KAJIURA, Stephen M*; FORNI, Jesica B; SUMMERS, Adam P; TYMINSKI, John P; WILLIAMS, Audrey T; Florida Atlantic University; University of California, Irvine; University of California, Irvine; Mote Marine Laboratory; Florida Atlantic University: Sexual dimorphism in elasmobranch fishes

Sexually dimorphic head shape is well documented in vertebrates from teleosts to mammals. However, only a single example of sexually dimorphic head shape is documented in the more basal chondrichthyan fishes. At sexual maturity, the rostral cartilages of male bonnethead sharks, Sphyrna tiburo, elongate concomitantly with the rod-like cartilages of the male intromittent organs (claspers) to create a distinct bulge along the anterior margin of the cephalofoil. We propose that the rostral cartilage elongation is a byproduct of endocrinological changes at the onset of sexual maturity that stimulate growth of the clasper cartilages. To determine if this phenomenon is unique to S. tiburo or widespread among the elasmobranchs, we examined rostral cartilage length and head shape among five coastal skate species (Rajidae) from the Atlantic basin. Twelve homologous landmarks on the head were digitized and a Procrustes analysis, thin plate spline, and principal components analysis were used to visualize and test for differences between the sexes. For all species, juveniles failed to demonstrate a sexual dimorphism in head shape whereas adults differed significantly. However, unlike S. tiburo, head shape differences among the skates were attributed to a suite of skeletal changes and not exclusively to an elongation of the rostral cartilages. Thus, sexually dimorphic head shape may be widespread among adults within the class Chondrichthyes and close examination of the rostral cartilages of other elasmobranch species may reveal previously overlooked differences. The basal location of the chondrichthyan fishes within the vertebrate clade extends the earliest appearance of cephalic sexual dimorphism among the vertebrates.