Marbled rock crab

The commonest crab in Malta and the one everybody has seen without knowing its name. The carapace is almost square, up to about 4 cm across, and carries a fine marbled pattern of dark green, olive, brown and cream that matches wet limestone almost exactly. The claws are short and stout with pale tips, the legs flattened and fringed, and the eyes sit right at the front corners of the shell.

It lives at the waterline, not under it. You find it on every rocky shore around the islands, on the splash-zone ledges at Sliema and Qawra, along the Gozo coast and around the salt pans, sitting just above the wash and dropping into the water when disturbed. It grazes algae and biofilm off the rock and scavenges whatever the sea leaves behind, and it tolerates a wide range of salinity and temperature, which is part of why it is everywhere.

For a diver it is mostly a surface-entry animal: the crab you notice while kitting up on the rocks, or in the first metre on a shore entry. It is extraordinarily fast, sprinting sideways and leaping small gaps, and it will hold position watching you until you get within about a metre.