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Creature Feature: Hermit Crab

by Brooklyn Bridge Park Conservancy 

Sneak a peek into the Environmental Education Center’s touch tank and you may notice what appears to be a sea snail making the rounds on nimble legs. Move in for a closer look, and in a flash the cautious animal will plunk down and vanish into its shell. Now all you may see is a pair of antennae gently moving in the water. Wait…a snail with legs and antennae?

The tips of two delicate antennae poking out from a sea snail shell are the giveaway that an empty shell has been repurposed as a hermit crab’s home. Stay still for a moment and the curious animal may sense that you pose no danger and re-emerge. The next thing you may notice is a pair of smaller antennae, called antennules. Antennae and antennules serve as sensory organs. Then look for the eyes, which are held on short stalks. Note the two pincers—the chelipeds. These sizable claws are used for defense and food handling. One claw is much larger than the other. It serves to close the entrance when the animal retreats into the safety of its adopted home.

While it is possible to pick up one of the aquatic hermits, the animal will instantly disappear deep into its shell. Continue watching and the crab may start to move around again on its two pairs of walking legs, which are attached to its thorax. What you will never see is the crab’s soft abdomen and the stubby fourth and fifth leg pairs (bringing the total number of legs to ten), which the hermit uses to hold onto its shell.

Hermit crabs are invertebrates. They do not have backbones. They have an exoskeleton, which takes the shape of an outer shell. However, hermit crab shells offer limited protection against predators. That’s where the sturdy sea snail shells come in. Over time, as hermit crabs get bigger, they outgrow their adopted shells and have to find a larger one. In the wild, sea snail shells, especially whelk shells, are hot commodities. Hermits will mill around and line up by size when they come across an abandoned one. Once a hermit finds the empty shell to be a good fit, it quickly moves in, freeing up its former shelter for a smaller crab. A moving day involving up to 20 hermits has been observed.

Hermit crab in a tank

Hermit crabs are omnivorous scavengers. In the Ed Center they are fed small chunks of mussel, fish, and bits of lettuce. So, when you visit during open hours, look closely at the snail shells in the touch tank. Can you see one with a pair of delicate antennae sticking out?

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