Dazzle to survive

Diversification of the stunningly colourful fairy wrasses was propelled by sea-level changes during the last ice age, according to an Australian study published in the journal Systematic Biology.

The resplendent coral fish, from the species-rich Cirrhilabrus genus of the Labridae family, emerged about 10 million years ago. But the genomic study found they diversified just 3–5 million years ago, during the Pliocene/Pleistocene when sea levels were in constant flux.

“The rise and fall of sea levels allowed for populations to mix, while also isolating populations,” says lead author Yi-Kai Tea, from the University of Sydney. “These repeated events promoted speciation by acting as a ‘species pump’.”

Tea suggests this likely drove the evolution of different colours and forms that resulted in the prolific species we see today – some even have all the colours of the rainbow on their tiny bodies.

Of course, it’s the males that are super colourful, says Tea. “The females are comparatively less attractive, and with smaller fins – they are still lovely and colourful in their own right, but the males are bananas.”

Complementing their flamboyant hues, the males perform lively courting displays, erecting their fins and changing their colours – at times even becoming iridescent – and swimming circles around prospective females while warding off rival males.

“On a reef where multiple fairy wrasse species often mix together, it is important for males to not only be attractive towards females, but also be attractive toward the correct females of the correct species,” Tea explains.

“We think that this intense selection has driven the evolution of colours and forms, resulting in so many species we see today – much like the birds of paradise.”

The fairy wrasses have 61 different species – of which Tea has described and named six so far – which comprise 10% of all known species in the family, making them the largest genus of the second-largest marine fish family.

But despite all their scientific attention, establishing the fishes’ history has proved challenging, and the team says there has never been an in-depth study to explore why there are so many species and how they evolved. 

“And this, to me, is where the real interesting story lies,” says Tea.

To investigate, they collected tissue samples from about 50 fairy wrasse species and extracted mitochondrial DNA to build a phylogenetic tree and show the evolutionary relationships among them on a genome-wide level – improving on single-gene trees.

Then they dated the tree to estimate when the various species diverged from their common ancestors – this allowed them to reconstruct historic events that led to their diversity.

They tracked the evolution of the species explosion to the western Pacific Ocean, from which the fairy wrasses radiated outwards, with multiple independent forays into the neighbouring Indian Ocean and even as far as the Red Sea.

As well as disentangling the fairy wrasses’ evolutionary history and establishing species boundaries, the study identified rare species which Tea hopes will help inform best practices for reef conservation and management.