Accidental Angels? Explaining the Phenomenon of Fish Hybridization on the Reef
26 Aug, 2020
All God’s angels come to us disguised.
—James Russell Lowell
Why do some fish species form hybrids while others don’t?
In attempting to answer this question, researchers in Australia have discovered that marine angelfishes may be the most prolific hybridizers in the communities of species found on coral reefs.
The research was led by Yi-Kai (Kai) Tea, PhD candidate at the University of Sydney’s School of Life and Environmental Sciences. Tea and his colleagues, Professors Nathan Lo and Simon Ho, Dr Joseph DiBattista from the Australian Museum, Jean-Paul Hobbs from the University of Queensland, and Federico Vitelli from Edith Cowan University, sought to explore why some fishes frequently create hybrids, and to look for the factors that facilitate this.
After deciding to focus on “one of the most charismatic and iconic groups of coral reef fishes,” they found that 42 species–nearly half of all known species of marine angelfishes—create hybrids.
“This is among the highest incidences of hybridization in coral reef fishes,” Tea said. The study has just been published in the open-access journal, Proceedings of the Royal Society B: Biological Sciences, August 5, 2020.
Tea continued: “We also found that hybrids are frequently produced even between angelfish species that are distantly related to each other; some separated by over 10 million years in evolutionary time.”
Other hybrids were found between species with over 12 percent pairwise distance in mitochondrial DNA. Pairwise distance is a measurement of differences in pairs of DNA sequences.
“This genetic separation is quite astounding, considering that hybrids are rarely reported between species that share more than 2 percent in genetic distance,” Mr Tea said. “Though coral reef fish hybrids are common; they are usually formed by closely-related species.”
A third key finding was that angelfish hybridize wherever different species exist. This contrasts with other coral reef fishes, which tend to only hybridize within certain zones of their shared habitats.
It is generally believed that hybridization between closely related sympatric species that share the same geographic area may accidentally produce hybrids by spawning at the same time. Because marine angelfishes are pelagic spawners, releasing their eggs and sperm in the water column, it is possible for a mixing of species without an intentional mating between males and females of that species. However, this remains to be documented.
“In terms of coral reef fish hybridisation, much remains unanswered, particularly in the context of why, and how hybrids are formed. We still don’t know why some species hybridize and others don’t,” Tea said. “For example, the regal angelfish, Pygoplites diacanthus, is found throughout the Indian and Pacific Oceans, yet no hybrids have ever been reported for this species.” He also has pointed out that the rare, deepwater Peppermint Angelfish, Centropyge boylei, has yet to be reported as involved in hybridization events.
From the paper, the authors offer this point of view:
“Curiously, no hybrids have been reported between Pa. multifasciata and Pa. boylei despite their distributional overlap in the French Polynesian Islands of Rarotonga and Moorea. However, unlike Pa. multifasciata and Pa. venusta, Pa. boylei has a preference for much deeper waters (53–120 m versus 7–70 m in Pa. multifasciata), making any overlap in the two species a rarity. A more parsimonious explanation is that hybrids between the two species have not yet been recorded, which is not unlikely given the geographical isolation and preference of Pa. boylei for very deep reefs. Nonetheless, our study provides novel insights into the outcomes of hybridization between divergent species of marine angelfishes, with results corroborating previous studies on hybridization between other divergent lineages of coral reef fishes.”
“In terms of cracking the secrets to hybridisation in coral reefs, we’ve only just scratched the surface.”References
Tea YK, Hobbs JPA, Vitelli F, DiBattista JD, Ho SYW, Lo N. 2020 Angels in disguise: sympatric hybridization in the marine angelfishes is widespread and occurs between deeply divergent lineages. Proceedings of the Royal Society B: 20201459.
Proceedings of the Royal Society B featuring the Peppermint Angelfish on a recent cover. Said the editors: “It commands star power due to a new study highlighting the remarkably high incidence of and tendency for hybridization in this family (even between divergent species), more so than in any other group of coral reef fishes.”
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