Competition Among Fish Leads to Species Differentiation

As fish compete with each other for food and territory, their competition breeds new species, according to scientists.

The research resonates with the ideas of Charles Darwin, who contended that competition played an important role in the evolution of new species. Darwin, however, was unable to provide empirical evidence that this was true, and little has been discovered since.

Writing in the journal Nature Communications, University of Bristol biologist Martin Genner and his colleagues report that they used population genetics and experimental evidence to show that competition leads to the differentiation of new species within cichlid fishes found in Lake Tanganyika in East Africa.

The fish Telmatochromis temporalis, Genner and his collaborators found, has two genetically distinct types that differ in appearance depending on its environment. Based on where T. temporalis is found in the lake, its body size is different.

"We found large-sized individuals living along the rocky shoreline of Lake Tanganyika and, in the vicinity of these shores, we found small-sized individuals, roughly half the size of the large ones, that live and breed in accumulations of empty snail shells found on sand," Genner said.

The larger fish outcompete the smaller ones, driving them away from the preferred rocky habitats and into the sand, where the fish shelter themselves and their eggs in empty snail shells.

"In effect, big and small fish use different habitats; and because of this habitat segregation, fish usually mate with individuals of similar size. There is virtually no genetic exchange between the large- and small-bodied ectomorphs," Genner said.

The fish essentially segregate themselves, and, despite being the same species, they do not interbreed between groups, the researchers learned. This non-random mating between larger and smaller fish of the same species sets the state for future evolution of the species.

"The relevance of our work is that it provides experimental evidence that competition for space drives differential mating in cichlid fish and, in time, leads to the formation of new species," Genner said. "Nature has its ways - from body size differences to the formation of new species. And clearly, size does matters for Telmatochromis and for fish diversity."