Why do some crabs live inside mussels?
Asks Lyra Caughey.
They are pea crabs, parasites which live inside the mussel’s shells. The mussel strains plankton out of the sea water for its food. The pea crab eats some of the plankton but it isn’t too greedy. It leaves enough plankton for the mussel to eat and grow, so there’s room for the growing crab.
The fat female pea crabs never leave their hosts. Sometimes you’ll find one carrying a mass of orange eggs under her tail. She will let them go into the seawater and some will find homes in other mussels. The smaller male pea crabs have flatter bodies to help them sneak in and out of mussel shells to find female crabs.