I had a bit of a surprise in my dinner over the weekend... after boiling up a bag of mussels, I found several odd-shaped crabs at the bottom of the water. Pea crabs, as it turns out... a not exactly uncommon but rarely seen member of Englands aquatic community.
Pea crabs and their relatives (Family Pinnotheridae) are best known as parasites of clams and mussels, although they have also been found affecting sea urchins, sea cucumbers, and tube worms. Most of what is known about these crabs comes from a few species that infest edible mussels and oysters, and because of this, the literature on the life history of pea crabs can be confusing and contradictory. The adult female pea crab is the best studied life stage, for the obvious reason that this is the stage most frequently encountered. Much less is known about the males and the early free-living stages, and what is known comes from captive animals observed in aquaria.
The general opinion is that pea crabs pass through several planktonic stages before moulting into a recognizable crab shape, although some species of pea crabs may hatch skip the planktonic stage. The young crab stage is characterized by a hard shell and flattened hairy legs for swimming. In this stage, both males and females have a free-living existance while looking for suitable hosts. Following entry into a host mussel, the female undergoes a transition (over several molts) into the familiar pea-shaped crab with a soft shell and limited mobility. Following these molts the female grows too large to leave its host, and its legs become smaller and weaker, sufficient to move around inside the mussel but not to live outside.
The male will continue to grow through several more molts, but is less dependant on a host mussel. In the British Pea Crab, the males remain small and can alternate between a soft-shelled form adapted to the inside of a host and a free-living hard-shelled form adapted for moving between hosts, with the season guiding their decision on whether to be soft-shelled or hard-shelled at any given time. Its less clear whether males of other pea crabs can switch between hard and soft shelled forms, however. The hard exoskeleton helps protect the crab both in the outside world and during the dangerous process of entering a mussel - the mussel can close down on a crab, potentially crushing it or snipping off legs, and males missing a leg or two show up with regularity inside mussels.
Female pea crabs are obligatory parasites once they settle in a host. The female doesn't actually feed on its host, however, rather it steals food from its host - though this has the effect of damaging the gills, causing local irritation of the mantle lining and reducing the growth rate of the host. The male can presumably also steal food from its host, but its less clear how dependent it is on the host mussel in the wild, and it may prove to have a largely free-living existance. The male sports fringes of hairs on its (flattened) legs which enable it to swim and hard shelled forms are restless, frequently leaving and re-entering hosts when in captivity. As all of the behavioral studies are based on captive individuals kept in aquaria with lots of mussels (and probably not much else), its not clear how much of their time hard-shelled males actually spend in hosts - it is probable that they are only part-time parasites.