CHICAGO, July 5 (Xinhua) -- A new study that analyzed the evolutionary relationships between more than 30,000 fish species showed that the fastest rates of species formation have occurred at the highest latitudes and in the coldest ocean waters.

Over the past several million years, cool-water and polar ocean fishes formed new species twice as fast as the average species of tropical fish, according to the study.

In the study, researchers from the University of Michigan (UM) and eight institutions tested the widely held assumption that species-formation rates are fastest in the tropics by examining the relationship between latitude, species richness and the rate of new species formation among marine fishes.

They assembled a time-calibrated evolutionary tree of all 31,526 ray-finned fish species, and then focused analysis on marine species worldwide.

Genetic data were available for more than one-third of the fish species analyzed in the study, and the evolutionary tree was time-calibrated using a database of 139 fossil taxa.

The researchers estimated geographic ranges for most of the marine fish species, including all species with genetic data. Then they used complex mathematical and statistical models to estimate the rates at which different groups of fishes split into new species.

Some of the fastest rates of new species formation occurred in Antarctic icefish and their relatives. Other temperate and polar groups with exceptionally high speciation rates include snailfish, eelpouts and rockfish.

Three of the largest coral reef-associated fish groups, wrasses, damselfish and gobies, showed low to moderate rates of species formation.

"Who would have thought that you'd have these really explosive rates of species formation happening in the coldest Antarctic waters, where water is literally at the freezing point and fish like the icefish have to have all kinds of really crazy adaptations to live there, like special antifreeze proteins in their blood to keep it from freezing," said UM evolutionary biologist Daniel Rabosky, lead author of the study.

"The fact that coral reefs support many more fish species than polar regions despite these lower rates may have a lot to do with their long history of connectivity and ability to act as a refugia," said co-author Peter Cowman of the Australia Research Council Centre of Excellence for Coral Reef Studies.

The study was published in Nature on Wednesday. Enditem

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