Bilateral asymmetry in craniofacial structures and kinematics of feeding attacks in the scale-eating cichlid fish, Perissodus microlepis

Cichlid fishes are a textbook example for adaptive radiations, since they diversified into several hundred highly specialized species in each of three great East African lakes. Even scale-eating, an extremely specialized feeding mode, evolved independently multiple times in these radiations and in Lake Tanganyika alone, six endemic scale-eating species occupy this extremely specialized ecological niche. Perissodus microlepis went a step further, by evolving bilaterally asymmetrical heads with an intra-specific polymorphism where left- and right-headed morphs predominantly scrape scales from the opposite sides of their prey. While the bilateral asymmetry of scale-eating cichlids has been known, exactly which craniofacial features explain the laterality of the heads remained unclear. Here we aimed, by utilizing micro-computed tomography (μCT), to resolve this issue of how bilateral symmetry in the skeletal structure is broken in scale-eating Perissodus. Our 3D geometric morphometrics analysis clearly separated and identified the two groups of either left- or right-headed fish. In addition, we observed consistent asymmetric volume changes in the premaxilla, maxilla, and mandible of the craniofacial structures, where left-headed fish have larger jaw elements on the right side, and vice versa. The bimodality implies that the effect sizes of environmental factors might be minor while genetics might be responsible to a larger extent for the asymmetry observed in their head morphology. High-speed video analyses of attacks by asymmetrical morphotypes revealed that they utilize their asymmetrical mouth protrusion, as well as lateralized behavior, to re-orientate the gape towards the preferred side of their prey fish to more efficiently scrape scales.