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.
Here’s where hermit crabs get comical. Besides scavenging for food, they are always shopping for a better shell. They will walk over and investigate any empty shell. After probing with their claws to be sure the shell is empty, they will maneuver close to the new shell and quickly lift themselves out of the old shell and insert their abdomen in the new. Then like a man trying on a new suit coat, they will lift the new shell, kind of move it around a bit to get a good fit, and even walk with it. But the process doesn’t stop there. They will go back to their old shell, put it back on, and go back and forth between the old and new shells several times. In the end, they may walk off in the new shell or go back to their old shell. During this whole process, they seem much undecided. They’re picky shoppers.
You can easily observe this behavior by selecting a variety of empty shells to present to the hermit crab. Collect several hermit crabs and put them in a marine aquarium, then select several shells that are about the same size. Try to get some with a hole in them left by an oyster drill. Put the shells in the tank and watch the hermit crabs continually investigate shells and try them on. If the crab is in a shell with a hole, you can tickle the crab to leave its shell by inserting a piece of straw through the hole. Once out of a protective shell, they are desperate to protect their soft abdomen. They will even wear a thimble as a shell if no shell is available. They will also fight over shells and get into a miniature tug of war with each other. The more dominant crabs will get the better shells. So here is a way to find out who is lowest on the hermit crab pecking order – look for the hermit crab going around in the shell of a left-handed whelk. Why? The hermit crab abdomen curves to the right like most snails, the shell of the left spiraling left-handed whelk is an uncomfortable fit and they will wear it only if no other shell is available to them.
Despite their defenses, hermit crabs are preyed on by blue crabs, whose claws can smash a small shell, birds and some fish. Pesticides in runoff from land kill their larvae. Yet one of their biggest threats is shell collectors. Small empty snail, conch or whelk shells are needed by the hermit crabs. And often the shells are not empty. The hermit crabs can withdraw out of sight into the shells. Many a shell collector has brought home shells only to find a dead hermit crab inside. They are far more entertaining to watch than to collect.
Tom Baird has been a fisheries biologist, high school and community college teacher (oceanography and microbiology), director of a science and environmental center, teacher of science and principal in Pinellas County as well as an educational consultant. He retired from the Florida Department of Education and he and his wife divide their time between Tallahassee and Cape San Blas.