Sea Shorts: Home-hopping hermit crabs

The photo below may be a tad blurry but your eyes aren't deceiving you. That is a tiny claw you see in the middle of that shell. This is a hermit crab. And yes, it's shell does look a little unusual for a crab.

 This fine hermit crab was spotted near a rock pool in Cornwall, England. Credit Samantha Andrews/Ocean Oculus

This fine hermit crab was spotted near a rock pool in Cornwall, England. Credit Samantha Andrews/Ocean Oculus

Unlike other crabs that you see running around rock pools (or at the seafood counter in your local fishmongers or supermarket), hermit crabs don't have a hard shell of their own. Instead they search for the perfect shell - not too big, not too small - to live in. Now their soft bodies are protected from those who would quite fancy a bit of crab for dinner. So why no hard shell? Well, part of the answer probably lies in the fact that despite their name, they aren't really crabs. Apparently they are more closely related to squat lobsters... which aren't really lobsters... that's taxonomy for you.

Well… I say shell… actually they aren't necessarily that fussy...

 This rather creative hermit crab was spotted in Saint Vincent and the Grenadines. Credit   Jenny Downing/Flickr  ( CC BY 2.0 )

This rather creative hermit crab was spotted in Saint Vincent and the Grenadines. Credit  Jenny Downing/Flickr (CC BY 2.0)

There are over 1,000 species of hermit crab in the world. Some are fairly small, some are pretty huge. Regardless of size, their chosen shell is not for life. Because they keep growing, hermit crabs eventually become too big for their borrowed homes and have to swap them for another.

The eagle-eyed among you may have spotted that my hermit crab's right claw is quite large compared to the one on the left (which is just peeking through). Hermit crabs often have one claw larger than the other. The larger claw is (unsurprisingly) used for defence - including blocking up the hole of the shell they have moved into - though this trick might not work for the crab in the glass jar.