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Cichlid fish transform their shape to handle environmental changes

Biologists at the University of Massachusetts Amherst are describing how cichlid fish changed their shape after a river in the Amazon was dammed. By studying fish in this context, experts can gain a better understanding of how living organisms, in general, may adapt to major human-induced environmental changes.

In 1984, a dam was closed in the Tocantins River in Brazil. The modification rapidly transformed this clear, flowing river into a deep, murky reservoir. The researchers theorized that fish would exhibit changes in body shape as they shifted their habitats and foraging behavior to adapt to the drastically different surroundings. 

“The once-historic rapids and streams that characterized the system have disappeared from the surrounding area, which in turn has affected the abundance and variety of food sources available to native fishes,” wrote the study authors.

The team analyzed museum collections of cichlid fish from before the dam was closed and compared the characteristics of these fish with specimens collected by fishermen 34 years later.

Study co-lead author Craig Albertson said that cichlid fish are known for their ability to alter, in as little as a single season, aspects of their body shape to match feeding conditions and other environmental shifts. 

The researchers evaluated changes in six native species of cichlid fish, ranging from large carnivores to small omnivores. The fish represented distinct local groups with body shapes that match their ecological roles.

“Our overarching hypothesis is that the damming of the Tocantins and subsequent formation of the Tucuruí reservoir has induced shifts in habitat and foraging behavior and that the anatomy of resident cichlid populations has change in ways that allow them to adapt to this novel environmental conditions. This study represents a first step toward assessing this hypothesis,” explained Albertson.

“Was anything surprising? Yes! While we expected to see changes in generalist species – those that are already predisposed toward living in a variety of habitats – we were surprised to see shape changes in the specialists as well,” said study co-lead author Chase Gilbert. 

“Evolving to specialize on a particular prey-type or habitat, can provide a competitive advantage in the near term, but it can also be an evolutionary dead end in the face of major environmental changes.”

According to the researchers, not only are these specialist species still found in the area, but they seem to be just as capable of changing body shape as the generalists.

Albertson said that changes across all species were associated with functionally relevant aspects of anatomy, such as the head, fin, and body shape. He noted that the parts of the body that changed over time are exactly those most likely to allow them to survive in their new environment.

The study is published in the journal Evolutionary Applications.

By Chrissy Sexton, Staff Writer

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