Ka Mo‘olelo Moana: Sleepy sponge crab lumbers around as if sleepwalking

While exploring Hawaii’s coral reefs one will discover a world bustling with activity.

Colorful fish flitter near the coral eating, cleaning and perhaps even hiding from perceived danger. Spiny sea urchins dot the reef, undulating their spines in a rhythm that seems to match the movement of the sunlight shining down from above. Tiny animals called coral polyps live grouped together in colonies and numbering in the thousands. They make up a single coral head. In Hawaii, there are about 50 different species of corals, and together they form a coral reef.

Hawaii’s coral reef is a strong stony matrix that provides protection to countless creatures as well as a hunting ground for others. Hiding in the dark nooks and crannies are lobsters and crabs, all known as crustaceans. They are nocturnal and will patiently wait for the sun to set before they venture out to forage. Makua o ka lipoa, or sleepy sponge crab (Dromia dormia), is one of about 190 species of crustaceans identified in Hawaii and is one of the largest.

This bulky crab is known as a true crab because it doesn’t require the use of a shell for protection as its cousin the hermit crab does. True crabs have large front legs that are claws and used as pinchers to help them to gather food. Claws are also a great defense against a predator or another intruding reef creature. Crabs have four pairs of walking legs but on the sleepy sponge crab the two rear sets of legs are shorter and point upward. The purpose of these four legs is to hold a sponge to the top of its body like a mule carrying a pack. They will carry around this sponge to help camouflage their body but from time to time, when food is scarce, they may nibble on their sponge. If no sponges are available these crabs have been seen carrying rubber slippers and even a crown of thorns sea star perfectly trimmed to fit.

The sleepy sponge crab moves slow and awkward on just two sets of legs from place to place, lumbering around as if it were sleepwalking. They are omnivores feeding on whatever they find and aggressively defending a resource when threatened. Instead of skin, this crab is encased in a hard fuzzy external skeleton called an exoskeleton that acts as a protective suit of armor. However, unlike our skeleton that grows with us, a crustacean must shed this skeleton as it grows. Molting is an extraordinary process when the sponge crab will start to absorb water causing it to swell. As the crab becomes too large to fit inside its skeleton, the carapace, or back-side, separates from the rest of its body enabling it to literally crawl out of its skin. The first thing the crab will do is retreat into a good hiding spot until its new exoskeleton hardens because it will be soft and vulnerable for a few days.

If you go out to look for this crab, you will need to search the dark caves or under coral ledges. Look carefully because unless they move their camouflage makes them very hard to see. If you can find one, they are pretty fun to watch but remember that they are aggressive so when they show you their claws it’s best to keep your distance.

* Erin Iberg is the community education manager at the Maui Ocean Center. “Ka Mo’olelo Moana,” or “The Ocean Story,” is a monthly column submitted by the Maui Ocean Center. It is open from 9 a.m. to 5 p.m. daily in Maalaea. For more information, call 270-7000.