. The clam, is known as the noble pen shell or pinna nobilis.

Pinna nobilis

• Pinna nobilis, whose common name is 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 120 cm of shell length. It produces a rare manganese-containing porphyrin protein known as pinnaglobin.
• The bivalve shell is usually 30–50 cm long, but can reach 120 cm.
• 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.
• 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.
• 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.
• This species is endemic to the Mediterranean Sea, where it lives offshore at depths ranging between 0.5 and 60 m.
• It could be found buried beneath soft-sediment areas (fine sand, mud, often anoxic).
• In December 2019, Pinna nobilis has entered the IUCN Red List as critically endangered.

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