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Two years ago, Joseph DiBattista pulled on flippers, bit down on a snorkel, and slid into the crystal blue waters of Socotra, a four-island archipelago about 380 kilometers south of Yemen. Through his dive mask, he spotted many fish he recognized—a blue-and-yellow surgeonfish here, a blue-and-white surgeonfish there—but his eyes were peeled for something stranger: fish bearing the blended coloring of two species. Before long, his hoped-for fish showed itself: a surgeonfish with blue, white, and yellow stripes. It was a hybrid, the offspring of the other two species.
It was the first of many hybrid reef fish that DiBattista, a marine biologist at Australia’s Curtin University, saw on the short dive. He will never forget that moment, he says, because it confirmed his suspicion that Socotra is an important hotspot for hybridization. Most reefs host few hybrids; Jean-Paul Hobbs, who was with DiBattista at Socotra and works with him at Curtin University, estimates that, at most reefs, he sees only one hybrid in a month of diving. But at Socotra, he saw one every 10 minutes, making it the most hybrid-rich coral reef in the world.
The researchers suspect that hybrid hotspots like Socotra are incubators for new coral reef species. Hybrid fish are born when unrelated species mate, a typically rare event that stirs the genetic pool. By jump-starting the genetic mixing that leads to the evolution of new species, hybrid hotspots could help explain why coral reefs host as many species as they do.
“Hybridization can either create species or it can dissolve two species,” says Hobbs. “It gives us a window into how species evolve.”
Even though they had never been to Socotra, the researchers had a hunch that hybrids flourished there based on its geography. It is situated at the confluence of three major water bodies: the Red Sea and Gulf of Aden to the west, the Arabian Sea to the east, and the Indian Ocean to the south. Each body of water hosts a different collection of species, some of which are closely related, and Socotra is where the fish meet.
And where the fish meet, so do the scientists. In October 2013, DiBattista, Hobbs, and other researchers descended upon the reef. They searched for fish with intermediate coloration, like the tricolor surgeonfish that caught DiBattista’s eye, or a yellow butterflyfish with an atypically colored tail.
In only six days, the researchers collected seven different types of hybrids, including butterflyfish, anemonefish, and surgeonfish, formed from 14 parent species. Clearly hybrids were common at Socotra, but the big question was whether they were fertile. If not, they go the way of the mule and their existence is an evolutionary dead end. However, the hybrids were fertile—their gonads had all the right parts—and the genetic analysis showed that the hybrids were passing on their genes. To DiBattista’s surprise, many of the “parent species” from Socotra and elsewhere also carried genes from their relatives; although they didn’t look the part, the parents were also hybrids.
This told DiBattista that hybridization is not a dead end. “It is an important process and it is going on in nature, even though we may not have noticed the extent to which it goes on before,” he says.
The researchers have traveled throughout the world searching for hybrids at other isolated reefs that, like Socotra, lie at the confluence of two or more oceans. Hobbs discovered the first known hybrid hotspot while doing research at Christmas Island and the Cocos (Keeling) Islands, which lie between the Pacific and Indian Oceans. He has several other leads to investigate, including one spot near Japan. Hobbs suspects that now that he’s started looking, ocean hybrid hotspots may be more common than previously suspected.
Because hybrids seemed rare, scientists wrote them off as unimportant to biodiversity, Hobbs says. But his discovery of two hybrid hotspots in less than a decade suggests that they’ve been there all along, and “we just hadn’t got in the water and looked properly. When you start to look for hybrids, you find hybrids.”