Zooming in on Bonnetheads: Quantifying Impacts of Maturity on Denticle Morphology

Shark skin is covered superficially by the crowns of dermal denticles, tooth-shaped scales, whose bases are embedded into the dermis. Denticles provide sharks with functional benefits, including protection during mating and enhanced drag reduction during high-speed swimming. In a few species, research showed that denticle morphology changes as a shark matures and differs between sexes, especially in regions where males bite females during mating. We quantified the denticle morphometrics of bonnethead sharks to investigate potential ontogenetic and body region differences as well as sexual dimorphisms. We examined abdominal skin from 24 bonnethead sharks (12 male and 12 female) between the first and second dorsal fins using scanning electron microscopy. Denticle morphometrics-including denticle shape (pointedness), crown width, crown length, % of overlap, ridge width, ridge angle, and the number of ridges-were measured using ImageJ. We hypothesized that embryonic sharks would have denticles shorter in length and width when compared to juvenile and mature sharks to follow a natural growth pattern, and that female sharks would have a higher % overlap and longer denticle crowns, to provide greater protection during mating. Denticle morphology varied significantly across maturity levels, with differences observed in the denticle shape, crown width, and % overlap. These data support our hypothesis that denticle morphology changes with development, potentially to optimize swimming performance. However, sex was only a significant factor for ridge angle and body region was not a significant effect for any variable measured here. These findings suggest that ontogeny drives denticle morphology variation more than sex or body region does for abdominal skin between the two dorsal fins. These results suggest that denticle shape remains consistent in the male and female adults but varies across maturity levels, possibly due to the hydrodynamic constraints of water flow during swimming or to provide a shark with greater protection as it matures. Understanding the developmental and functional adaptations of shark denticles provides insights into their evolutionary role in facilitating survival, locomotion, and reproductive methods.