Meet the hermit crab

Photo by warrenski. Retrieved from Flickr on 02-23-2023.

My partner and I are in the process of buying a house.

It is not fun.

So, this week, we’re escaping to the world of an animal that always has its house with it: the hermit crab.

A widespread species for aquarium tide pool exhibits, there are over 800 species of hermit crabs worldwide. The smallest is just a fraction of an inch, while the largest reaches the size of a coconut! Most of these live in the ocean, typically in tropical regions.

There are around a dozen terrestrial hermit crabs. These species also live in tropical regions, and although they live on land, they also need access to some wetness. They breathe with reduced gills, which they keep wet through the humid air or stopping by a puddle.

Not a crab

Or at least, hermit crabs are not true crabs.

What makes them different? Hermit crabs don’t have a hard exoskeleton over their entire body. The front pairs of legs (the ones sticking out of their shell) have a hard exoskeleton; the back pairs (the ones in the shell) don’t.

Photo by Sergio Boscaino. Retrieved from Flickr on 02-23-2023.

Instead, those back pairs of legs are much softer. If you ever see a hermit crab changing shells, you’ll see that they are actually pretty long and have a back end that looks like a lobster.

The ‘hermit’ part of their name is also incorrect. Some hermit crab species can be found in groups of 100 or more!

There’s no place like home

Since hermit crabs have a squishy back end, they need a way to protect themselves. So, they make their homes in discarded shells.

Hermit crab bodies are well-suited for this living arrangement. Their abdomens twist to fit nicely in the spiral snail shells they live in. Once inside, a hermit crab can anchor itself to the shell with a few pairs of legs and its tail. It also has a large appendage called a uropod that anchors itself to the center post of the shell.

Hermit crab without its shell. Photo by Scott Kinmartin. Retrieved from Flickr on 02-23-2023.

As hermit crabs grow, they need to find larger shells. After all, it’s not great protection if you can’t fit inside! Hermit crabs won’t kill snails to take over their shells; they’ll wait until the snail is already dead to take over. They find dead and dying snails by following the trail of chemicals that comes off a dying body.

But not just any shell will do! A hermit crab carefully inspects a potential home before taking it over. A hermit crab will ensure the potential shell is sturdy and moveable using its eyes and feelers. It will also inspect the shell’s interior since it needs to be big enough to get inside quickly.

Shell conga line

Finding the right-sized shell is extremely important. Hermit crabs will even fight each other to the death over similar-sized shells. However, fighting is very low and even nonexistent when there are many different sizes of hermit crabs in one area. Fighting over a shell that is way too big for you makes no sense!

Sometimes, when there are many hermit crabs of varying sizes, a shell swap will occur. The hermit crabs will line up in size from smallest to biggest. As the larger hermit crab vacates its old shell for a bigger one, the next-sized hermit crab is right there to take its place.

Adding some flair

Some hermit crabs decide to decorate their shells with anemones. This results in a mutualistic relationship between the crab and the anemone.

The hermit crab gets protection from predators through the anemone’s sting. Being covered with anemones also gives the hermit crab added camouflage. The anemone gets to eat scraps of food that the hermit crab drops and gets a free ride to new areas in the ocean.

For the jeweled hermit crab, these anemones are special. When they change shells, the hermit crabs take the time to take their anemones with them! I’ll leave you with a video of this very thing.


National Geographic

Seattle Aquarium

Smithsonian’s National Zoo

Marine Science Institute Blog

Discover Wildlife

Thought Co.

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