Watch out for scrambling shells

Geetha Iyer

If you happen to be spending an evening near the ocean, close to estuaries or mud flats, or if you are in one of the islands that are such favourite tourist destinations, look out for shells that seem to be walking or scrambling about in the sands in shallow waters. This amusing spectacle will be that of the hermit crabs going about their life! They carry their ‘home’ with them, a shell into which they can quickly withdraw when in danger.

Not true crabs
coenobita-sp The hermit crab is a crustacean like the true crabs but is only distantly related to them. True crabs have a thick calcified carapace to cover and protect their bodies. Hermit crabs lack this covering over their entire abdomen. So to protect their tender abdomen which holds all the organs necessary for their survival, they seek refuge inside the empty shells of Gastropods1. Although they live inside these gastropod shells they are neither related to Mollusca nor do they make these shells.

Hermit crab diversity
Worldwide there are close to 1100 species of hermit crabs. They come in different sizes from about a quarter of an inch to the size of a soft ball; but the largest among them is the coconut crab, which also holds the distinction of being the largest terrestrial invertebrate. They also vary in colour, from red to brown to purple with dots, stripes, and patterns over their body. Some are terrestrial while more numbers of them are found in ocean waters, in the intertidal regions. The terrestrial ones will not survive long if left in water, just as the marine ones will die out of water. Whether terrestrial or aquatic they need water to breed. So the female terrestrial hermit crab moves into water to lay her eggs.

The author is a consultant for science and environment education. She can be reached at

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