Head morphology

Bonnethead sharks are small coastal sphyrnids that demonstrate the least amount of lateral expansion of their cephalofoil. Bonnetheads from the Pacific were previously described as a separate species or subspecies from Atlantic specimens based upon their different head shapes. Current research on comparative head morphology demonstrates that this difference is strongly supported and that there is an additional sexual dimorphism in head shape at least among specimens from the Atlantic.

Male and female bonnethead sharks do not differ in head morphology between the sexes as embryos and juveniles. As adults, female bonnethead sharks are characterized by a broadly rounded head morphology whereas males possess a distinct bulge along the anterior margin of the cephalofoil. This bulge is formed by the elongation of the rostral cartilages of the males at the onset of sexual maturity and corresponds temporally with the elongation of the clasper cartilages.

Tooth morphology

Male elasmobranchs bite and grip the body of their female mates during courtship and copulation. In batoids (skates & rays), this gripping action is facilitated by the development of sharp, cuspidate dentition on the males only. Female dentition is generally molariform but, despite the differences in tooth morphology, the diets of the sexes remain the same. Only during the mating season does the male dentition develop the cuspidate tooth morphology; during the non-mating season the male dentition is molariform, similar in shape to the female dentition which does not change seasonally.

In conjunction with studies of the tooth morphology, the resultant mating wounds on female rays were quantified. Females have a dermis that is 50% thicker than comparably sized males, presumably to provide protection from the vigorous mating bites of the males. The mating wounds closely follow the seasonal appearance of the cuspidate male dentition.

Kajiura, SM, JP Tyminski, JB Forni & AP Summers. 2005. The sexually dimorphic cephalofoil of bonnethead sharks, Sphyrna tiburo. Biological Bulletin 209: 1-5. (with cover)

Kajiura, SM, AP Sebastian & TC Tricas. 2000. Dermal bite wounds as indicators of reproductive seasonality and behaviour in the Atlantic stingray, Dasyatis sabina. Environmental Biology of Fishes 58(1): 23-31.

Kajiura, SM & TC Tricas. 1996. Seasonal dynamics of dental sexual dimorphism in the Atlantic stingray, Dasyatis sabina. Journal of Experimental Biology 199(10): 2297-9306.