Facts on Hermit Crabs in Coral Reefs

Housing

• The hermit crab has an exoskeleton; its abdomen just has a softer one. As the hermit crab grows, it outgrows its exoskeleton and must molt or shed the old one and start looking for a larger shell. It uses discarded snail, winkle, whelk and other single-shell mollusks' shells for protection. A hermit crab will fight for a shell it desires and will kick others out. It will then take the new shell for a test ride before wholly stuffing itself into it.

Unusual Body Parts

• Although not a fish, hermit crabs have gills which are ensconced in the shell. The crab uses its gills for breathing air and cannot breathe under water. However, the gills must be kept moist.

  Since he is a crustacean, the hermit crab has two sets of antennae. Located just below the eyes, he has one long set and one short set of feelers.

  Of the eight legs on its body, the hermit crab only uses four for walking. The hind-most pair stays in the shell to anchor the body.

  The mouth of the hermit crab has three appendages that work like little hands that hold and tear food.

  If the hermit crab loses an appendage and the conditions are right (such as safe environment) the appendage will grow back.

Hitch-Hikers

• Many hermit crabs have anemones attached to their shells. These invertebrates either attach themselves to the shell or the hermit crab attaches them. Anemones and their host hermit crab have a symbiotic relationship in that the anemone eats the crab's leftover food and protects the crab by using the stinging cells on its tentacles to ward off the hermit crab's enemies, such as yellow-fin tuna, octopuses, turtles, larger true crabs and other hermit crabs. Hermit crabs often take their anemones with them when they move to a new shell. Barnacles and sea sponges often ride on hermit crabs, too.

Myths

• Hermit crabs are not really hermits. On coral reefs, they live in communities of over 100 residents.

  Many people think that hermit crabs eat coral reefs. Actually they eat the coral polyps, which make the reef. They also feast on algae and detritus that they find on and in the coral.

Uncertain Future

• As oceans become more acidic, the number of hermit crabs living on coral reefs is declining due to their need for shells, which dissolve in the acidification.