Comparative scale morphology in the adaptive radiation of cichlid fishes (Perciformes: Cichlidae) from Lake Tanganyika

The morphology of fish scales has been investigated for > 200 years, but research on evolutionary patterns of scale morphology is scarce. Here, we study scale morphology and its evolution in the adaptive radiation of cichlid fishes from Lake Tanganyika, which are known for their exceptional diversity in habitat use, feeding ecology and morphology. Based on a geometric morphometric approach on eight scales per specimen (covering different body regions), we quantify scale types and morphology across nearly all ~240 species of the cichlid adaptive radiation in Lake Tanganyika. We first show that scale type, shape and ctenii coverage vary along the body, which is probably attributable to adaptations to different functional demands on the respective scales. Our comparative analyses reveal that flank scale size is tightly linked to phylogeny, whereas scale shape and ctenii coverage can be explained only in part by phylogenetic history and/or our proxy for ecology (stable isotopes and body shape), suggesting an additional adaptive component. We also show that our measured scale characteristics can help to assign an individual scale to a taxonomic group or ecotype. Thus, our data may serve as a valuable resource for taxonomic studies and to interpret fossil finds.