Meet the Land Hermit Crab

Hermit crabs are interesting, clean, and easy-to-care-for animals that are available in many pet stores or even small gift shops along the beach. Thousands of people worldwide, both young and old, keep land hermit crabs. Land hermits can live for a decade or more and thrive in the most basic of housing when properly kept.

The name “hermit crab” is misleading because in the wild, these crustaceans live and travel in large groups, roaming the shorelines at night in search of food. The “hermit” label actually refers to the solitary existence of the crab in the shell it takes as its home. The land hermit’s shell is not really its own but a discarded snail shell or similar object that the crab claims for itself. Nearly all varieties of hermit crabs must hide their soft abdomen in a shell or similar hard object for protection.

Hermit crabs are crustaceans that belong to the class Crustacea, as do blue crabs, shrimp, and lobsters. There are several different families of hermit crabs and hundreds of species within each family. Contrary to their name, hermit crabs are more closely related to lobsters than to blue crabs.

The few species of terrestrial hermit crabs (land hermits) all belong to the family Coenobitidae and two genera, Birgus and Coenobita, the latter of which is typically sold as a pet. There are about a dozen species of Coenobita, but they are relatively difficult to distinguish from one another. The most common is the Caribbean land hermit (Coenobita clypeatus), whose natural range is near the Caribbean islands and southern Florida.

Other species in this genus include Coenobita compressus, Coenobita rugosus, and Coenobita perlatus, all found near the shorelines of the Indian and Pacific oceans. They are seldom available as pets. (For more information on the different types of hermit crabs, see Land Hermit Crab Varieties.)

Of the hundreds of known species of hermit crabs, most live on coral reefs, in the deep sea, or along coastal brackish and salt shores. Land hermit crabs are the exception to this rule: they are terrestrial crabs that live near the water but otherwise remain on land, except when females go to the water to lay their eggs. The hatchlings spend the first few weeks of their lives in the water until they’re developed enough to move onto dry land and seek out their first shell.

Adult land hermits are known to migrate in large numbers over long distances, but they generally stay within 100 yards (91 m) of the shoreline at all times. They live almost anywhere, from sandy beaches to the tops of small trees and steep cliffs, as long as they’re relatively close to a water source. Few predators feed on land hermits, and they are relatively long-lived, which means that the land hermit population can reach gigantic numbers in suitable areas if left undisturbed.

Although land hermits may be active at any hour, they are nocturnal by nature. During the day, land hermits in the wild hide under leaves and other litter or in burrows that are sometimes many yards from the sea. At night, the crabs leave their hiding places and venture to the shore to eat and replenish their water supply. You’re likely to notice similar behavior in your terrarium: your crab will eat, drink, and generally become more active at night.

All hermit crabs have a similar shape: a pair of claws at the ends of their first pair of legs, long antennae, eyes on fairly long stalks, and a soft abdomen that’s asymmetrical in shape, with smaller pleopods (leg-like appendages used for swimming and brooding eggs) than those of a regular shrimp or crab. The abdomen is long and curved, allowing the crab to fit inside a shell’s curved interior.

Crabs vary widely in size—the smallest crab you’re likely to see for sale will be barely larger than a pea, while the largest may be big enough to inhabit a 10" (25 cm) shell.

Like other related crustaceans, hermit crabs have two major sections to their body. The front part is the cephalothorax, which consists of the head area fused to the thorax. The eyes, antennae, and mouth parts are at the head (the front end of the cephalothorax), which is marked by a thickened area called the shield. The gills and walking legs are under the back part of the cephalothorax.

The rest of the crab’s body consists of the abdomen, which has only a few traces of the wide segments that mark the abdomen of a shrimp or lobster. The pleopods are reduced in size compared to other crustaceans’, and some crabs have no pleopods at all, which allows them to slip into and out of a shell more easily.

Land hermits have poor eyesight, so they get most of their sensory information through smell and from vibrations in the soil, to which they’re very sensitive. The eyes themselves are located on long, flattened stalks attached to the head.

Like other crustaceans, land hermits have two pairs of antennae. The outer pair is long, while the inner pair is shorter, flattened, and ends in flattened sensory segments. These antennae are their most important sensory organs—they help the crabs find their food.

