European spider crab

A big, long-legged crab with a roughly pear-shaped or triangular carapace that narrows to two blunt horns at the front, the whole shell covered in short stiff hairs and studded with spines. Base colour is a dull orange-brown to brick red, but you rarely see it clean: the hairs hold scraps of algae, sponge and hydroid that the crab attaches deliberately, so most animals look like a slowly moving patch of reef. Carapace length reaches about 20 cm and the leg span can approach 70 cm, which makes a full-grown one the biggest crab most Maltese divers will see.

It lives on rocky reef, over sand and rubble and along the edges of Posidonia meadows, recorded from the shallows to about 150 m but most often met by divers between 5 and 50 m. It is a slow, deliberate grazer and scavenger, working over algae and picking at carrion and small invertebrates, and it usually walks away rather than confronting you.

The crab is mostly solitary, but around moulting time animals gather, sometimes in heaps, because a freshly moulted crab is soft and defenceless and safety comes in numbers. Females carry a large orange egg mass tucked under the folded abdomen.