Sea cucumbers

Seven species of Holothuria have been recorded from the Maltese islands and telling them apart properly needs a microscope, but three are easy enough underwater. Holothuria tubulosa is the big brown one lying out in the open in seagrass and on sand: a leathery sausage twenty to forty centimetres long, tan to dark chocolate, its upper surface studded with soft conical bumps that give it a warty outline. Holothuria poli is smaller, paler and greyer, usually tucked half under a rock with rows of white-tipped tube feet showing along its flanks. Holothuria forskali is the glossy black one on rocky reef, smoother-looking with yellow-tipped papillae, and the one to be a little careful with.

They live from just below the surface down past forty metres, and you will find them on almost every Maltese shore dive: strewn across the sand between Posidonia clumps, wedged into boulder gaps, and lying on the silty flats inside sheltered bays. At the front end a ring of short, branching, mop-like tentacles works constantly, shovelling sand and sediment into the mouth.

Everything they eat is sediment. They strip the organic film, bacteria and detritus off sand grains and pass the clean sand out the other end as neat casts, which is why the trails behind them look like someone piped toothpaste along the seabed. A large Maltese meadow can have a startling density of them, and between them they turn over a serious volume of sand every year.