Creature Feature

The Original House Flippers

Hermit crabs take real estate seriously.

Story & Photos By Kelly Currington

Hermit crabs’ unusual name comes from the fact that they carry their home on their back, inferring that they are reclusive and live in solitude, but this is far from true. They can actually be very social animals and create communities in the wild. 

Hermit crabs are one-of-a-kind evolutionary crustacean anomalies. Although they are called crabs, they aren’t true crabs, which have thick chitinous exoskeletons. Instead, hermit crabs have soft, long, spiral-coiled abdomens that end in an asymmetrically hooked tail, which they use to secure themselves in an abandoned mollusk shell. This protects their soft and vulnerable bodies, which have no natural protection from the elements or predators.

This stareye hermit crab has carried its home onto a purple rope sponge.

Before becoming anything resembling their adult appearance, hermit crabs go through several stages of change. The eggs will develop into tiny, free-swimming larvae called zoea, which have spiny carapaces and rudimentary limbs on the abdomen and thorax. From this stage they molt numerous times before reaching the next stage of growth, the megalops, where appendages appear, the abdomen lengthens, and the eyes enlarge. After several more moltings, megalops becomes juvenile hermit crabs, little replicas of adults. 

Over their lifetime, hermit crabs molt regularly as they grow. The smaller the crab, the more often it molts. Tiny ones (0.7 inch in diameter) will bury themselves in the sand for about two weeks for the molting process, and they will do this several times a year. Then, as they reach about 0.96 inch in diameter, the process takes about a month and happens three or four times per year. The next phase (1.68 inches in diameter) doubles in duration but only happens once or twice a year. Finally, when a hermit crab reaches adulthood, the process still takes about two months, but decreases in frequency to once every eighteen months or so. All these moltings are part of the aging process, but unlike some other crustaceans, there is little difference in their appearance pre- and post- molting.

Another fun fact about these interesting critters is that they will change shells after each molting, usually upgrading to slightly more space than their previous home. Sometimes this consists of very complex shell exchanges with dozens of post-molting hermit crabs. One crab will leave their current shell and a smaller one will move into the vacant real estate. This event can be one or two hermit crabs moving homes, or an entire procession of home-swappers moving into little efficiency shells that are less than an inch in size all the way up to giant hermits in the grandioso shells of Queen conch and Triton trumpets. 

You can also get an idea of their age by the thickness of their antennae and the little sharp nodules on their grasping claw, which is what they use to snag food. They aren’t the pickiest eaters and will dine on just about anything that is near them—small fish, invertebrates like worms, plankton, and floating food particles in the water that pass by them. Keeping your fingers out of reach is advised because they just might resemble little fishy bits to near-sighted hermits!

These hardy characters can live thirty years or more in their natural environment, and with over 800 species, and most of them marine, you are bound to encounter them on the shoreline, in shallow tide pools, or on reef dives. All hermit crabs start life in the sea. Land hermits will always be close to the shoreline. They need sea water to wet their gills and the inside of their shells. They will slip into a tide pool or shallow water pockets to saturate their gills and shell lining before moseying back to the sandy shore. 

A hermit crab has five pairs of legs.

As with most wildlife, if you approach them slowly and have patience, you can observe them digging in the sand and shoveling it into their mouths where they extract the nutrients and excrete the remnants. Their eyestalks are always watching for danger and when they feel threatened, they will very quickly retreat into their shell and the opening will fall face down on the sand. Sometimes they will scuttle away quickly to avoid contact, but more often than not, they just slam the door in your face. Don’t take this personally; they are merely protecting themselves and waiting for any perceived danger to pass. If you are very still, steady your breathing, and wait, they will usually slowly place their chelipeds (grasping claws) in the sand and lift their shell so they can peak out and see if it’s safe to open the door again. If so, they will prop the door open and continue their feeding. This is a such a treat to witness. 

Hermit crabs are everywhere on the reef. You have probably swum over them and never noticed unless they were out in the sand and obvious. If you slow down and take your time checking out each brightly colored sponge and coral head, you will see them in the bottom of the sponges and in the nooks and crannies of the coral heads. Often there are clusters of the tiny juveniles in the bowls of sponges. Don’t pass up the sea fans, but very carefully check them over because you will see hermit crabs clinging on as the fan sways with the ebb and flow of the sea. 

A cool treasure hunt you can do on a dive is tracking them. You will see their tracks in the sand, kind of like tire tracks. You can see the individual foot marks and the shell drag. Try to follow the tracks until you find the driver! (Always keep your dive orientation and never leave your dive buddy when doing this.) If you are correct in your direction, at the end of the tracks you will find a perfect little hermit crab! 

Giant hermit crabs often inhabit conch shells, and have been known to eat the conch inside.

When you find them, it is tempting to pick them up, but please resist this urge as it traumatizes them, as well as possibly introducing harmful bacteria from your hands onto their shells. It is always best to just observe, admire, photograph, and learn. If you are lucky, you may see two rivals squabbling over shells, their legs extended and pulling on the opposer’s shell, trying to force each to abandon their home so they can claim it as theirs—house flipping at its finest! 

People love to take hermit crabs home and keep them as pets, or even worse, buy them from pet stores, but this is a death sentence for the tiny crustaceans. Even with the best of intentions and care, they usually only live a few months at the most in captivity, which is equivalent to mere seconds in their thirty-year lifespan in the wild. Respecting their personal space, their place in the eco-system, and their right to the longest and most rewarding life possible is our responsibility. 

The next time you are strolling along the beach or diving on a reef, take a little time to search for these little gems and observe them doing what they do best—living free, being adorable, and maintaining their place in the ranks.

There are so many beautiful and fascinating creatures in the sea, and each one plays a crucial role in keeping the reefs healthy. The more we learn about each one, the more we learn how to protect them, and in turn, protect the oceans.

Author Kelly Currington says, “My first dip below the service of the sea changed my life in an instant. I will spend every minute I can in that underwater world.”

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