Pea crabs are small pea-size crustaceans that hang on to the larger oysters and mussels as a form of stabilization selection. You see, alone they may not be able to survive. However, with a strong host, these lucky crabs have a chance of continuing their life and producing offspring. It becomes a kind of natural selection where the crabs that are able to hang on best to a host survives and gets to reproduce.
A survival of the fittest with an evolutionary pattern. It is explanatory that the oyster and mussels provide the pea crabs with protection as well as food that nourishes them as they grow stronger on the coat of an oyster.