When is a tuna more like a seahorse than a marlin?
The first comprehensive phylogeny of the "spiny-rayed fish," a group that includes about a third of all living vertebrate species, has been published in the Proceedings of the National Academy of Sciences.
The spiny-rayed fish are an incredibly diverse group, including tuna and billfish, tiny gobies and seahorses, and oddities such as pufferfish and anglerfish. The fish occupy every aquatic environment from coral reefs and open oceans to lakes and ponds and the group includes all the major commercially fished species.
The paper also says that after roaring along for their first 100 million years, the pace of evolution of the spiny-rayed fish down-shifted about 50 million years ago.
But exactly how the more than 18,000 species in 650 families are related to each other was unclear.
"There has been a 'bush' at the top of the family tree leading to the rest of the vertebrates," said Peter Wainwright, professor and chair of evolution and ecology at the University of California, Davis and senior author on the paper.. "Now we have this beautiful phylogeny of one-third of all vertebrates."
Some groups of fish have gone along steadily for millennia; others have gone through bursts of rapid evolution. Overall, the researchers found that the rate at which new species formed was fairly constant across the group from their origin to about 50 million years ago, then dropped about five-fold and has remained at that level since.
That might mean that these fish have essentially filled the available spaces, Wainwright said.
"It's not uncommon in evolution to see a rapid diversification followed by a slowdown, but it's never been seen on such a scale before," he said.
Wainwright's laboratory worked with the lab of Yale University's corresponding author Tom Near and colleagues at the University of Tennessee, The Field Museum in Chicago, Florida Atlantic University and CUNY Staten Island to construct the family tree. Matt Friedman, a paleontologist at the University of Oxford, England, added fossils that helped set dates for branches of the tree.
The researchers looked at 10 genes in more than 500 fish species representing most of the families of spiny-rayed fish. They used the genetic data to construct a tree, grouping related families together. They also looked at the pace of evolution -- the rate at which new species formed -- in different branches, and across the group as a whole.
The spiny-rayed fish originated about 150 million years ago, separating from more primitive fish, such as lampreys, sharks and sturgeon, and from the ancestors of salmon and trout. Since then, they have spread into every aquatic habitat on Earth.
The tree shows some interesting relationships. For example, tuna are more closely related to seahorses than to swordfish or barracuda. The oddly shaped pufferfishes are related to anglerfish, the only fishes whose bodies are wider than they are deep.
Cichlids, a family that includes about 2,000 species of freshwater fish known for brooding their young in their mouths and a favorite for studies of evolution, are related to the engineer gobies, an obscure family of just two species that live on coral reefs and raise their young in a nest.
Wainwright's special interest is in the evolution of fish jaws. Fish have two sets of jawbones, an outer jaw and "pharyngeal jaws" in the throat that adapted to different functions. In some fish, the lower pharyngeal jaw is fused into a single solid bone that can be used to crush prey such as shellfish.
Biologists had assumed that this fused jaw had evolved once and then spread into different groups of fish. Instead, the new tree shows that this structure evolved at least six times in different groups of fish.
Citation: Thomas J. Near, Alex Dornburg, Ron I. Eytan, Benjamin P. Keck, W. Leo Smith, Kristen L. Kuhn, Jon A. Moore, Samantha A. Price, Frank T. Burbrink, Matt Friedman, and Peter C. Wainwright, 'Phylogeny and tempo of diversification in the superradiation of spiny-rayed fishes', PNAS 2013, July 15th, 2013, doi:10.1073/pnas.1304661110