When the American painter Bob Ross said, “There are no mistakes, only happy accidents,” he evidently referred to painting on a canvas.
But the mantra of Mr. Ross is just as true for coral reef fish, where the eggs of one fish species and the sperm of another can sometimes combine to produce hybrid offspring with colors that are even more striking than those of their parents. Think of them as the happy little bushes of speciation.
Take for example the multibarred angelfish (Paracentropyge multifasciata), which has black-and-white stripes like window-blind slats, and the purple masked angelfish (Paracentropyge venusta), which resembles a lemon drop with a brilliant pear-blue back. When the two fish feed, they produce a blue-and-yellow hybrid, whirled with white, almost like a piece of baby.
When Yi-Kai Tea, a graduate student at the University of Sydney and the Australian Museum Research Institute, first saw this shrinking anglerfish, he thought it was a hybrid. Although coral reefs can look like a panoply of light shades, many coral reefs have as distinctive color patterns as postage stamps. So when a hybrid happens, it’s easy to spot.
When Mr. Tea was looking for other examples of the apparent hybrid, he came up with the idea for a comprehensive survey of all known, naturally occurring marine angelfish hybrids, which he and his colleagues describe in a paper published this month in Proceedings of the Royal Society B.
To find potential subjects, the researchers researched through photographs, ancient research and museum archives. Mr. Tea even asked someone to see his pet fish. After comparing the mitochondrial DNA of 37 hybrids with that of their parent species, the team found that an airy 48 percent of marine angelfish can hybridize, more than any other group of coral reefs. This data downplays the previous record holders, the butterflyfish, a family in which more than a third of the species are capable of producing hybrids.
“Hybridization in reef fish has long been a conundrum,” Hugo Harrison, a molecular ecologist at James Cook University in Australia who was not affiliated with the paper, wrote in an email.
The hybrid process was historically dismissed as a rare phenomenon among coral reef fish, but a growing body of research in the past few decades has discovered a surprising abundance of hybrid reef, according to a 2015 paper in Current Zoology.
In the case of butterflyfish, most hybrids occur in the narrow edges where two different populations meet. If a butterflyfish strays too far from its own species, it can have trouble finding a mate of its own species. “That they pair with someone who looks the most like them,” said Luiz Rocha, fish curator at the California Academy of Sciences, who provided examples of hybrids for the paper.
But the new paper found that the majority of anglerfish hybrids occur between species that live on the same reefs. At first glance, sympathetic hybridization makes no evolutionary sense, and even a threat to species diversity. If a fish of one species could repeatedly produce viable offspring with another species living on the same reef, the two species could be eroded.
One possible explanation is that anglerfish reject monogamy. Unlike butterfly fish, which mate their lives, angelfish live in harems where multiple females mate with a single male.
“If the male identifies one female in his harem, they will rise in the water column and release their sperm and eggs,” said Mr. Tea. Once adrift is an egg fair game for fertilization by a sperm of any other kind. While this can technically happen to all reef fish, the chances of spotting are much higher with different angler species sitting on the same reef. “Accidents happen in the ocean,” said Drs. Rocha.
One of the paper’s most surprising findings is that anglerfish can produce hybrid offspring with species that have as much as an 11 percent difference in mitochondrial DNA. “This is extremely remarkable,” said Mr. Tea, noting that this evolutionary distance was equivalent to a liar (but not quite as far as the sturgeon).
Reef fish rarely hybridize in more than 6 percent difference, Mr. added. Tea.
“What’s fascinating about this study is that hybrids are so common, not just geographically, but also across species lines.” Dr. Harrison said.
Although the paper reveals a surprising abundance of angelfish hybrids, the mechanism itself – how and why hybrids form – remains a conundrum, the researchers write. One eccentricity, for example, is the fact that researchers could not find a single example of a hybrid regal anglerfish, a tropical fish that swims almost anywhere in a tropical ocean.
The 11 percent hybrid in the new paper is a mix between the emperor Angelfish, as Pomacanthus imperator, and the flowering angelfish, Pomacanthus annularis.
The juvenile emperor resembles tree rings of porcelain, concentrated blue and white circles rippling from its tail, while the juvenile flowering sports chevroned blue and white stripes. In their hybrid offspring, amazingly enough, these circles and stripes transmog in a labyrinth that may be found in a child’s puzzle book. If only the secrets of hybridization were as easily solved.
