Exploring feeding behaviour in deep‐sea dragonfishes (Teleostei: Stomiidae): jaw biomechanics and functional significance of a loosejaw

Deep‐sea dragonfishes (family Stomiidae) possess spectacular morphologies adapted to capturing large prey items in a seascape largely devoid of biomass, including large fang‐like teeth set on extremely long jaws. Perhaps the most intriguing aspect of dragonfish morphology is a lack of a floor to the oral cavity (i.e. there is no skin between the mandibular rami) in species of three dragonfish genera. The present study aimed to investigate the kinematic properties and performance of lower‐jaw adduction in stomiid fishes and to infer what functional advantages or constraints the ‘loosejaw’ confers. A computation model based on dynamic equilibrium predicted very fast jaw adduction for all species at gapes ranging from 90–120° in 66.6–103 ms. Simulations demonstrated that forces resisting lower‐jaw adduction in dragonfishes, and long‐jawed fishes in general, are substantially greater than those in fishes with shorter jaws. These forces constrain inlever length, resulting in relatively high mechanical advantages to attain fast adduction velocities. By reducing the surface area of the lower‐jaw system, loosejaws drastically reduce resistive forces. This has permitted loosejaw dragonfishes to evolve lower mechanical advantages that produce high displacement velocities with an extremely long jaw, a distinct asset in capturing large and scarce resources in the deep‐sea. In addition, loosejaws require a substantially reduced adductor mass to close long jaws at high velocities. These results reveal that the loosejaw condition is an adaptation that expands the morphological boundaries imposed by the dynamic limitations of a long jaw. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 106 , 224–240.