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Hawaiian Marine Life Profiles: Invertebrates

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Common Name:

Anemone Hermit Crab

Hawaiian Name:

unauna

Scientific Name:

Dardanus pedunculatus

Found in Exhibit:

Mid Reef



While quietly resting at home, you are startled by loud, rapid knocking at your door. You open it to find your neighbor and his five kids standing outside, demanding that your families exchange houses. Since his family is bigger than yours, and frankly, he's a little intimidating, you pack up and move into your neighbor's smaller house. Sound crazy? In the human world this sort of behavior might be unusual, but not for hermit crabs.

Hermit crabs, or unauna, live in abandoned snail shells to protect their soft abdomens, and need to find larger shells as they grow. As these homes can be hard to find, fierce battles may ensue. If two crabs are interested in the same shell, they may engage in a game of tug-of-war with it. If a hermit crab desires a shell that is already occupied by another crab, it begins knocking on the shell with its own, in hopes that the current resident will eventually move out.

One particular species of hermit crab relies not only on a hard shell for protection, but also carries anemones on its mobile home. These anemones are venomous to most predators and may provide additional camouflage and protection from parasites. The hermit crab, aptly named the anemone hermit crab, in turn provides transportation and occasional food scraps to the anemones. This relationship, in which both species benefit, is called symbiotic. When the anemone hermit crab changes shells, it will carefully transplant the anemones to their new home. There are usually two species of anemones found on the shell: a few larger, intricately patterned anemones and several small, white anemones, often found above the crab's head. Both of these anemones are almost always found living on a hermit's shell.

Hermit crabs have five pairs of legs, each specialized for a different task. The first pair has claws, with one claw always larger than the other. The second and third pair are used for walking. The fourth pair holds on to the edge of the shell when the crab moves forward in the shell, and the last pair firmly grips the inner cavities of the shell. The front of the crab is protected by a hard exoskeleton, and if the crab is being attacked, it can retreat inside of its home, leaving only the large claw visible. The only time that they leave their shell is to move to another, which can be accomplished quite rapidly.

Though the bodies of hermit crabs leave them extremely vulnerable, they have evolved various ways of protecting themselves. Some species are specialized to live in hollow bamboo tubes, some have given up mobility to live in fixed worm holes, and some individual hermit crabs will live in just about anything that they can fit into and move around. Hermit crabs are truly wonders of nature, but the next time that you hear a knock on your door, be glad that you don't live in their world.

PDF: hermit_crab.pdf