Scotland's seas, which constitute an area greater than that of the seas around the rest of the UK, are among the most biologically productive in the world. They are home to a third of the world's whale and dolphin species, most of the UK's maerl, (a collective term for several species of calcified red seaweed, and an important marine habitat), Horsemussel ("Modiolus modiolus") and seagrass beds, and distinctive species like the tall sea pen, "Funiculina quadrangularis". It is estimated that the total number of Scottish marine species exceeds 40,000. This includes 250 species of fish, the most numerous inshore variety being saithe, and deeper water creatures such as the dogfish, porbeagle and blue shark, European eel, sea bass, Atlantic halibut and various rays. There are four species of sea turtle, the leatherback, loggerhead, Kemp's ridley and green turtle. Scottish waters contain around 2,500 crustacean species and 700 molluscs and in 2012 a bed of 100 million flame shells was found during a survey of Loch Alsh.