Constructional morphology within the head of hammerhead sharks (sphyrnidae)

The study of functional trade-offs is important if a structure, such as the cranium, serves multiple biological roles, and is, therefore, shaped by multiple selective pressures. The sphyrnid cephalofoil presents an excellent model for investigating potential trade-offs among sensory, neural, and feeding structures. In this study, hammerhead shark species were chosen to represent differences in head form through phylogeny. A combination of surface-based geometric morphometrics, computed tomography (CT) volumetric analysis, and phylogenetic analyses were utilized to investigate potential trade-offs within the head. Hammerhead sharks display a diversity of cranial morphologies where the position of the eyes and nares vary among species, with only minor changes in shape, position, and volume of the feeding apparatus through phylogeny. The basal winghead shark, Eusphyra blochii, has small anteriorly positioned eyes. Through phylogeny, the relative size and position of the eyes change, such that derived species have larger, more medially positioned eyes. The lateral position of the external nares is highly variable, showing no phylogenetic trend. Mouth size and position are conserved, remaining relatively unchanged. Volumetric CT analyses reveal no trade-offs between the feeding apparatus and the remaining cranial structures. The few trade-offs were isolated to the nasal capsule volume's inverse correlation with braincase, chondrocranial, and total cephalofoil volume. Eye volume also decreased as cephalofoil width increased. These data indicate that despite considerable changes in head shape, much of the head is morphologically conserved through sphyrnid phylogeny, particularly the jaw cartilages and their associated feeding muscles, with shape change and morphological trade-offs being primarily confined to the lateral wings of the cephalofoil and their associated sensory structures.