Hermit Crabs: Mobile Home in a Snail Shell

Contrary to popular belief, a beach is far from desolate when humans are not walking on it. These are “living beaches”, bustling with creatures digging, scuttling, wading, actively foraging, hunting, and fighting around the tidal clock. And some of the most industrious of these beach residents are hermit crabs.

A hermit crab, though related to crabs, is not a “true” crab. It is asymmetrical and has an abdomen that looks like a coiled-up ice cream cone. Unlike other crabs, its exoskeleton is not strong, and offers little protection against predators. To stay safe, a hermit crab must spend its life looking for an external object that is roomy enough to house its entire body yet light enough to carry around. This life of a hermit crab is one of an obligatory scavenger of abandoned snail shells.

Based on fossil evidence from the Cretaceous and Jurassic periods, paleo-scientists believe that hermit crabs were symmetrical animals until some point in their prehistoric past. They would occupy shells made by ammonites (the now extinct cousin of squids and octopuses). After the extinction of ammonites, hermit crabs turned to gastropod shells (made by snails) for cover. For this strategy to succeed, hermit crabs had to adapt their body to fit the shape, contours, and interior design of asymmetrical, spiralling shells. They use the curved end of their abdomens to hug the shell’s columella (central axis) and hold onto the shells for dear life. 

Taxonomically, hermit crabs are decapod crustaceans — belonging to the same larger group as lobsters, shrimps, crayfish, and true crabs. Within Decapoda is another order called Anomura. This includes hermit crabs alongside porcelain crabs, mole crabs, squat lobsters, and king crabs. Some hermit crabs are terrestrial in their habits, living most of their lives on land, needing access to water only to keep their gills wet and lay eggs when it is time to breed. There is also diversity among fully aquatic hermit crabs: ranging from deep-sea and decorator hermit crabs to coral hermit crabs that have shed their nomadic lifestyle altogether for permanent burrows left in the reef by worms, corals, and sponges.

For the spectator, watching a hermit crab is fascinating. Head to the nearest beach known for these backpacking crabs, find a comfortable spot to sit down and watch their lives unfold before you. If at first it feels like nobody is around, that’s probably because hermit crabs retreat into their shells at the slightest sense of threat or human footsteps. They curl up, let their shells drop to the ground, and lie motionless. Give them about 60 seconds and they resume full activity, inspecting their surroundings, one peep and one limb at a time. 

While some individuals forage, others may congregate around a meal — a juicy seed pod, ripe fruit, or perhaps a rotting beached fish. Quite often, hermit crabs can be found in the middle of a brawl. To understand why, here is some context.

The quest for a shell is not as straightforward as coming across a shell and adorning it. Shells need to be abundantly available and in a diversity of sizes. This depends on the diversity and abundance of snails in the area. Shells also need to be in good shape to house and protect hermit crabs. This becomes a function of what kinds of predators of snails occur in the vicinity and what state of damage they leave the shell in. Some reef fish crush snail shells to get to the flesh. Hermit crabs benefit most from shells that are as intact as possible. For instance, predators like moon snails and murex snails drill a neat hole in their prey’s shell and slurp up the flesh without damaging it. Cone snails inject conotoxins into their prey, not needing to negotiate with the shell. These make the best hermit homes. 

Hermit crabs spend much of their time walking around vast beaches looking for shells, but this does not have to be a wild goose chase. They use chemical signals to hone in on vacant shells. Hermit crabs can tell when a shell becomes vacant based on the odours released from the dying animal. If upon reaching an empty shell a hermit crab finds it is a bit too big, it might wait for up to several hours for another crab to inquire after the same shell in the hope that it can climb into their discarded shell instead.

The wait-and-watch strategy can often lead to an entire marketplace of hermit crabs assembling at the site of a vacant shell, eagerly waiting for the right candidate to fit the shell and unlock a series of shell exchanges amongst the rest. This is referred to as a “vacancy chain”, where each crab leaves its shell for a slightly bigger one left behind by another crab looking to scale up.

Hermit crab lives are closely linked to numerous environmental processes. When threats arise, the impacts they feel are profound. This could range from the lack of shells due to overfished snail populations to a complete absence of shells in some areas because of overharvesting by beachcombers and shell collectors. Hermit crabs have been found to resort to using beer cans, bottle caps, and other trash as surrogates for shells. Most often, these substitutes do not work. Plastic containers either do not fit well or are too conspicuous, increasing the chances of predation. Scientists have also come across another disturbing trend — where hermit crabs entering marine debris are unable to get out, leading to injury, starvation, and death. And as we now understand, a dying hermit crab is a cue for “vacant shell available” to the next lot of hermit crabs who sadly fall into the same trap.

There are steps that can be taken to minimise the stress these animals face. There are two critical and actionable steps we can take on the next trip to the beach. Leave the shells behind and resort to picking up the trash instead!