Snorkeling with a Hawaiian Box Crab is Lots of Fun!

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Rough Box Crab at Kaiona Beach Park Oahu while Snorkeling from Kenny Muise on Vimeo.
 

You never know what you may see when snorkeling in Hawaii and one of the greatest thrills is to see something that you’ve never seen before.This past weekend we did our snorkeling at Kaiona Beach Park on Oahu. It’s a chill beach with some great snorkeling and a great place to see turtles on most days. It was gorgeous this weekend and we had a blast.

Towards the end of the day I saw the girlfriend in a particular spot for a little bit so I swam back in to where she was. She told me that there was a huge crab that she had seen but had lost because it had burrowed itself into the sand. While I was moving rock and some coral around, this little Hawaiian box crab popped out and started to make a break for it. (The girlfriend swears there was a larger one too, but we just got video of the smaller one.)

So, I decided to take the video of the small box crab. At the beginning of the video you can tell from the top how much it looks like a rock or piece of coral on the sand floor, particularly when it burrows itself into the sand. And if that wasn’t enough, you can surely tell how well this little box crab is camouflaged because you see it run away, burrow, and disappear next in the sand like a ghost! Notice how the smart little bugger gets himself right next to an actual piece of coral into the sand and blends right in.

I’m telling you, snorkeling reveals an entire world to discover and marvel over. It doesn’t always have to be turtles and dolphins. Sometimes, the smallest (and funniest things) leave you in awe.

Hawaiian Box Crab

  • Scientific Name: Calappa hepatica or hepatic box crab
  • Its oval carapace is mottled tan and white and is covered with raised knobs.  This species is 3 to 4 inches wide and is most commonly found from the water’s edge on shallow reef flats to depths of 150 feet.

From the Waikiki Aquarium:

Box crabs are efficient burrowers and dig into the sand rapidly:  alternating, they push the sharp back edge and sides of the carapace down and backward into the sand, then shove the flattened claws forward against the sand.  Once buried, the claws are held tightly against the front of the body and the incoming respiratory water current is filtered through hairy inner edges of the claws.  Ridges between the eyes form two tube-like channels through which water is drawn down to the gill chamber within the carapace.

The out flow of expired water can be seen as a small fountain of water arching from the front of the crab’s head.  The armored body seems well designed for defense as well as digging.  The eyes, antennae, and respiratory openings are located on the highest points of the carapace, allowing the rest of the crab’s body to remain hidden from predators.  The crab’s mottled or sandy color blends with the background and gravel in which it lives, and makes the animal difficult to see unless it moves.  The wing-like extensions of the carapace extend over the walking legs.  And, the large flattened claws protect the head and delicate feeding structures from predators and abrasive sand.

The claws or chelipeds of the box crab are specialized tools for feeding on snails.  The right and left claws are equally sized, but of different design.  The right claw is equipped with a pronounced hook and heavy crushing knobs and functions somewhat like a can opener.  The hook is inserted into the opening of a snail’s shell and the edge of the shell is crushed between the knobs.  The box crab snips more and more of the shell until the soft body of the prey is exposed.  The left claw is more delicate, with a pointed, moveable pincer that is used to pluck the flesh from the crushed snail.  Some of the most common prey includes auger snails that also live in sandy-bottomed areas.  Box crabs, like most crustaceans, are probably most active at night, but can sometimes be seen during the day.

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