Caribbean reefs are graced with incredible beauty and rich biodiversity. More than 100 species of coral coexist in these waters; all spawn simultaneously every summer during the same few full moonlit nights, forming a thin pink slick of intermingled eggs and sperm on the waters surface. Scientists have wondered for some time how, with this extraordinary opportunity for cross-species breeding, these creatures manage to preserve their identities. Research published today in the journal Science is providing some answers.

Steven Vollmer of Harvard University and his colleagues examined what they initially thought were three distinct coral species of the genus Acropora in the Caribbean Sea. But on further genetic characterization, the team discovered that in fact only two distinct species exist. The third is a hybrid of the first two and is for the most part sterile: it cannot interbreed with its two parent species, which would dilute the reef's biodiversity. Members of this group can, however, reproduce by cloning themselves, generating what researchers call immortal mules—animals that possess genetic makeups identical to those of the parent hybrid and that can survive for a very long time. Intriguingly, the hybrids come in two different physical forms, "bushy" and "palmate." The investigators found that the creature's morphology depends on which species the egg comes from. It seems that maternal mitochondrial DNA and egg cytoplasm determine the form the progeny takes. In this way, not only do cross-species offspring not detract from the local biodiversity, they actually enhance the morphological and ecological variation of the reef.

Can the new findings help to explain biodiversity in other coral-filled waters? The team posits that the same effects they observed in the Caribbean may also occur elsewhere. "The obvious next step," Vollmer says, "is to look for immortal mules on the hyperdiverse indo-pacific coral reefs and determine how they might be affecting the biodiversity and ecology of those reefs."