Renowned journal Proceedings of the Royal Society B has a new front coverfish: the marine angelfish. It commands star power due to a new studyhighlighting the remarkably high incidence of and tendency for hybridisationin this family – even between divergent angelfish species.

Hybridisation refers to the process by which two different species mate andproduce hybrid offspring. Angelfishes hybridise more so than in any othergroup of coral reef fishes.

The research was led by Yi-Kai (Kai) Tea, PhD candidate at the University ofSydney’s School of Life and Environmental Sciences.

He and his colleagues, Professors Nathan Lo and Simon Ho, Dr Joseph DiBattistafrom the Australian Museum, Jean-Paul Hobbs from the University of Queensland,and Federico Vitelli from Edith Cowan University, sought to explore why onlysome fishes create hybrids, and the factors that facilitate this.

After deciding to focus on “one of the most charismatic and iconic groups ofcoral reef fishes”, the marine angelfish, they found that 42 species – nearlyhalf of all known species of marine angelfishes – create hybrids.

“This is among the highest incidences of hybridisation in coral reef fishes,”Mr Tea said.

A juvenile Pomacanthus imperator x P. annularis hybrid with strikingintermediate patterns. Photograph by Y.K. Tea.

Near-opposites attract

“We also found that hybrids are frequently produced even between angelfishspecies that are distantly related to each other; some separated by over 10million years in evolutionary time.”

Other hybrids were found between species with over 12 percent pairwisedistance in mitochondrial DNA. Pairwise distance is a measurement ofdifferences in pairs of DNA sequences.

“This genetic separation is quite astounding, considering that hybrids arerarely reported between species that share more than 2 percent in geneticdistance,” Mr Tea said. “Though coral reef fish hybrids are common; they areusually formed by closely-related species.”

Boundless hybridisation

A third key finding was that angelfish hybridise wherever different speciesexist. This contrasts with other coral reef fishes, which tend to onlyhybridise within certain zones of their shared habitats.

“In terms of coral reef fish hybridisation, much remains unanswered,particularly in the context of why, and how hybrids are formed. We still don’tknow why some species hybridise and others don’t. For example, the regalangelfish, Pygoplites diacanthus , is found throughout the Indian andPacific Oceans, yet no hybrids have ever been reported for this species,” MrTea said.

“In terms of cracking the secrets to hybridisation in coral reefs, we’ve onlyjust scratched the surface.”

Citation and link to study: Tea YK, Hobbs JPA, Vitelli F, DiBattista JD,Ho SYW, Lo N. 2020 Angels in disguise: sympatric hybridisation in the marineangelfishes is widespread and occurs between deeply divergent lineages.Proceedings of the Royal Society B: 20201459. (live after embargo lifts).

Image: Hybrids of marine angelfishes are often easily recognised due to theirstriking coloration. Offspring with hybrid ancestry often have varying levelsof intermediate coloration patterns aligning with both parent species.Photographs by Y.K. Tea.

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