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Underwater Photo of the Week - March 19, 2018

Sleepy Sponge Crab

POTW March 19, 2018

Hi Gang!        

This week’s POTW is a shot of a crustacean commonly known as the sleepy sponge crab. Spotted first by Captain Steve, the crab was sitting on a ledge inside of one of Maui’s iconic dive sites, 1st Cathedral. The Cathedrals (1st and 2nd) are actually located off Lanai, but they are dived by Captain Steve and the Hawaiian Rafting Adventures gang on a regular basis.

The bulky bodied sleepy sponge crab, Dromia dormia, is one of roughly 190 species of crustaceans that occur in Hawaiian waters. A typical adult has a carapace that is not quite as wide as a soccer ball. Males tend to be slightly larger than females. Individuals usually seek cover in the latticework of the reef during the day, and tend to be more active under the cover of darkness.

The sleepy sponge crab and its close relatives are often referred to as “true crabs” as opposed to other crustaceans such as hermit crabs that have the word “crab” in their common name. True crabs are equipped with large, clawed front legs used in food gathering and defense. True crabs are also equipped with a natural hard shell, and do not depend on the abandoned shells of mollusks for their protection as is the case with hermit crabs and their close relatives.

In addition, true crabs have four pairs of legs that are used for walking across the substrate. However, in the case of the sleepy sponge crab, the two pairs of rear legs serve in a different capacity. These shorter legs point upwards instead of toward the sea floor, and they are used to hold pieces of sponge over the top of the crab’s carapace. You can see the pieces of yellow-green sponge in the photo that illustrates this [iece. The chunks of sponge are used primarily to help the crustaceans camouflage themselves. However, sleepy sponge crabs sometimes feed on the sponges.

When sponges are scarce, sleepy sponge crabs have been seen using animals and items that range from crown-of-thorn sea stars to the soles of discarded tennis shoes to help mask their presence.

I hope you enjoy this week’s POTW!

See you next week,


See you next week,