There are many species of parasitic barnacles, distributed around the world. The rhizocephalans parasitize crabs; they are sometimes found on king crabs in our area.
A rhizocephalan larvae lands on a host crab, gets rid of its shell, and moves inside the crab host. Then it spreads throughout the crab’s body, digesting the crab’s reproductive organs, invading the nervous system, changing the crab’s behavior, and stunting the growth of the crab. The first female larva to invade a crab causes the crab to produce a sac-like structure (called an ‘externa’) on its abdomen. This sac holds the parasite’s unfertilized eggs. The barnacle causes the crab to release hormones that draw in male barnacle larvae. At least one of these new arrivals penetrates the externa, where the cells of the new larva become sperm and fertilize the eggs. The resident female parasite controls the crab’s behavior, causing the host to care for the externa as it would its own offspring. Then even a male crab behaves like a female crab and broods the externa on its abdomen, under its tail flap.
• Mary F. Willson is a retired professor of ecology.
