A head of the curve: Bonnethead shark ( Sphyrna tiburo ) cephalofoil morphology and trophic ecology

Abstract Notable characteristics of bonnethead sharks (Sphyrna tiburo) include an omnivorous diet and sexual dimorphism in cephalofoil morphology, with males exhibiting a more prominent tip at the anterior of their snout, which one study hypothesized develops with sexual maturity. However, it is not known if this characteristic is prevalent in all bonnethead subpopulations, nor whether it is consistent across their range. The potential ecological or physiological function (mating, feeding, swimming energetics) of this difference is also unknown. In this study, cephalofoil morphology was assessed by sex and maturity for bonnethead sharks in Biscayne Bay and Tampa Bay, Florida. Additionally, muscle biopsies were analyzed for stable isotope composition to explore potential associations with diet. Sharks were caught between May 2022 and May 2023 via longline (Biscayne) and gillnet (Tampa). Body measurements and dorsal and ventral photographs of the head were taken as part of a scientific workup and sharks were promptly released. Images were analyzed in ImageJ to determine a best-fit parabolic curve; comparisons of cephalofoil curvature were based on the absolute “a” coefficient from the parabolic curve equation y = ax2 + bx + c via ANOVA and PERMANOVA. Shape coordinates were also analyzed with a Procrustes ANCOVA to test for differences among groups while controlling for covariates. In both Biscayne Bay and Tampa Bay, males and females had significantly different head shapes, but, contrary to previous hypotheses, the cephalofoils of both sexes became rounder at maturity. There was greater rounding in female cephalofoils than male, and females experienced greater changes in cephalofoil shape as they matured. Diet, as examined through δ13C and δ15N values of muscle biopsies, showed isotopically distinct food webs with differing ontogenetic dietary patterns between Biscayne Bay and Tampa Bay following ANOVA and ANCOVA tests, but was not correlated to bonnetheads’ cephalic sexual dimorphism. From this result, we hypothesize that younger and smaller bonnethead sharks of both sexes may benefit from the pointed cephalofoil for enhanced swimming performance, and that larger adult females may outgrow these benefits through development of physical characteristics, including larger body sizes necessary for reproduction.