Fish Relatives Surprise Scientists
July 18, 2013

Surprises In The Fish Family Tree

Susan Bowen for - Your Universe Online

Scientists have used genetic data to create a comprehensive evolutionary family tree, or phylogeny, for "spiny-rayed fish," a category that encompasses about a third of all living vertebrate species. They were quite surprised to find out just who was related to whom in the fish world.

The researchers, who published their findings in the Proceedings of the National Academy of Sciences looked at 10 genes in more than 500 fish species representing most of the families of spiny-rayed fish. They found some family connections that they were not expecting. The tuna is more closely related to the seahorse than it is to marlins, swordfish or barracudas. Puffer fish and angler fish, the only two species whose bodies are wider than they are deep, are closely related. Cichlids, who brood their young in their mouths, are related to engineer gobies, who raise their young in a nest.

Another surprise has to do with fish jaws. Fish have two sets of jaws. The second one, called the pharyngeal jaw, has adapted for different purposes in different species. In some fish, this second jaw is fused into a solid bone that they use to crush prey such as shellfish. Biologists had assumed that all of these fish had a common ancestor, but instead, they have found that this structure evolved independently in at least six different groups of fish.

The spiny-rayed fish species occupy all aquatic environments from coral reefs to open oceans to lakes. They originated about 150 million years ago when they separated from more primitive fish such as sharks, lampreys and sturgeons and from the ancestors of soft-rayed fish such as salmon and trout. They include more than 18 thousand species and 650 families.

Peter Wainwright, the lead author of the paper, is a professor and chair of evolution and ecology at the University of California, Davis. Matt Friedman, a paleontologist at the University of Oxford, England, added fossils that helped set dates for branches of the tree. They found that the pace of evolution within this group was quite rapid up until about 50 million years ago. At that time, it decreased about five-fold and has remained at that level since then. This degree of slowdown is unusual.

In a related article, reported by Thomas J. Near of Yale University, the researchers state, "Our analyses, which use multiple nuclear gene sequences in conjunction with 36 fossil age constraints, result in a well-supported phylogeny of all major ray-finned fish lineages and molecular age estimates that are generally consistent with the fossil record."

He describes how this new study compares with prior research. "Contrasting our divergence time estimates with studies using a single nuclear gene or whole mitochondrial genomes, we find that the former underestimates ages of the oldest ray-finned fish divergences, but the latter dramatically overestimates ages for derived teleost lineages." Teleosts are fish with bony spines. Their findings place much of the diversification within this group during the late Mesozoic and early Cenozoic eras. Thus, they designate this time period as the "Second Age of Fishes."