Piscatory paradox: Frigid polar oceans, not balmy coral reefs, are species-formation hot spots for marine fishes
Tropical oceans teem with the dazzle and flash of colorful reef fishes and contain far more species than the cold ocean waters found at high latitudes. This well-known “latitudinal diversity gradient” is one of the most famous patterns in biology, and scientists have puzzled over its causes for more than 200 years.
One frequently advanced explanation is that warm reef environments serve as evolutionary hot spots for species formation. But a new study that analyzed the evolutionary relationships between more than 30,000 fish species concludes 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 new study, which was published July 4, 2018 in the journal Nature.
“These findings are both surprising and paradoxical,” said University of Michigan evolutionary biologist Daniel Rabosky, lead author of the study. “A number of hypotheses explain extreme tropical diversity as the result of faster rates of species formation, but it’s never been tested in fishes.
“Our results are counterintuitive and unexpected, because we find that speciation is actually fastest in the geographic regions with the lowest species richness.”
The authors admit they cannot fully explain their results, which are incompatible with the idea that the tropics serve as an evolutionary cradle for marine fish diversity. The findings also raise questions about whether the rapid cold-ocean speciation the team documented reflects a recent and ongoing expansion of marine diversity there.
In the study, Rabosky and colleagues from 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, then focused their analysis on marine species worldwide.
“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,” Rabosky said.
Rabosky is an associate professor in the U-M Department of Ecology and Evolutionary Biology and an associate curator at the U-M Museum of Zoology. Coauthors include EEB graduate student Pascal Title and Matt Friedman, associate professor in the Department of Earth and Environmental Sciences and director of the U-M Museum of Paleontology, among others.