Near-sterile hybrids boost coral diversity. (Marine Mules).

Some of the considerable biodiversity of corals may come from underwater versions of mules, say researchers.

Genetic analysis of what were once considered three species of Caribbean Acropora coral confirms that one of them is actually a collection of first-generation hybrids of the other two, says Steven V. Vollmer of Harvard University. The hybrid corals can reproduce asexually and create big, craggy colonies. They can also produce eggs and sperm in the lab, but genetic analysis of coral in the wild indicates that the hybrids seldom succeed at sexual reproduction, Vollmer and his Harvard coworker Stephen R. Palumbi report in the June 14 Science.

The researchers compare the parent corals with horses and donkeys that inter-breed and produce sterile mules. Among corals, the hybrid offspring vary in shape depending on which parent contributed the egg, the researchers report. "It's a new twist on how you get biodiversity," says Vollmer.

Coral variety has long puzzled researchers, Vollmer says. Many coral species on a reef release their eggs and sperm at the same time. In Australia's Great Barrier Reef, some 105 corals in 36 genera spawn synchronously. "It's pretty messy in the water," Vollmer says.

"First of all, we thought with all these corals spawning at the same time, there must be some mechanism keeping the species from fertilizing the wrong mate," says Carden C. Wallace of the Museum of Tropical Queensland in Townsville, Australia. "Then, it turned out that this was not really so."

Experimental crossbreeding in the past 15 years showed that many of these mass-spawning species could hybridize in laboratories. Whether this happens in natural reefs remained a mystery. Researchers didn't even know whether the lab hybrids could reproduce sexually because a coral can take 7 years to mature.

Some biologists proposed that corals in the real world hybridize wildly and form complexes of sexually reproducing, gene-trading species. Wallace welcomes the new paper as an alternative view of coral diversity.

The idea that the two parent corals hybridize has been suggested before, says Madeleine van Oppen of the Australian Institute of Marine Science in Townsville. In 2000, she and her colleagues published genetic data indicating--as the new paper does--that Acropora prolifera is actually a hybrid of Acropora cervicornis and Acropora palmata.

Vollmer and Palumbi confirmed the finding with a more extensive analysis, considering two variable regions of coral DNA in the nucleus and one in the cell's mitochondria. Also, the hybrids appear to backcross with only one of their parent species,A, cervicornis. Using an elaborate computer simulation of the way genes can spread between and within populations, the team concluded that backcrossing is probably very rare.

The genetic study of wild specimens showed which parent species contributed the egg to the variously shaped hybrids. Those colonies with A, cervicornis mothers grew "really beefy, like small Volkswagens underwater," Vollmer says. A. palmata mothers led to hybrids at most "as big as your knee."

"It's a really elegant study," says Nancy Knowlton of the Smithsonian Tropical Research Institute in Balboa, Panama. Whether other mule corals turn up remains to be seen, though.