Hard prey, soft jaws and the ontogeny of feeding mechanics in the spotted ratfishHydrolagus colliei

Abstract

The spotted ratfishHydrolagus collieiis a holocephalan fish that consumes hard prey (durophagy) but lacks many morphological characters associated with durophagy in other cartilaginous fishes. We investigated its feeding biomechanics and biting performance to determine whether it can generate bite forces comparable with other durophagous elasmobranchs, how biting performance changes over ontogeny (21–44 cm SL) and whether biomechanical modelling can accurately predict feeding performance in holocephalans.Hydrolagus collieican generate absolute and mass-specific bite forces comparable with other durophagous elasmobranchs (anterior=104 N, posterior=191 N) and has the highest jaw leverage of any cartilaginous fish studied. Modelling indicated that cranial geometry stabilizes the jaw joint by equitably distributing forces throughout the feeding mechanism and that positive allometry of bite force is due to hyperallometric mechanical advantage. However, bite forces measured through tetanic stimulation of the adductor musculature increased isometrically. The jaw adductors ofH. collieifatigued more rapidly than those of the piscivorous spiny dogfishSqualus acanthiasas well. The feeding mechanism ofH. collieiis a volume-constrained system in which negative allometry of cranial dimensions leaves relatively less room for musculature. Jaw adductor force, however, is maintained through ontogenetic changes in muscle architecture.