Hermit crabs

Hermit crabs are a common sight by the seaside, most often spied crawling around in rock pools. Adults vary in size from around 15 to 150 millimetres. Their most prominent features are their shells, and yet these are not really part of the crab at all. Most crabs have a hard carapace that they produce themselves, but hermit crabs have soft, easily-damaged bodies and would quickly be killed without further protection. Because of this they search out the empty shells of gastropods such as whelks and periwinkles. As they grow they have to find bigger shells that can accommodate their increased body size. There is often much competition for the best shells, but once a crab has successfully obtained one it will insert its asymmetrically twisted abdomen far inside until only the front two pairs of legs remain out in the open.

Technically the crabs have five pairs of legs, but the back two of these are too small and puny to be much use for anything other than holding on to the shell, and so remain tucked away inside. The second and third sets are larger and stronger and so are used for walking, but even these are dwarfed by the first set. The front right leg has a large claw that is used for holding food and for fighting, and the smaller claw on the front left leg is used to help with eating.

Hermit crabs aren't fussy about what they eat; if they can catch it they'll have it. Any animal and vegetable matter scavenged from the seafloor will do. The crabs often have a symbiotic relationship with other creatures such as sea anemones which come and live on their shells. The crabs get camouflage and protection and in return the anemones get to eat any leftover food. Sometimes this relationship becomes so established that the anemone will 'move house' along with the crab when it changes shells.

Male hermit crabs can be quite aggressive when the time comes to mate, and will grab hold of female crabs, fighting off any competing suitors with their large right claws. Male reproductive organs are located behind their rearmost legs whereas those of the female are on the third pair. On the left-hand side of the female abdomen are structures known as pleopods which are used to carry the fertilised eggs. The baby hermit crabs do a fair amount of their development whilst tucked away here inside the eggs. When they eventually hatch and venture out into the sea they are tiny, shrimp-like creatures in desperate need of a shell.

This hermit crab lives in a tank at the Anglesey Sea Zoo, a small aquarium that only has native sea creatures on display. It is well worth a visit, with its lobster breeding programme being a particular highlight. The zoo is also a good reminder of what a great range of sea life we have in the UK, and reinforces the fact that creatures don't have to be exotic to be interesting. Great satisfaction can be derived from learning about the animals 'indoors' and then going and finding the wild versions just outside.

Anglesey Sea Zoo