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

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.