Hermit Crabs

Hermit Crab

Hermit crab with symbiotic hydroid

Published: Thursday, May 15, 2014 at 10:03 AM.

By Tom Baird

 

You are wandering along in the shallows at the edge of the salt marsh, or maybe snorkeling in the clear water off the beach. You see a shell moving along, but it’s moving too fast for a conch or whelk. You reach down to pick it up and it quickly stops. As you lift it up, you see something quickly withdraw inside. As you turn it over, maybe there are the tips of a pair of pincher claws just visible. You just picked up a foraging hermit crab, one of the most comical of sea creatures.

One of the common hermit crabs in St Joseph Bay is the Dwarf Hermit Crab (Pagurus longicarpus), also known as the Long-clawed Hermit Crab. Like all hermit crabs, it is a scavenger ambling along picking up bits of food.  In the clear shallows, put out some small pieces of fish or scallops you have just cleaned and watch for a few minutes. Pretty soon a little parade of hermit crabs will converge on the meal and start tearing it apart. As you watch, you’ll notice a few things. First, the hermit crabs inhabit different types of shells.  The second thing is that some shells look fuzzy or furry.  Others will have something fleshy on top of the shell.

The ones with furry shells have a hydroid (Hydractinia echinata) living on the shell. This does not harm the hermit crab and helps camouflage it. In turn, the hermit crab moves the hydroid colony around enabling it the better filter food from the sea water and keeping it underwater as the tide goes out. Those crabs with something fleshy on them have an anemone riding along. The Cloak Anemone (Calliactis tricolor) especially likes to live on the shells of hermit crabs. Like the hydroids, the hermit crab gives the otherwise sessile anemone mobility and food. The anemone helps protect the hermit crab from predators by virtue of their stinging nematocysts on their tentacles. Hermit crabs will even fight over anemones and try to steal them from each other.

Why you find hermit crabs in different shells is because they need to find larger and larger shells as they grow. Hermit crabs are always on the lookout for a more suitable shell. Hermit crabs do not kill the snails, whelks and conchs whose shells they appropriate. They utilize the empty shells after the snail’s death. The little Dwarf Hermit Crab so regularly found in the shallows of the bay uses the small empty shells of salt marsh periwinkles, oyster drills and mud snails. Other hermit crab species utilize larger shells, such as those of Moon Snails, Left-handed Whelks and Horse Conchs.

So why do they need a shell at all? Other Crustaceans don’t hide their bodies in snail shells.  The abdomen of a hermit crab is a soft, unarmored thing, unlike the abdomen of a lobster or a shrimp for instance. The abdomen of a lobster has hard chitin plates that you have to cut through to get at the meat inside. The hermit crab abdomen is soft.  It is vulnerable to predators.  So like a man climbing into an army tank, the hermit crab picks up its armor and carries it around.  But the snail shell isn’t straight, it spirals. So to, the abdomen of the hermit crab is curved to fit the right hand spiral of most snail shells.



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