Sunday, 8 June 2008

Creature clones itself to avoid becoming fish food

  • 18:00 13 March 2008
  • NewScientist.com news service
  • Catherine Brahic
Bottom-dwelling sand dollars are related to sea urchins and sea stars (Image: GNU licence)
Bottom-dwelling sand dollars are related to sea urchins and sea stars (Image: GNU licence)
Cloned larvae (right) are smaller than non-cloned larvae (scale bar: 0.2 mm) (Image: Dawn Vaughn)
Cloned larvae (right) are smaller than non-cloned larvae (scale bar: 0.2 mm) (Image: Dawn Vaughn)

It's a novel escape route that makes a mockery of the status of the individual: if you run the risk of being eaten, just clone yourself. That is the approach taken by the larvae of sand dollars – marine animals related to sea urchins.

Fish are voracious predators of sand dollar larvae. Dawn Vaughn and Richard Strathmann of the University of Washington, Seattle, found that when they put four-day-old sand dollar larvae in a tank with fish mucus, the larvae cloned themselves.

They did this by either splitting in two or by producing a small bud which detached itself and developed into a new larva. Either way, the clones were smaller than the original larvae. Sand dollars did not clone themselves if there was no fish mucus in the water.

Vaughn and Strathmann believe the larvae sense the mucus, interpret it as a sign that fish are nearby, and respond by producing clones.

Better odds

There is no parental care in this species. "[The mother] is on the seafloor," says Vaughn. "She has no idea what types of risk her offspring are going to meet, so the larvae respond to cues from predators."

Cloning gives the larvae's genes a greater chance of survival in two ways. With two copies around one is statistically more likely to escape predation. Also, the smaller clones are less likely to be detected by predators.

"Studies indicated there is an optimal size below which fish do not detect their prey," says James McClintock, an ecologist at the University of Alabama at Birmingham, US, who studies how "things that can't run away from predators defend themselves".

Cloning does have a cost, however. Vaughn and Strathmann found that cloned larvae developed into smaller sand dollars: just before metamorphosing into adults, the clones were on average one-third the volume of non-cloned individuals.

Unique response

This could make it more difficult for clones to survive as adults, which only reproduce sexually, but suggests that overall, the benefits outweigh the risks.

Cloning has been seen before in echinoderms – a group of marine organisms that have a five-axis symmetry, including brittle stars, sea stars and sea urchins, and sand dollars. But in all cases, it has been in response to conditions that are favourable for growth, such as plentiful food or favourable temperatures.

"This is the first demonstration that cloning can be induced by predation," says McClintock. "What is so unique in Vaughn's study is that this defence becomes a reproductive response – making more of yourself in anticipation of risk."

While this is the first demonstration in nature, cloning as a response to predation is common in science fiction – it's how Cylons in the TV show Battlestar Galactica, for example, cheat death.

Journal reference: Science (vol. 319 p. 1503)

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