The biodiversity of the Earth’s oceans is disproportionately concentrated in coral reefs, findings from a new study led by researchers at Yale University has found, offering the first insights into these ecosystems are so rich in rapid species diversification.

A new study from Yale has shed light on how coral reefs became biodiversity hotspots by charting the evolutionary trajectories of the coral-reef fish wrasses and parrotfishes – two fish ranked among the most species-rich and ecologically diverse lineages of reef fishes, accounting for more than 650 species.

According to the paper, published in the scientific journal Science Advances earlier this month, these fish – which comprise the family Labridae – experienced an explosion of “evolutionary innovation” and “accelerated species diversification” during the early Miocene, some 20 million years ago.

“Multiple lineages of wrasses and parrotfishes diversified and dispersed rapidly and recently,” said Chase D Brownstein, a graduate student in Yale’s Graduate School of Arts and Sciences and the study’s lead author. “This explosion of diversity resulted from multiple, independent events happening simultaneously across the wrasse and parrotfish Tree of Life.

“Our results tie these diversification events to changes that were occurring within coral reef systems.”

To draw their conclusions, the researchers analysed genomic data from nearly 400 wrasse and parrotfish species. Using fossil specimens, they created a time-calibrated evolutionary tree that delineates the lineages of all major groups of the fishes, allowing them to identify when the rapid diversification began. 

“The reef fish started developing all sorts of fascinating innovations and dietary behaviours. All this diversification stems from what was happening within the reefs themselves,” said Brownstein. 

Key to bringing this study together was the collaborative effort between several natural history museums. It was by pooling collections in each biological repository that researchers were able to obtain the coverage necessary to “reveal evolutionary relationships” among wrasses and parrotfishes.

Ruling out climate change and the numerous novel innovations to jaws and teeth among subgroups of the fishes, researchers eventually concluded that the burst in evolutionary innovation were associated with conditions in the coral reefs the fish inhabited.

“The key factor is that wrasses and parrotfish lineages associated with coral reefs began diversifying like gangbusters while those unassociated with reefs did not,” said Brownstein. “And the reef fishes started developing all sorts of fascinating innovations and dietary behaviours. All this diversification stems from what was happening within the reefs themselves.

“For example, we suggest that the extinction and turnover of corals, and other changes to the structure and location of reefs, was driving species diversification.”

There is – it turns out – mounting evidence that profound changes took place in reef systems beginning about 20 million years ago. A recent study has showed that the Indo-Pacific Coral Triangle (the region with the highest marine biodiversity on the planet) began forming during this period. 

According to Thomas Near, professor of ecology and evolutionary biology in Yale’s Faculty of Arts and Sciences, the research here will be vital to predicting how climate change and other human disturbances will affect coral reef biodiversity hotspots.

“This work informs efforts to protect the Earth’s biodiversity,” he said. “We show that the planet’s most species-rich marine ecosystems have developed through long legacies of subtle changes. We need to be aware of those legacies to predict how climate change and other human disturbances will affect these biodiversity hotspots.”

The study, ‘Phylogenomic establishes an Early Miocene reconstruction of reef vertebrate diversity’ has now been published in the journal, Science Advances.

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