Pinna rudis has a distinctive shell that typically reaches lengths of 25 to 30 cm. The shell is composed of two very fragile, long, triangular, wedge-shaped valves covered with large, protruding scales arranged in regular rows. These scales are especially prominent near the shell’s opening. The shell features five to ten low ribs radiating from the pointed end, running the length of the valves, which are almost symmetrical, toothless, and transparent at the ends. The Rough Pen Shell’s color is usually reddish-brown, and the rear edge of the shell is rounded and free.
The Rough Pen Shell inhabits the Mediterranean Sea, the Black Sea, and parts of the eastern Atlantic Ocean, including the waters around Malta. They can be found anchored vertically to rocks or firm sediment using strong, iridescent byssus threads, famously referred to as the „golden fleece“ in Greek mythology. These shells are often located in small patches of sand on rocky bottoms or in rock crevices, at depths ranging from the surface to 70 meters, though they are more commonly found between 20 and 70 meters in areas protected from strong water movement.
The Rough Pen Shell has a unique lifestyle and historical significance. Its byssus threads, known as sea silk, have been historically woven into fine yarn to create luxurious items such as gloves and stockings. This species is often confused with juveniles of Pinna nobilis, but Pinna rudis is more triangular and robust, with larger protruding scales and a browner or pink-orange color. Pinna rudis is a hermaphrodite, producing both sperm and ova, and its larvae are planktonic, drifting with currents until they find a suitable settlement site. This bivalve is also a filter feeder, drawing water into its shell to filter out plankton and other small particles for sustenance.
Divers can witness the Rough Pen Shell at various dive sites around Malta. One notable location is Hekka Point on Gozo’s north coast, where these intriguing clams thrive amid the diverse marine life. As you explore the depths of Malta’s waters, keep an eye out for these remarkable bivalves, which add to the rich biodiversity and historical allure of the region’s underwater world.
