Freshwater fish at the top of the food chain evolve m...( Durham NC For avid fishermen and an...)

Durham, NC For avid fishermen and anglers, the largemouth bass is a favorite freshwater fish with an appetite for minnows. A new study finds that once they evolved to eat other fish, largemouth bass and fellow fish-feeders have remained relatively unchanged compared with their insect- and snail-eating cousins. As these fishes became top predators in aquatic ecosystems, natural selection put the breaks on evolution, say researchers.

A highly sought-after game fish, the largemouth bass belongs to a group of roughly 30 freshwater fishes known as centrarchids. Centrarchids are native to North America but have since been introduced into lakes, rivers and streams worldwide. This group of fishes eats a wide range of aquatic animals, says first author David Collar. "There's a good deal of diet diversity in the group," says Collar, a postdoctoral researcher at Harvard University. "Some species feed on insects, snails, or small crustaceans, and others feed primarily on fish."

In terms of nutritional value, fish are loaded with fats and proteins needed for growth, explain the researchers. "Fish make great fish food," says co-author Brian O'Meara of the National Evolutionary Synthesis Center. "But they're hard to catch," says O'Meara.

Biologists have long known that certain head and body shapes make some centrarchids better at catching fish than others. To catch, kill, and swallow fish prey, it helps to have a supersized mouth. "There are a lot of different sizes and shapes that will be fairly good at feeding on insects," Collar explains. "But there's really only one way to be good at feeding on fish you need a large mouth that can engulf the prey." The largemouth bass is a prime example: "There's no fish out there that's a better fish-feeder," says co-author Peter Wainwright of the University of California at Davis.

One key to feeding on fish is to have a large mouth, but the other part of the equation is speed, the researchers e
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