CRITTER: Papa ‘i ako ‘ako’ a the Rainbow Swimming Crab

Scuba diving in a cave under the reef at 40 feet deep, then looking up at the dark ceiling of the cave right into the psychedelic eyes of a 7-inch wide blue and orange crab is quite exciting!

The first time I saw one of these fierce-looking predator crabs I was not quite sure what I was looking at. They walk slowly backward and guard their face with large powerful pincers that are covered with spikes. They look like they could be right out of a scene in an old Star Wars movie! As I approached the crab with my bright video lights on to get a close-up, the crab let go of the lava cave wall and swam rapidly through the open water to the sea floor then scurried away into a crack.

What is unique about the Hawaiian swimming crabs is their back legs are modified into swimming paddles that can be used to propel themselves through the open water quickly.

It is quite unusual to see a slow-moving rock crab all of a sudden become a fast-moving swimming crab. They swim backward so their powerful pinchers are always facing a potential predator.

The rainbow swimming crab is more active at night and can be found in calm bays out feeding on whatever it can catch. They will eat everything from live fish to sea urchins and they also feed on dead animals. The rainbow swimming crab is related to the larger Samoan crab that is often trapped for food here in Hawai‘i but papa ‘i ako ‘ako ‘a is not large enough to offer any food value.

You can see the rainbow swimming crab in action in my underwater educational movie series on my web at www.underwater2web.com and my YouTube channel at Underwater2web.

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Terry Lilley is a marine biologist living in Hanalei Kaua‘i and co-founder of Reef Guardians Hawai‘i, a nonprofit on a mission to provide education and resources to protect the coral reef. To donate to Reef Guardians Hawaii go to www.reefguardianshawaii.org.