Freshwater fish at top of food chain evolve more slowly

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Top predators adopt an evolutionary holding pattern

Since evolving to eat other fish, freshwater fish at the top of the food chain have remained relatively unchanged compared with their insect- and snail-eating cousins, according to new research.

Scientists report in the journal Evolution that once these fish, known as centrarchids, became top predators in aquatic ecosystems, natural selection put the brakes on their evolution.

“Throughout the tree of life, we find examples of groups whose species take on a wide variety of shapes and sizes, as well as groups of organisms that are surprisingly undiverse,” says first author David C. Collar, a postdoctoral researcher in Harvard’s Department of Organismic and Evolutionary Biology. “Our study shows that the evolutionary origin of some ways of making a living can cause diversification to slow, resulting in morphologically similar species. Of course, focusing on centrarchids, our study examines this pattern on a smaller scale, but the principles apply broadly to the diversity of life.”

Native to North America but now found in lakes, rivers, and streams world-wide, centrarchids eat a wide range of aquatic animals, including insects, snails, small crustaceans, and other fish. 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, like the largemouth bass.

“There are a lot of different sizes and shapes that will be fairly good at feeding on insects,” Collar says. “But there’s really only one way to be good at feeding on fish – you need a large mouth that can engulf the prey.”
While one key to making it to the top of the food chain is to have a large mouth, the other part of the equation is speed, Collar and his colleagues explain.

“A largemouth bass mostly relies on swimming to overtake its prey, and at the last moment will pop open its mouth — kind of like popping open an umbrella — and inhale the prey item,” says co-author Peter C. Wainwright of the University of California, Davis. “They’re able to strike very quickly and inhale a huge volume of water, which allows them to catch these big elusive prey.”

Collar, Wainwright, and co-authors Brian C. O’Meara of the National Evolutionary Synthesis Center in Durham, N.C., and Thomas J. Near of Yale University wanted to know how this feeding strategy affected the pace and shape of evolution among largemouth bass and other species that feed primarily on fish. 

To find out, they examined museum specimens representing 29 species of centrarchid fishes. Using a chemical process to stain and visualize the bones, muscles, and connective tissue, they measured the fine parts of the head and mouth.

By mapping these measurements onto the centrarchid family tree — together with data on what each fish eats — the researchers were able to reconstruct how diet and head shape have changed over time. “It looks as if the variety of head shapes and sizes in centrarchids is strongly in-fluenced by what they eat — primarily whether they eat other fish or not,” Collar says.

More importantly, when they compared fish-feeders with species that eat other types of prey, the researchers found that bass and other centrarchids that feed primarily on fish have remained relatively unchanged over time. Once they evolved the optimal size and shape for catching fish — roughly 20 million years ago — natural selection seems to have kept them in an evolutionary holding pattern, the researchers say.

“At some point in the history of this group, some of them started feeding on fish,” says Wainwright. “And once they achieved a morphology that was good at feeding on fish, they tended not to evolve away from that. They were already good at catching the best thing out there. Why should they diversify any more? Life was good.”
Collar, Wainwright, O’Meara, and Near’s research was funded by the National Science Foundation.