The Barnacle and the Poor Green Crab

An Essay by Katherine L. Dean

Crabs and barnacles are both arthropods. Find out more about arthropods.

The barnacle is a sea animal with a hard body called a crustacean. It is the only crustacean that cements itself to something solid and remains there forever. Baby barnacles swim around in the water but soon attach themselves to rocks, driftwood, and other crustaceans such as crabs and snails. Sometimes so many of them attach themselves to boats and ships that they slow the vessel down. Even our whales carry these small white "shells" on their great bodies.

When they are under water, barnacles eat plantlike parts of almost invisible swimming animals called phytoplankton and zooplankton. When the barnacles are not under the water they have to wait until the tide comes in before having dinner!

Most barnacles are harmless unless stepped on with bare feet. They are very sharp! There is, however, a parasitic barnacle which attaches itself to the ordinary tidemarsh green crab. It is called a sacculine barnacle. This barnacle baby swims around freely until it finds a green crab. It then attaches itself to the fuzzy coating on the top of the crab's shell. It puts part of its cells into the crab through this coating. Eventually these cells get into the bloodstream of the crab and come to rest in its stomach. The barnacle continues to grow inside the shell eating all the interior organs of the crab. By the time the crab sheds its old shell, a process called moulting, the parasitic barnacle is living inside of it.

The green crab isn't a green crab anymore. It doesn't lay crab eggs. It lays sacculina barnacle eggs. It looks like a green crab but it isn't. It has been taken over by the barnacle.

• Roberts, Mervin F., The Tide Marsh Guide. New York, E.P. Dutton.

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