Â鶹ÒùÔº

The biodiversity of Earth's oceans is disproportionately concentrated in coral reefs—the vibrant undersea ecosystems where thousands of known marine species reside.

A new Yale-led study offers insight into the origins of this exceptional biodiversity. It charts the evolutionary trajectories of wrasses and parrotfishes, which rank among the most species-rich and ecologically diverse lineages of reef fishes, accounting for more than 650 species.

These fish, which comprise the family Labridae, experienced an explosion of evolutionary innovation and accelerated species diversification during the early Miocene, about 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."

The results are in the journal Science Advances.

The researchers analyzed 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.

"Museum collections are essential to understanding the evolutionary history of life on Earth," said study co-author Luiz Rocha, curator of ichthyology at the California Academy of Sciences.

"Working with such a diverse group of fishes required deep collaboration between several natural history museums, and it was only by pooling collections in our biological repositories that we obtained the coverage necessary to reveal evolutionary relationships among wrasses and parrotfishes."

While climate fluctuations can fuel evolutionary change, the explosive species diversification of wrasses and parrotfishes occurred during a time of global climate stability, so the researchers explored other explanations.

The reef fishes' common ancestor possessed pharyngeal jaws—a second set of jaws in the throat—that enabled its ecological success, said Brownstein, who is part of Yale's Department of Ecology and Evolutionary Biology.

Knowing this, the researchers explored whether the evolution of specific traits in wrasse and parrotfish lineages drove diversification. They tested several characteristics considered ecologically important in subgroups of the fishes, including novel innovations to jaws and teeth, and determined that none of them explain the rapid diversification.

Instead, researchers concluded, the burst in evolutionary innovation was likely associated with conditions in the coral reefs the fishes 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," Brownstein said.

"And the reef fishes started developing all sorts of fascinating innovations and dietary behaviors. 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 mounting evidence that profound changes took place in reef systems beginning about 20 million years ago, Brownstein said. A recent study showed that the Indo-Pacific Coral Triangle, the region with the highest marine biodiversity on the planet, began forming during this period.

The new findings demonstrate the value of using genomic data and the fossil record to better understand how Earth's biodiversity developed, said Thomas Near, professor of ecology and evolutionary biology in Yale's Faculty of Arts and Sciences.

"This work informs efforts to protect Earth's biodiversity," Near 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."