The hermit crab is constantly on the move

StoryImage( ‘/Images/Story//Auto-img-11720853757581.jpg’, ‘Photo courtesy of‘, ‘Hermit crabs are omnivores (eating plants and animals) and scavengers (eating dead animals they find). They eat worms, plankton and organic debris.‘);
StoryImage( ‘/Images/Story//Auto-img-117208543426232.jpg’, ‘Photo by Dr. Robert A. Hedeen’, ‘As the hermit crab grows, it must find a larger home‘);

In the distant past, the hermit crab figured out the wisdom of the snail in carrying its home on its back, so the crustacean became willing to burden itself with a snail’s shell for the protection it offers. This peculiar habit of appropriating unoccupied snail shells in which to live has brought about curious modifications in the body structure of this crab.

The abdomen is soft, unlike most other crabs, and is twisted to fit into the coils of the shell. One pair of abdominal legs develops into hook-like structures that anchor the body within the shell; the other abdominal legs are degenerate. Anyone who has ever attempted to extract a hermit crab from its shell knows it is next to impossible to do without tearing off the animal’s abdomen. I have found, however, that a lighted match applied to the end of the shell is often an effective, if not a sporting, way of inducing a crab to release its tenacious hold and vacate its shell.

One difficulty encountered by the hermit crab is that of their growth, which makes it necessary for it to go house hunting. As the crab increases in size, it becomes increasingly cramped in its home, and sooner or later must move to larger quarters. After much scurrying about, a larger snail shell is found, and the crab does not waste any time moving in. In the blink of an eye, it darts from the old to the new so as to be exposed as short a time as possible to predators.

As the crab nestles into its new quarters, the right-handed pincer is used to close the mouth of the shell. The smaller left-handed pincer is used to fill any crevice left in the opening by the other claw. One would think the burden of transporting a heavy, calcareous shell on its back would slow the movement of a hermit crab to a snail’s pace, but such is not the case. It readily and rapidly moves along the bottom while it scavenges for bits of food.

One of the first things the crab does after occupying a new shell is to find a group of colonial hydroids (relatives of jellyfish and sea anemones) called sea fur and place the primitive animals on top of the new home. Some members of the sea fur colony possess stinging cells that capture prey for the host crab to drag in and also to offer protection. Other members of the colony do not sting but enlarge the crab’s home by building on the shell’s free edge. In return for these favors, the crab offers the hydroids a place of secure anchorage and efficient transportation to and from feeding areas. This sort of relationship between two different species, where each benefits from the presence of the other, is known as commensalism and is quite common in the Animal Kingdom. The lives of the two species are not so intimately linked, so that one species cannot live without the other.

When an association between two species occurs that is so intimate that one cannot live without the presence of the other, such as the association of an alga and a fungus to form a lichen, it is called symbiosis. The next time you are at the seashore, you will probably encounter a hermit crab and note the fuzzy appearance on the shell. If a hand lens is available, you will be able to discern the individual polyps making up the “fur.”

A hermit crab is not as docile a creature as one might suppose and readily engages in mortal combat with any creature it thinks it can overcome, including its own kind. In the face of a more powerful opponent, however, the hermit quickly retreats to the interior of its fortress, slams the door, and becomes a recluse until the danger has passed.

Hermit crabs make interesting pets for children and biologically-minded adults, and are usually available for purchase in souvenir shops along the coast. Likewise, there are several sites on the internet that offer them for sale with live delivery guaranteed. They may be kept in a terrarium, but a constant supply of water must be made available as it is needed to be stored in the shell to keep the gills moist.

A final note for surf fishermen: I have found hermit crabs to be outstanding bait for many fish venturing into the surf. You may get the crabs to leave their shells by placing them in warm water and gradually increasing the temperature.

Dr. Robert Hedeen is a former resident of Maryland’s eastern shore and resided in the Chicago area from 1960 to 1971. He is a retired professor emeritus of biological sciences in the University of Maryland system. He has published more than 30 scientific papers, has written numerous magazine articles, and is the author of two books on the natural history of the Chesapeake Bay.

From the Feb. 21-27, 2007, issue

Enjoy The Rock River Times? Help spread the word!