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Want to avoid being eaten? The solution is simple: clone yourself, only smaller!
20 MARCH 2008

Large white disk with star shape in center Sand dollar, clone thyself!
Most of us know the sand dollar from the bleached, disc-shaped shells we find on the beach. But the sand dollar, which, like its sea star (AKA starfish) and sea urchin cousins belongs to the echinoderm class of marine animals, begins life as a tiny floating larva.

Like all animals, the goal of the sand dollar is to survive and pass its genes to a new generation. To accomplish that feat, it must survive adolescence in the open ocean, a tasty morsel in a sea of famished fish. Grazing and growing until it is about six weeks old, the sand dollar larva then settles to the ocean floor where it completes development in the sand and muck and assumes the familiar sand dollar form.

It turns out, however, that the sand dollar has evolved a nifty trick to confound predators when it is at a vulnerable early stage of life. As soon as the precocious age of four days, the sand dollar larva can quickly make a smaller copy of itself in response to the presence of fish mucous, a general cue that predators are cruising nearby. The sand dollar clone and the larva from which it buds are both Mini-Me versions of the original, about half the size, providing a less obvious target for the predatory fish that feast on sand dollar larvae.

Two black and white blobs, one large and one small, small appendages poking out from dark centers
The two larvae shown here will soon be juvenile sand dollars but differ greatly in size. The smaller larva is a clone resulting from exposure to predator cues. Photo: Dawn Vaughn, University of Washington

"We think that by reducing their size they also reduce their visibility to predators," explains Dawn Vaughn, a University of Washington graduate student and the lead author of a paper describing the sand dollar cloning phenomenon in the journal Science. "They are both smaller than sibling larvae that did not clone."

Large white disk with star shape in center rests on top of large grained sand
The familiar sand dollar we all know. Photo: US Fish & Wildlife Service

Why Make Mini-Mes?
Cloning is a common reproductive theme in plants and some animals and their larvae. Benefits, scientists think, might include a reproductive edge, better dispersal and an opportunity to salvage material lost when the animal undergoes metamorphosis. The new National Science Foundation-supported study, conducted by Vaughn and Richard R. Strathmann at the University of Washington's Friday Harbor Laboratories, suggests another possibility: making a smaller version of yourself can help you escape the jaws of predation.

Black and white image of small v-shaped organism, a small spherical shape budding from its center
Exposed to cues from fish predators, a sand dollar larva is caught in the act of cloning. The embryo-like "bud" was soon released and developed into a second smaller sand dollar larva. Photo: Dawn Vaughn, University of Washington

"Fish might have a harder time detecting the clones because they are smaller," Vaughn notes. "The assumption is small size would protect the larvae against visual predators."

To test the hypothesis, Vaughn placed individual 4-day-old sand dollar larvae into shot glasses and exposed the animals to fish mucous, the slimy coating found on fish skin. She observed that larvae exposed to fish mucous formed clone buds within 24 hours. Once the clones detached from the larvae, the original versions shrank to about half their pre-clone size. Larvae not exposed to fish slime did not clone themselves. (There is no word on the effects of exposure to Jack Daniels or Stolichnaya.)

Disk-shaped organism next to blob-like organism without a defined shape
At times, clones were not much bigger than an unfertilized sand dollar egg. Photo: Dawn Vaughn, University of Washington

Vaughn told The Why Files it is possible that there is a price to pay for cloning sand dollar Mini-Mes. The act of cloning, she says, "implies a big trade off." For instance, being smaller could make the animal more vulnerable to predators that use senses other than sight to detect prey. Also, reduced size could make it harder for the sand dollar to avoid predation later in life, as, in many species of animals, being bigger is one way to keep potential predators at bay. How the smaller sand dollars fare in the benthos, the silty or sandy layer at the bottom of the sea, is unknown. Answering that question, says the University of Washington biologist, is the next step in her research.

-Terry Devitt

Related Why Files
Cloning Conundrum
Brave New World
Dolly: Ten Years Later
The Ethics of Cloning

Bibliography
• Predators Induce Cloning in Echinoderm Larvae, Dawn Vaughn and Richard R. Strathmann, Science, 14 March 2008


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