Variation in hydrodynamic performance among boxfish carapace shapes

The clade of boxfish (Ostraciidae and Aracanidae) are reef-dwelling fish that share a unique feature, the carapace. The body of Ostraciidae is completely covered by a bony shell of fused hexagonal plates. In Aracanidae, the carapace also covers the body, but is not completely closed at the caudal peduncle for some of the species. The carapace provides protection against predators, but its shape also defines their hydrodynamic characteristics. Among the many extant species, shapes vary between boxes with major lateral keels, over wedges with dorsal and ventrolateral keels, to more disc-like shapes with major dorsoventral keels. Here, we studied how hydrodynamic performance differs among twenty-four species of boxfish. Drag forces and moments were calculated using computational fluid dynamics simulations based on 3D laser-scans of museum specimens, which were all digitally rescaled to the same carapace volume. Up to 40% difference was observed between boxfish species with the lowest drag force and those with the highest drag force. We found a strong positive correlation between drag force and frontal-projected area and a negative correlation between drag force and carapace length. The strong variability in drag force related to the degree of axial compression of boxfish shapes, together with the relatively high coefficients of drag in boxfish in general, suggests that weak selection pressures on drag-reduction are common in this group.