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After a breakup, coral embryos live on as clones

Afloat in the ocean, the newly dividing cell masses shatter easy, die hard

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2:57pm, March 1, 2012
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Choppy waters and even mellow surf can knock drifting coral embryos to bits. But it takes more than shattering to kill these resilient young animals. The fragments turn out to have the power to keep on growing as clones.

Many corals start life adrift in open water, forming when eggs and sperm released by their settled parents float to the sea surface and mingle. The new embryos often face at least somewhat rough water on about half of the spawning nights in the central Great Barrier Reef. Lab tests mimicking these conditions split apart 45 percent of coral embryos just starting to divide, says Andrew Negri with the Australian Institute of Marine Science in Townsville. Yet checking on these remnants showed plenty of survivors that remained small but matured normally, Negri and Institute colleague Andrew Heyward of Perth report in the March 2 Science.

“A pretty cool observation,” says coral biologist Nancy Knowlton of the Smithsonian Institution’s National Museum of Natural History in Washington, D.C. “What makes it neat is not that the developing embryos can clone per se, but they are likely to do it under natural circumstances.”

Undamaged, older larvae of sea stars and brittle stars (as well as some of their echinoderm relatives) sometimes clone themselves. In these animals, however, the very youngest larvae get some protection from their environment. Right after fertilization, a membrane forms around the echinoderm embryo as cells start to divide. Corals, in contrast, start naked.

Many coral species release their buoyant bundles of sex cells during highly synchronized mass spawnings. “It looks like upside-down, pink rain,” Negri says. As for the odor, “I can’t really describe it, but once you’ve smelled it, you never forget it.”

To test very young corals under realistic conditions, Negri and his colleagues focused on moderate winds greater than 11 knots (12.7 miles per hour), which nudge water into waves at least 30 centimeters high. The researchers poured young embryos from that height — twice. Studying the survivors, researchers found a mix of embryo sizes as smaller remnants kept on dividing. Like their intact brethren, if a smidge littler, these cloned bits developed into brisk swimmers and then selected places to settle down for some reef-building adulthood.

Finding normal development despite disaster opens up a new range for experiments, says Negri. Coral versions of human twin studies might compare some environmental effects on genetically identical individuals, or environmental tests might determine the vulnerabilities of youngsters of different sizes. The discovery also raises evolutionary questions about the possible advantages of cloning for sea-tossed corals. After roughly 240 million years, the coral lineages certainly have had time to evolve a protective membrane, he says.

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