Hermit Crabs, Facts And Photos - National Geographic

Common Name: Hermit crabsScientific Name: PaguroideaType: InvertebratesDiet: OmnivoreAverage Life Span In The Wild: Up to 40 yearsWeight: 1.3 ounces to 11 pounds

What is a hermit crab?

There are over 800 species of hermit crabs worldwide, and almost all are ocean dwellers—though people are likely most familiar with the dozen semi-terrestrial species, called land hermit crabs, which are often kept as pets. There’s only one freshwater hermit crab, Clibanarius fonticola, which is native to Vanuatu.

Hermit crabs are omnivorous scavengers, eating microscopic mussels and clams, bits of dead animals, and macroalgae.

These crustaceans have been misnamed for two reasons: First, they’re not true crabs, like blue crabs, in that they don’t have a uniformly hard exoskeleton and can’t grow their own shells. Instead, hermit crabs have a hard exoskeleton on the front part of their bodies but a soft tail on the other half, which they protect using the discarded shells of other animals, like whelks. They’re more closely related to certain kinds of lobsters than to true crabs.

Hermit crabs have a curled tail with a hook that enables their bodies to fit inside these borrowed shells. Sometimes when a new shell turns up, hermit crabs will form a line, biggest to smallest, to see which animal fits the new shell. The next smallest will take that crab’s hand-me-down home, and so on.

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This behavior of sheltering in shells alone is actually what gives them their name. But hermit crab is a misnomer for these social crabs, which sometimes live in large groups of a hundred or more in the wild.

Mating and reproduction

Hermit crabs vary in their mating habits. The Caribbean hermit crab, for example, lives in wetlands, but when it’s time to mate, will head for the seashore in huge masses.

Amid the chaos, males and females find each other, coming partly out of their shells so the male can transfer a sperm packet to the female, which fertilizes her eggs. She later carries her eggs to the water’s edge, where contact with seawater causes the eggs to burst and the larvae to float away.

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The larval crabs will live on ocean plankton, molting through several stages before acquiring a shell of their own and coming back to land.

Threats to hermit crabs

Hermit crabs don’t breed well in captivity, and so the numerous land hermit crabs seen in pet stores and tourist shops are taken from the wild, which is considered an unsustainable practice.

Plastic pollution is also a problem for hermit crabs, which often mistake a plastic bottle cap or container for a new home. A 2020 study in the Journal of Hazardous Materials found that around 570,000 hermit crabs die annually from getting caught in plastic debris on two tropical islands in the South Pacific.

When these trapped crabs die, they release a pheromone signal to other crabs that there may be a shell available, which lures even more crabs into a death trap.

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Hermit Crab vs. Horse Conch

The 11-pound horse conch might seem invincible against most threats. But the tables are turned a bit when hermit crabs stage an invasion of the dinner-snatchers.