FRANCIS, A.W.; Villanova University, PA: Phylogeny and Feeding Biomechanics of Pleuronectiform Fishes

Mechanical advantage of lower jaw depression and elevation was determined for five families and 10 genera of pleuronectiform fishes. Of the 11 species examined, five were sinistral and six dextral. Mechanical advantage for both lower jaw depression and elevation was lowest for species of Psettodidae and Paralichthyidae. For species of Pleuronectidae, Achiridae, and Cynoglossidae, mechanical advantage of lower jaw depression and elevation was greater. When bilateral measurements of mechanical advantage were compared, opening and closing lever ratios for the ocular side (left in sinistral species and right in dextral species) were always greater than the opposing blind side. Thus, greater speed is generated on the ocular side while greater force is generated on the blind side for all pleuronectiforms examined. The phylogenetic pattern is for basal pleuronectiforms, like Psettodidae and Paralichthyidae, to generate greater force at the tip of the lower jaw by both ocular and blind sides when opening and closing the mandible in pursuit of fish. For more derived pleuronectiforms, like Pleuronectidae, Achiridae and Cynoglossidae, the lower jaw of both ocular and blind sides opens and closes rapidly in pursuit of benthic invertebrates. Thus, there appears to be a shift from basal force generating piscivores to derived velocity generating invertebrate feeders. This trend is also reflected in the degree of asymmetry between ocular and blind side mechanical advantage, with the least asymmetry in more basal pleuronectiforms and greatest in more derived pleuronectiforms.