Jaw-dropping sculpins : comparative functional morphology and evolution of the Cottoid feeding apparatus

Fifty-five species of sculpin (Scorpaeniformes: Cottoidea) inhabit the Salish Sea of Washington, USA and British Columbia, Canada, and their heads range in shape from short and squat to long and sleek to lumpy and oblong. This study focuses on jaw leverage, or the mechanical relationship between the movement of muscles and jaws, in five of these species. I measured 14 anatomical and nine kinematic variables describing jaw form and function, and I phylogenetically corrected the data to track how jaws evolve. Anatomical leverage was measured in dissected specimens, and kinematic leverage in live feeding fish. Both lines of data revealed that the jaw of one species, the crab-eating ambush predator Hemilepidotus hemilepidotus, provides leverage needed for a significantly faster, weaker jaw than sympatric sculpins. Phylogenetic independent contrast showed that evolutionary shifts to higher gear ratios correlated with shifts to shorter muscles across all five species. Coevolution of leverage and muscle length allows diverse jaw structures to maintain similar muscle strain magnitudes, a possible strategy for preserving high feeding performance. This morphological integration may help explain how dozens of sculpin species maintain distinct head morphologies and coexist in the Salish Sea.