Boxfish (Ostraciidae) have peculiar body shapes with conspicuous keels formed by their bony carapace. Previous studies have proposed various hydrodynamic roles for these keels including: reducing drag during swimming, contributing to passive stabilisation of the swimming course, or providing resistance against roll rotations. Here, we tested these hypotheses using computational fluid dynamics simulations of five species of Ostraciidae with a range of carapace shapes. The hydrodynamic performance of the original carapace surface models, obtained from laser scanning of museum specimens, were compared with models in which the keels were digitally reduced. The original carapaces showed no reduced drag or increased passive stability against pitch and yaw compared to the reduced-keel models. However, consistently for all studied species, a strong increase in roll drag and added mass was observed as a result of the presence of keels, despite the relatively small reductions in keel height. Especially the damping of roll by resistive drag torques increased considerably. Our results suggest that the shape of the boxfish carapace is important to enable the observed roll-free forward swimming of boxfish, and that it may facilitate the control of maneuvers.

ANSYS Workbench 19.1 simulation output.