Monthly Archives: July 2015

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

 

 

Lousy weather paves way for some unexpected fun

Published July 20, 2015 in the “Ocean Watch” column, Honolulu Star-Advertiser ©2015 Susan Scott

flats

Pancake Creek’s sand flats stretch at least a mile. ©2015 Susan Scott

Pancake Creek, Queensland, Australia » One good thing about crummy sailing weather is that it channels me to areas I might not otherwise visit. I had heard about this anchorage, but it’s up a creek and that didn’t match my vision of sailing the Great Barrier Reef.

That’s what’s wrong with preconceived notions. Besides offering a sheltered and scenic haven for Honu, this estuary ecosystem hosts the wildlife of my dreams. It also has the best national park sign I’ve ever seen.

After several blustery days at Lady Musgrave Atoll, Craig and I set sail for the mainland’s Pancake Creek. With no road access, the large creek is a boats-only haven inside Eurim­bula National Park. We share the ample anchorage with 22 other boats, as well as sea and land birds from cormorants to kookaburras.

That’s enough pleasure right there, but when the tide goes out here, it’s a whole new world. The receding water reveals a mile or more of golden sand flats with tiny ponds, rippling streams and sand art in intricate patterns.

sand

These masterpieces of nature are even more fascinating because of the creatures the ocean brings in at high tide and leaves behind in the ebb. I’m the first to take my dinghy ashore each day at low tide to check out the new arrivals.

I never see rays but I see their beds, each pit the precise shape of the ray that rested there. An astonishing number of rays sleep (and likely eat crabs) on the sand during high tide, their beds at low tide forming puddles that stretch across the flats.

raybed

Inside these and other miniature pools, stranded until the water returns, I’ve found gelatinous snail eggs, green polychete worms 2 feet long, white sea pens, red and white sea urchins and a bright blue mystery creature that my best guess is entwined ribbon worms.

Mystery worms_sm

On the sand around the puddles are millions of bubbler crab food balls and soldier crab armies, marine animals so remarkable they deserve a column of their own (next week).

One hiking trail around the creek led us to a remote beach camp containing tents, kitchen gear, kayaks and life jackets, all provided by the national park service. A large sign explains that the camp is for public use, and lists some do’s and don’ts, mostly safety advice.

The sign ends with this: “We cannot guard against stupidity, however, if you are stupid and kill yourself, we will fly your body out and that will be the end of our obligation. Have a nice day!”

When I set out for the GBR, I never dreamed I would sail up a creek and have so much fun. Thank heaven for lousy sailing weather.


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

©2015 Susan Scott

 

Atoll’s abundant wildlife, towering trees worth a visit

Published July 13, 2015 in the “Ocean Watch” column, Honolulu Star-Advertiser ©2015 Susan Scott

Lady Musgrave Atoll, Great Barrier Reef, Australia >> Last week Craig and I sailed to this atoll, 50 miles offshore from Bundaberg. Even though the trades were still blowing at 25 knots, bringing with them the usual waves and rain, residents assured us that the anchorage inside the atoll was safe. As good as the wildlife was in the marina, I wanted coral, and we set sail.

The trip was not what I expected.

After entering the narrow channel into the atoll, we dodged a leaping manta ray and multiple coral heads and dropped anchor. But even though the water was relatively calm inside the circular reef, we had a problem. Honu’s anchor chain had so rusted in our absence that we couldn’t trust it to hold the boat.

An hour later we had a second anchor off the bow and a safety line tied to the first. With the boat triple-leashed to the bottom, we launched the dinghy to visit the atoll’s famous island, a circle of land that takes about 20 minutes to walk across and an hour to walk around. Lady Musgrave’s island is remarkable because it holds an underground lens of fresh water that supports a mature Pisonia forest.

The Pisonia is not your average tree. Historically, the species dominated coral atolls throughout the Pacific, but after centuries of human habitation, introduced insects, rats and weeds, Pisonia forests became rarer than ever.

Today at Lady Musgrave Atoll (as well as at our own Palmyra Atoll), it’s possible to see Pisonia forests in all their grandeur.

The trees grow to 100 feet tall and have smooth, light gray trunks, some as big around as a compact car. The towering hand-shaped leaves form a whispering canopy that blocks wind and discourages undergrowth, yet the ashen trunks give the forest a light and airy feel. If ever there existed an elven glen filled with sentient trees, this is it.

From a windy, wave-washed beach we entered the forest on a park path and were instantly transported to a shelter of green and gray. When rain fell from passing squalls, the trees kept us mostly dry, their layers of leaves making music of the drops that trickled through.

Fearless, rust-colored rails (birds related to coots and moorhens) skittered here and there, eating insects and tiny crabs from the forest floor.

On the leeward side of the island, we emerged from the tree haven to see humpback whales breaching just outside the reef. It’s winter here in the Southern Hemisphere, and the Southern Ocean’s humpback whales have migrated to waters inside the Great Barrier Reef to mate and give birth.

It’s true that Lady Musgrave Atoll protects boats from rough seas, but at high tide, rowdy waves rose over the coral walls and rocked the boat so hard I got queasy. That, plus the shaky anchor situation, and weather too cold to go snorkeling, made us decide to leave Lady Musgrave for a more sheltered part of the park on the mainland.

As we sailed from the atoll, a white-bellied sea eagle hovered overhead and a pod of dolphins raced to the boat, all seeming to bid us farewell.

No, my experience at Lady Musgrave Atoll was not what I expected. It was better.


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

©2015 Susan Scott