Hermit Crabs

By Alex Scott

Every living organism on planet earth evolved in unique environments and inhabits its own ecological niche, perfectly suited to whatever ecosystem its ancestors found itself in. But one family of organisms is unique, an evolutionary one-of-a-kind.

The hermit crab is a crustacean and a misnomer: it is not actually a true crab, which has a thick chitinous exoskeleton that even humans need a special tool to crack open. Hermit crabs, on the other hand, have a hard exoskeleton around their front and legs but are soft around their abdomen, requiring them to use gastropod (usually sea snail) shells to protect themselves from predators and the environment. There are many other examples of symbiosis (life forms working together) in the natural world, but no other organism relies on another to make their protection for them. What makes the hermit crab so unique from not only its crab relatives, but every other creature on the planet?

The first crustaceans evolved about 500 million years ago in the Middle Cambrian period, when life in the world’s oceans was rapidly diversifying due to environmental and biological causes. The first gastropods and their signature coiled shells also appeared in this era. Almost 300 million years later, the first crab-like organisms appeared in the Late Triassic period, and soon after, true crabs in the early Jurassic. This group of crab-like organisms, named Anomura, is named for their asymmetrical tails, and includes the ancestors of the hermit crab. Approximately 150 million years ago, these crabs used that asymmetrical tail to enter a coiled gastropod shell, which due to its shape is inaccessible to other crabs, and wear the shell as protection.

Although relying on an entirely different group of animals for survival does not seem like an easy way to live, hermit crabs have survived and spread for the past 150 million years doing just that, now comprising over 800 species. Biologists are still studying why exactly this ancient group of animals decided that using other animals’ shells was more beneficial, but it’s worked for them so far. Some octopuses use shells as temporary protection from predators, but no other creature uses another creature’s shell full time. And that’s not the only animal hermit crabs use as protection: some hermit crabs attach sea anemones to their shell to protect from predators overhead, while the anemone gets access to food as the crab walks along the sea floor. In this way, the hermit crab is truly unique in the animal kingdom.

Because it cannot modify the shell it inhabits, a hermit crab must change shells throughout its life as it grows. Females are especially reliant on shell size, because they must find shells large enough to hold their eggs during breeding, which usually occurs within the first year of life. Depending on the species, hermit crabs can live from 10 to 70 years, and are always growing, albeit very slowly by the end of their lives. This means they are always on the lookout for a larger shell to move into.

Hermit crabs are generally social creatures and gather in large groups in order to form a vacancy chain, a term originally coined by social scientists to describe humans’ distribution of resources. When a hermit crab decides it needs a larger shell, it will look for an empty one in the area. If that shell doesn’t quite fit, it will wait near the shell until other hermit crabs with similar ideas walk by. Then the hermit crabs will physically line themselves up in descending order of shell size and simultaneously leave their shell for the crab behind them and take the bigger shell from the crab in front of them. Not every swap goes perfectly, and sometimes a hermit crab will end up with a shell of the same size or no shell at all, but most of the time every crab gets a bigger home that will last them until the next shell swap. It’s the perfect way to ensure that every animal has a home without wasting a single shell, and although it’s only recently been observed in the wild by biologists, it’s not hard to imagine this system being used by hermit crabs long in the past.

Many creatures throughout the animal kingdom are capable of showing ingenuity and creativity in their natural habitats, but no animal is creative like the hermit crab. Using another animal’s shell as a home, and trading homes between fellow hermit crabs, is a skill like no other. So when you are walking on the beach admiring the beautiful shells that wash up on the shore, remember to leave as many as you take for hermit crabs of every size!

See how hermit crabs swap their shells with each other

This article appeared in the Beachcombing Magazine September/October 2021 issue.

No live shelling: Be sure shells are empty and sand dollars, sea stars, and sea urchins are no longer alive before you bring them home.