To unusual species of crab, life’s just a series of spheres

Published July 27, 2015 in the “Ocean Watch” column, Honolulu Star-Advertiser ©2015 Susan Scott

flats

The round, blue-and-white body of a soldier crab resembles a glass marble. ©2015 Susan Scott

On some Australian beaches magic occurs twice a day. During ebb tides the water leaves behind a smooth sandy surface that slowly transforms. A few hours later that formerly flat plane contains millions of sand balls, so dense their sides touch.

sand

Clearly something was making the balls, but I could not see what.

I initially thought the rollers were soldier crabs, described in one guide as “one of the most loved crabs of Australia.”

How could you not love them? The crabs have eight long, tan legs with dark red joints, and their round blue-and-white bodies resemble glass marbles. Juveniles are about the size of small marbles; adults are jumbo marbles, about an inch across. Soldier crabs walk forward rather than sideways, and look for all the world like tiny wind-up toys.

Their military moniker comes from platoons the crabs form while foraging on Pacific and Indian Ocean tropical beaches. Individuals gather by size and sex, and scurry across the sand in troops that change shape like schools of fish.


Soldier crabs at low tide, Eurimbula National Park, Pancake Creek, Queensland Australia.

The crabs’ fluid group advances confuse bird predators and charm human observers. Step too close, though, and the parade is over. Alarmed soldiers dig down and disappear.

During marches, each crab scoops wet sand with its front claws, rolls it against its mouth parts to extract nutrients and discards the sand in a ball.

soldiers

Soldier Crabs ©2015 Susan Scott

Soldier crabs entertained me for hours, but their sand balls were bigger and less compact than the millions of perfect pea-size spheres that blanketed the tidal flats.

Curious, I took a beach chair from my sailboat to the beach at the start of an ebbing tide and set up an observation post. For 30 minutes I sat staring at the smooth sand in front of me. Nothing. But when I turned around … balls! There lay dozens of perfect spheres fanned out around tiny holes.

A flash of movement revealed a flat, sand-colored crab about the width of a squashed pea. And when I moved to take a picture – mystery solved. The crab curled up, froze and “played ball,” perfectly mimicking its own castoff sand bubbles.

sand bubbler

Sand Bubbler Crab ©2015 Susan Scott

I say “bubbles” because the crabs in the invisibility cloaks are called sand bubbler crabs. Twice a day, with soldier crabs, they work like mad filtering sand and sifting silt before the next tide comes in. When that happens, both species dig holes, hunker down and wait for the next feast.

Even after solving the mystery of the sand balls, I still think it’s magic. Magic by nature is the best kind.

Check out bubbler crabs at work at bit.ly/1JeFZ0o.

waiting

Susan waiting for the tide to go out.©2015 Susan Scott


Marine biologist Susan Scott writes the newspaper column, “Ocean
Watch”, for the Honolulu Star-Advertiser, www.staradvertiser.com

©2015 Susan Scott