Martinez C.M., F.J. Rohlf and M.G. Frisk (In review). Role of reproductive strategy in pectoral fin dimorphism in sister species of skate, Leucoraja erinacea and Leucoraja ocellata, Evolution & Development.
Instances of sexual dimorphism occur in a great variety of forms and manifestations. In some skates (Batoidea: Rajoidei), pectoral fins of mature males create a distinct bell-shaped body form not seen in females, although the pattern has never been evaluated in detail. This particular form of dimorphism is present in each of the sister species Leucoraja erinacea and Leucoraja ocellata, but differences between sexes are much greater in the former. In order to understand the nature and potential causes of pectoral dimorphism, we used geometric morphometrics to investigate allometry of fin shape in L. erinacea and L. ocellata and its relationship to the development of reproductive organs, based on previous work on bonnethead sharks, Sphyrna tiburo. We found that allometric trajectories of overall pectoral shape were different in both species of skate, but only L. erinacea varied significantly with respect to endoskeleton development. Male maturation was characterized by a number of sex-specific morphological changes, which appeared concurrently in developmental timing with elongation of cartilage supported claspers. We suggest that external sexual dimorphism of pectoral fins in skates is a product of skeletal growth needed for clasper development. Further, the magnitude of male shape change appears to be linked to the differential life histories of species. This work reports for first time that pectoral dimorphism is a persistent feature in rajoid fishes, occurring in varying degrees across several genera. Lastly, our results suggest that pectoral morphology may be useful as a relative indicator of reproductive strategy in some species.