The pea crab (Pinnotheres pisum), also known as the mussel flea. This tiny animal is one of the smallest crab species (a female rarely measures more than 12 mm across) and is a parasite of mussels and some other bivalve mollusks. Mussels are a tough nut to crack: their thick shells and lightning reflexes protect them from most small predators. But the pea crab has taken on this challenge and has dedicated its life to breaking down these defenses to live within the protective enclosure of the shell. As the mussel inhales seawater, the gills filter the edible particles and collect them in a mucous rope, before passing along a conveyor belt of ciliary hairs. The pea crabs are simply served alone. The Life Cycle of the Pea Crab The female crab never leaves the mussel. Almost twice as large as the male, it is practically spherical. When carrying eggs, the large mass of eggs clinging to its underside is so large that it almost doubles its thickness and walking becomes impossible. His life of leisure also means he doesn’t need a hard exoskeleton. The male, on the other hand, is a different beast. During the early stages of his life, he also resides in the inner sanctum of the mussels. But as he matures and the appeal of sexual calls, he takes risks. Adult females attract males by emitting a pheromone that comes out of the mussel from its exhaling siphon. A masculine current will then emerge from its homely host to meet her. To facilitate his exit and subsequent entry into his lady’s lair, the male is not only smaller, but also flatter. It is also an excellent swimmer, having stripes of long hairs on its legs that act as oars. Because of these study trips into the big wide world, it has many of the usual crustacean adaptations: a hardened exoskeleton and cryptic markings of browns and grays are helpful in minimizing the chances of harm. However, experimental studies in captivity with other pea crab species have shown that even entering and exiting host mollusks is risky. Many males have their legs trapped and amputated or, more seriously, crushed. Still, this odyssey is worth the risk for males, as a mating could mean a considerable investment in the next generation. A single female pea crab can produce thousands of eggs; More than 9,000 have been registered. The larger the host mussel, the more food it absorbs; and more food for the crab means that it can also grow and produce more eggs. After about a month, the female’s eggs hatch. Free-swimming crab larvae emerge from their mussel home into the surrounding seawater and join the plankton community. In their final free-floating stage, pea crab larvae, known as megalopae, have swimming legs called pleopods along the tail, as well as large chemoreceptors. They are thought to use these adaptations to detect and swim towards their final hosts, the mussels, before happily being absorbed along with the mussel’s food to settle down. Main image © Peter David Scott/ The Art Agency
