More than 50 species of crab are found in UK waters. The largest, the edible crab, can reach almost 30cm across the carapace and weigh more than 6kg. Along with animals such as lobsters and prawns, crabs are crustaceans, a group characterised by their all-encasing suits of armour. In addition to giving excellent protection, the armour provides firm anchorage points for powerful muscles.

Spiny spider crabs like the one here congregate to form huge mating mounds. These can be several feet high and there are reports of them containing over 1000 individuals. Moulting crabs are protected in the centre of the heap and males are always on standby for the receptive females. Mounds can stay in one place for months over the summer.

Some crabs play unwilling host to a specially adapted shell-less barnacle. It sends its branching roots through the crabs tissues, preventing many normal processes such as mating. The process is known as parasitic castration. Infested crabs tend to be smothered with encrusting animals, as the barnacle also prevents moulting.

Female crabs carrying fertilised eggs in a mass tucked beneath their abdomen (tail flap) are said to be berried. The eggs hatch into little larvae that have the appearance of bizarre extra-terrestrial creatures and float with the plankton. They become increasingly crab-like before settling on the seabed.

Crabs must moult to grow. The dead crabs washed up on beaches are usually empty suits of armour cast off by crabs when they moult - you can tell by the lack of smell. Having slipped out of its old suit, a crab will swell up before forming more armour. The period when it is soft is fraught with danger.

In most crab species, the female can mate only while still soft, just after moulting. Males grab a female that is shortly to moult and carry her in a pre-mating embrace to ensure that they are first on the scene when the time arrives. Pairs that are actually mating can be distinguished because the female is then upside-down.

The two claws of a crab are usually different in shape, size and function. The smaller one will be for grabbing, cutting or prising, while the larger one serves as a crusher. The muscle responsible for closing the crushing claw is almost always the crabs biggest single muscle.