The Pea Crab is an unusual little parasite which you may just meet face-to-face. Pea Crabs live inside clams, mussels, and oysters, and people sitting down to a seafood meal occasionally find them. Those in the know consider them delicacies.
Pea Crabs fit their name. The largest females measure under an inch across. Full-grown males are much smaller. These tiny crabs live in the part of clams and similar creatures called the mantle, which, among other things, sifts food and oxygen from sea water. Positioned atop the mantle's gills, Pea Crabs snag bits of food, get oxygen, and enjoy the protection of their host's hard shell.
Once full-grown, female Pea Crabs stay put, one per clam, oyster, or mussel. The crabs don't even develop hard shells of their own because they don't need to. The little male Pea Crabs wander, going from clam to clam. Around Puget Sound the big gaper, or horse, clams host 3 species of these little parasites. Young crabs may live in smaller bivalve species, moving to bigger ones like the gapers as they grow. Some kinds of Pea Crabs live in worm burrows or inside other types of marine creatures.
Especially for Pea Crabs living inside bivalves, "parasite" is the right label. The crab affects its host and not for the better. Besides diverting some of the clam's food, the crab damages the host's feeding apparatus as it grabs edible bits. This affects the bivalve's growth in various ways.
Adult Life in a Mussel
Throughout their adult lives, pea crabs live within green-lipped mussels. They lead a solitary life, with just one crab usually inhabiting each mussel shell.
Adult pea crabs rely on their mussel hosts for both food and protection. When mussels catch phytoplankton on their gills (as part of the process of filter feeding), pea crabs intercept some of the phytoplankton and eat it themselves. Also, the hard shell of the mussel helps keep pea crabs safe from predators.
A female pea crab never leaves her host mussel. As she is protected from predators throughout her adult life, she has no need for her own hard shell. Instead, her shell is soft and rounded.
Males, on the other hand, do leave their host mussel on occasion (to seek out a female and fertilise her eggs), so they have a hard shell and are relatively flat from top to bottom. This shape helps them with getting into and out of mussels through the narrow gap between shells that is used by mussels to pump seawater in and out. Their shell colouring also provides camouflage to avoid predation by fish such as spotties, which are common around mussel reefs and mussel farms.
Pea crab females produce far more eggs than non-parasitic crabs. Their protected lifestyle means they don’t need to move to find food or escape from predators, so they can use their extra energy on egg production. Unlike non-parasitic crabs, female pea crabs produce eggs constantly all year round.
After her eggs have been fertilised by a male crab, the female holds the eggs under her abdomen while they develop. As they grow, they form into an enormous egg mass that’s as big as the crab’s body – it’s so large that she would have trouble moving even if she wanted to! After about a month, the eggs hatch and the swimming larval crabs move out of the mussel into the surrounding seawater.
Sourced From/More Information:
Article 1 & Images: https://www.skagitbeaches.org/creatures-list/172-pea-crab.html
Article 2 & Images: https://www.sciencelearn.org.nz/resources/755-life-of-a-pea-crab