Bite force and performance in the durophagous bonnethead shark, Sphyrna tiburo

Abstract

Bite force, a measure of performance, can be used to link anatomical form and function. Earlier studies have shown bite force to have a significant influence on dietary constraints and ontogenetic shifts in resource utilization. The bonnethead shark, Sphyrna tiburo, is a durophagous member of the family Sphyrnidae. Its diet in South Florida waters consists almost entirely of blue crabs, which are crushed or ingested whole. This abundant coastal predator's feeding mechanism is specialized for the consumption of hard prey, including a modified biting pattern and molariform teeth. The goals of this research were to (1) characterize the mechanical function of the feeding mechanism of S. tiburo through biomechanical modeling of biting and in vivo bite force measurements; (2) compare the bite force of S. tiburo with those of other fishes; and (3) identify functional constraints on prey capture by comparing the bite force of S. tiburo with the fracture properties of its primary prey item, blue crabs. Maximum theoretical bite force ranged from 25.7 N anteriorly to 107.9 N posteriorly. S. tiburo has the second lowest mass specific bite force for any fish studied to date, and its posterior mechanical advantage of 0.88 is lower than other durophagous chondrichthyans, indicating that this independent evolutionary acquisition of durophagy was not accompanied by the associated morphological changes found in other durophagous cartilaginous fishes. Blue crab fracture forces (30.0-490.0 N) range well above the maximum bite force of S. tiburo, suggesting that prey material properties functionally constrain dietary ecology to some degree.