The role of gill rakers in the adaptation and habitat partitioning of Nicaraguan crater lake cichlid fishes

The coexistence of species is facilitated by niche partitioning. Resource competition is a major cause of phenotypic differentiation.The major axes of differentiation in closely related species occur along the two ecological dimensions habitat and diet. Adaptive radiations typically follow these axes of variation. Some of the most important traits in the radiations of freshwater fish are body size, body shape, and a few trophic features such as mouth position, teeth type or gill rakers. In some of the most famous temperate freshwater radiations (i.e., sticklebacks, whitefish) gill rakers play a pivotal role in the adaptation to novel prey types. Cichlids are the most famous textbook examples of fast and divergent adaptive radiations. Often species differ in adaptation to alternative prey types associated to particular trophic morphologies. Pharyngeal jaw shape, oral and pharyngeal teeth and mouth position are the main traits typically studied in cichlid radiations. Gill rakers have however rarely been consider when trying to understand cichlid adaptation to the environment. Here we assess the variation of gill rakers in the cichlid assemblage inhabiting the Nicaraguan large and crater lakes. We found marked differences among species, and found interesting correlations between gill raker traits and environmental features demonstrating their role in adaptation and diversification.