Ocean Watch

Monday, February 2, 1998

Mole crabs love feasting
on man-of-war tentacles

DURING a recent visit to Midway Atoll, a worker there told me an odd type of jellyfish had been washing up on the beaches.

He looked in several books but could not find the creature.

"They have bubbles like Portuguese man-of-war, but no tentacles."

"Mole crabs probably ate the tentacles," I told him.

"I didn't see any crabs."

"You can't see mole crabs. They live beneath the sand."

He raised an eyebrow.

"Well, if these things are just Portuguese man-of-war with their tentacles eaten off, they're still really weird ones."

He was right.

What I found lining the beach at the high-tide line were great rafts of Portuguese man-of-war stuck together to form mats the size of breadboards. And I mean stuck together. They were so dense I could pick up the entire raft from one side and it held together.

Normally, handling such a mass would be foolhardy, given the sting their tentacles can deliver.

But the worker was right about the tentacles: They were completely gone -- nibbled to nubs, I suspected, by hungry little mole crabs.

Mole crabs are common on Hawaii's beaches, but most people rarely see them. These sand-colored, 1-inch-long crabs live just under the sand near the surf line.

The crab digs backward into the sand until only its antennae and stalked eyes remain above the surface. When something good to eat drifts past, the crab grabs it and hauls it under the sand to eat.

Rarely, when something really good drifts past but is too large to drag down, the crabs emerge for a feast. I saw this once on Kailua Beach where several mole crabs munched on a dead file fish lying at the water line.

In Hawaii people often call these crabs sand turtles, but that name is confusing. Once, I used this term to a local person who thought I meant the reptiles. The conversation quickly became absurd.

The best way to find mole crabs is to walk along the surf line at dawn or dusk and watch for little V's in the receding water. This is the shape the crabs' antennae make as the water rushes past.

If you're patient, you can sometimes see the crabs washed out of their hiding places by the force of a wave, then quickly re-enter the sand. They dig holes with such speed that only the most observant watcher will see it.

Such efficient backward burrowing is aided by the crabs' ability to flex their abdomens, a trait not seen in most crabs.

Mole crabs are beneficial because they clean the beaches of dead animal matter, including Portuguese man-of-war tentacles.

After strong onshore winds, look for beached Portuguese man-of-war with their tentacles stuck in the sand. A closer look will usually reveal a crab beneath the surface winding up the long blue string.