WILGA, C.D.*; HUETER, R.E.; WAINWRIGHT, P.C.; MOTTA, P.J.: FUNCTION AND EVOLUTION OF UPPER JAW PROTRUSION MECHANISMS IN ELASMOBRANCHS

Upper jaw protrusion is an important component of the feeding mechanism in most elasmobranchs. To quantify the mechanics of feeding, particularly those of upper jaw protrusion, in four diverse species of elasmobranchs we used anatomical dissection, electromyography and high-speed video recordings. Our results show that while certain muscles show a common pattern of motor activity among the four species during feeding, other muscles are functionally and evolutionarily plastic. Our observations of elasmobranch upper jaw protrusion mechanisms suggests a mosaic of character changes over the course of evolution that involve anatomical changes in all cases and modifications of muscle activation patterns in some cases. The muscles involved in upper jaw retraction also show modulation in some species and may be active during jaw opening as well as in jaw retraction. The primary jaw closing muscle in a durophagous shark exhibits an extended double-burst pattern of motor activity during crushing of hard-shelled prey. Within the evolution of feeding mechanisms of elasmobranchs, there are several structural changes that retain a conserved motor pattern and behavior. At least one instance of structural modification is accompanied by an alteration in the motor pattern which led to a change in behavior. Finally, several instances of plasticity in activation of certain muscles exist.