By Sarah Ward
Living Seas Officer
At the end of every Shoresearch season, all the species records we’ve collected at the various sites are totted up, giving us a good overview of what we’ve seen over the course of the season, and where.
Every year we have a list of the most commonly seen species, which always includes seaweeds like wracks, periwinkles, and the green shore crab. Most years, we find the Green Shore Crab, Carcinus maenas, has been recorded on every single Shoresearch survey we’ve undertaken, and is also usually found at other rock-pooling events we run, as well as by Seasearch divers on shallower dives.
This crab is so named because it is typically found on the shore, and is usually green in colour, although juvenile crabs often show various patterns in brown and white, and older individuals can be identified by having a more orange-red shell.
Crabs are part of the sub-phylum crustacea, which is a large group of animals including crabs, lobsters, prawns, barnacles and amphipods – mostly exclusively marine. Crustaceans all have segmented bodies and limbs, and many have an external body covering or ‘exoskeleton’. More specifically, crabs are decapods – meaning they are 10-legged.
Because of their hard exoskeleton, shore crabs are unable to grow in the same way that soft-bodied animals can. They must shed their shell when their soft body underneath has outgrown the shell; this is known as ‘moulting’. Following a moult, the crab is then without its protective shell, meaning it is more susceptible to predation and damage – until a new shell hardens over the course of a few days. Male crabs will often ‘guard’ a female who is about to moult, which affords the female protection, but the male will have other intentions – as once the female has moulted, he will then be able to mate with her.
It’s great to look for green shore crabs on the seashore, but remember to treat them with care, particularly if you find a male guarding a female, or indeed a berried female (meaning she’s carrying eggs). Check out this guide to eco-friendly crabbing for more information!