Like other crustaceans, land hermits use their gills to breathe. Hermits’ gills are smaller than those of other crustaceans because their cephalothorax is narrower. The land hermit is able to live on land because it can store small amounts of water in its shell, carrying its breathing water with it wherever it goes.

Hermit crabs are decapods: they have 10 legs. The first pair, called the chelipeds, ends in claws (pincers) that differ in size. The left leg has a large pincer, called the chela, that a crab uses for climbing and self-defense. This enlarged pincer also serves to block the opening to the shell when the crab withdraws inside of it, not only protecting itself from predators but also conserving moisture within the shell. The smaller right pincer is used for eating and climbing.

The second and third pairs of legs are called ambulatory legs and allow the crab to walk. The fourth and fifth pairs of legs never extend outside the shell under normal circumstances and are used by the crab to hold itself inside the shell to maneuver it while walking. These legs are very short, asymmetrical, and have roughened areas on their edges to help the crab hold onto the shell.

Land hermit crabs are relatively placid and can live comfortably among other land hermits. Land hermits of many sizes often live together in the wild. However, the crabs don’t tend to interact much unless they’re mating or trading shells. In most cases, multiple crabs kept together are likely to ignore one another. If one crab encounters another while moving, it’s likely to climb over it as though it were just an empty shell—unless it wants the other crab’s shell for its own.

In the wild, land hermit crabs often congregate on a beach or other open area, and for several days the hermits swap shells back and forth as each tries to find one that’s perfect for its body. Especially large gatherings can attract tens of thousands of land hermits as they swap shells, climb trees, and mate along the beach. In locations where suitable shells are not especially diverse or common, a shell often passes from crab to crab over several generations until the interior supporting structure of the shell wears away.

If you keep multiple land hermits, you may observe a shell trade in your own terrarium. If a smaller crab finds an empty, oversized shell and moves into it to improve on what it had before, a larger crab in an undersized shell might attempt to trade with the smaller crab. Sometimes two crabs can come to a mutual agreement to trade, but it’s more likely that the larger crab asserts itself against the smaller one and forces the trade by using its claws to coerce the smaller crab out of its shell.

A land hermit crab has an exoskeleton (a hard, protective covering), but the skeleton doesn’t offer sufficient protection on its own, which is why a hermit requires a shell for protection. The exoskeleton doesn’t grow as the soft tissues underneath it do, but a new exoskeleton is constantly developing underneath the hermit’s existing one. Every few months—the exact time period depends on temperature, food supply, and the crab’s age—a hermit sheds its old exoskeleton and discards or eats it. This process is known as molting.

During the molting period, the crab also regenerates any legs it has lost since its last molting (although the new legs may be smaller). However, because the muscles of the crab fasten to the inside of the exoskeleton, they aren’t as effective when the exoskeleton is soft, and the crab is unable to move properly until the exoskeleton hardens. This process takes roughly 5 to 10 days, and during this period, the molting crab is likely to go into hiding to protect itself from predators—including other hermit crabs.

If you notice your crab burrowing in the substrate of the terrarium, it may be doing so because it’s defenseless during its molting and quite vulnerable to dehydration as well as to predators. (Land hermits may also burrow in the substrate if their enclosure is too hot, but if the terrarium temperature is normal, and you notice your pet exhibiting this behavior, it’s likely in the process of molting.)

Don’t disturb your crab during molting, and keep it separate from all the other crabs in the enclosure for a week or two, until its new exoskeleton has been fully formed.

Wild land hermit crabs sometimes communicate by making a whirring, chirping sound. This noise can be heard during aggressive encounters in the hermits’ native environment but is seldom heard in captivity. Aggressive behavior is rare in land hermits—the only observed aggressive behavior in these crabs has been when one crab shakes or rocks another to drive it from a shell that the aggressor wants. Even when engaging in this act, neither crab is harmed.

However, soft, recently molted crabs are in danger when they’re among other crabs because the other crabs may attack and eat the molting hermit. This isn’t a form of social aggression: hermit crabs are scavengers, eating whatever they come across that’s edible—including other crabs.