A hybrid in a mixed school of fish Tane Sinclair-Taylor
They were oceans apart for thousands of years, but now they are reunited. Fish from the Indian and Pacific Oceans that have become separate species are getting it on at island retreats, and having hybrid babies.
The powder blue surgeonfish (Acanthurus leucosternon) from the Indian Ocean and the whitecheek surgeonfish (Acanthurus nigricans) from the Pacific Ocean are two very different-coloured species of surgeonfish that were once one. A. leucosternon has a distinctive single yellow stripe on its blue flank, while A. nigricans has two yellow stripes and a white cheek mark.
Acanthurus leucosternon, whose ancestors hung out in the Indian Ocean Tane Sinclair-Taylor
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Some 600,000 years ago, a sea level drop separated the two oceans. The fish on either side of the land barrier began to evolve differently, and are now classed as distinct species.
Acanthurus nigricans, a denizen of the Pacific Tane Sinclair-Taylor
The seas returned to present day levels about 6000 years ago, allowing the two reef fish to meet at islands along the Indo-Pacific border, including Christmas Island and the Cocos (Keeling) Islands.
Reef relations
The reefs around these islands are hotbeds of interspecies sex, says of Curtin University in Western Australia.
In fact, when Di Battista and his colleagues studied the DNA of A. leucosternon and A. nigricans from these areas, they couldn鈥檛 find a single pure-bred specimen 鈥 all the fish they caught were genetic hybrids. 鈥淭hese guys are actually choosing to mate with each other, even though they鈥檙e different species,鈥 Di Battista says.
Many of the fish also had a mix of colourings and features from the two parent species. It鈥檚 unclear how long the two species have been pairing up and why, Di Battista says. 鈥淵ou can get hybrids in the ocean when one fish species is more abundant than another, he says. 鈥淭he rarer fish goes for the next best thing he can find 鈥 a different species but one that he鈥檚 closely related to.鈥
A hybrid of Acanthurus leucosternon and Acanthurus nigricans Tane Sinclair-Taylor
of James Cook University in Queensland, Australia, agrees. 鈥淚ndividuals of the rare species may choose to mate with another closely related species owing to a scarcity of conspecific partners.鈥
Long-lost cousins
Ocean fish hybrids were once thought to be rare, but scientists are finding more and more areas of species overlap where interbreeding is common.
鈥淭he occurrence of hybridisation is usually underestimated and more cases are probably at Christmas and Cocos Islands,鈥 says van Herwerden.
And they might become more common now climate change is driving fish into new territories.
鈥淎s the ranges of fish begin to shift, they鈥檙e going to come in contact with new species that they may be closely related to but with which they鈥檝e never had contact before,鈥 says Di Battista. 鈥淚 don鈥檛 know if it will be a good thing or a bad thing, but the mating opportunity will be there.鈥
Journal Reference: Molecular Phylogenetics and Evolution,
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