Under threat, sand dollar grows some small change
The odds of growing up aren't good for baby sand dollars. Smaller than the head of a pin, the larvae drift in the ocean — easy prey...
Seattle Times science reporter
The lowly sand dollar
What are they? As echinoderms, they are closely related to sea urchins, sea stars and sea cucumbers. The white shells found on shore are exoskeletons of dead animals.
What do they eat? Detritus, small crustaceans, copepods and algae.
Where do they live? In sandy or muddy areas, from the intertidal zone to waters 130 feet deep.
How long do they live? Up to 10 years.
What eats them? Sea stars, crabs and sea gulls.
Source: Monterey Bay Aquarium; San Francisco State University
The odds of growing up aren't good for baby sand dollars.
Smaller than the head of a pin, the larvae drift in the ocean — easy prey for anything with a mouth.
But a University of Washington graduate student has discovered the tiny animal has a surprising survival strategy: Faced with the threat of being gobbled up, it makes like Dr. Evil from the "Austin Powers" movies and clones itself.
The resulting "mini-me" may escape hungry fish because it is even teenier than the original — and harder to see.
"If you are eaten, but the smaller version of you survives, you're still a winner from an evolutionary standpoint," said Dawn Vaughn.
Vaughn's discovery is reported in this week's issue of the journal Science, a distinction most researchers rank higher than an appearance on "Oprah." It's unusual for a student to make the cut for one of the world's top science publications, but Vaughn's doctoral adviser urged her to aim high.
"I expected the paper to be rejected in record time," Vaughn said, laughing.
But what editor could resist a tale of fish slime and plucky invertebrates?
"They really are cool," said Vaughn, whose fascination with the tango of predator and prey was inspired by nature shows. She never expected to fall in love with creatures so small it takes a microscope to see them.
Familiar inhabitants of Washington's subtidal zone, sand dollars start life through the chance encounter of sperm and egg, simultaneously released into the water by mature adults. The larvae free-float for about six weeks before metamorphosing into miniature sand dollars that settle in colonies and eventually grow to full size.
"They're so tiny, we call them sand dimes," Vaughn said.
The white shells that wash up on the beach are the creatures' external skeletons. Living sand dollars are covered with velvety, purple spines used to grab food particles.
Vaughn knew many other marine invertebrates shift their shape to avoid being eaten. Colonial animals called bryozoans grow spikes when voracious sea slugs crawl across them. Barnacles take on a bent posture to repel snails. Vaughn's previous research showed periwinkle larvae narrow their shell openings to keep out marauding crab larvae.
Sand-dollar larvae, which look nothing like the adults, have multiple, spiky arms. Vaughn suspected those spikes might lengthen when predators were in the neighborhood.
She set out to test the notion at the UW's Friday Harbor Laboratories on San Juan Island, where sand dollars are in plentiful supply.
"They are pretty much the lab rats of larval biology," Vaughn explained. "They're really easy to rear."
But how to fool the infant sand dollars into thinking a predator was near?
Vaughn decided to add fish slime to the shot glasses where she kept her larvae.
Scraped from the back of a species called Dover sole — or slime sole — the mucus might be a chemical cue the larvae would recognize and respond to, Vaughn reasoned.
Still, she was floored when she came into her lab the next day and saw the larvae budding off clones.
"I remember saying out loud: 'It worked!' "
The larvae that were not exposed to fish slime did not clone themselves.
Over the next six weeks, she tracked development of the clones and the original larvae.
One of the most startling things was that by creating clones, the larvae were actually shrinking themselves down instead of trying to look bigger.
"The general assumption is that bigger is safer," said UW marine biologist Richard Strathmann, Vaughn's co-author and adviser. "But on the other hand, if there are a lot of predators out there that need to see their prey to eat it, being smaller might be safer."
The theory still needs a lot of work, both researchers agreed.
Thomas Ebert, an expert on marine invertebrates who discovered some sea urchins can live more than 100 years, cautioned against reading too much into the evolutionary significance of the clones.
"It could simply be a response to a harsh stimulus in the environment," said Ebert, professor emeritus at San Diego State University.
Vaughn was heading back to her Friday Harbor lab this week, where she'll try to figure out how fast the larvae can bud off clones, and whether the clones really are more likely to escape being eaten by fish.
She'd also like to get out of the lab more to enjoy the splendor of the San Juans. But with so many questions yet unanswered, that may not happen.
"I spend most of my time looking through a microscope."
Sandi Doughton: 206-464-2491 or email@example.com
Copyright © 2008 The Seattle Times Company
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