DEAN, M.N.; MOTTA, P.J.; Univ. of S. Florida; Univ. of S. Florida: Hoover vacuums and scissor jacks: A novel jaw protrusion mechanism in batoids

Protrusion of the jaws during feeding is common in members of the Batoidea (rays, skates, sawfishes and guitarfishes), members of which possess a highly modified jaw suspension. The lesser electric ray Narcine brasiliensis preys primarily on polychaete annelids using an extreme and unique mechanism for jaw protraction. The ray captures its prey by shoving its jaws (protruding in an �oral tube� more than 10% of body length) beneath the substrate and generating negative oral pressures to suck worms into its mouth. The suction pressures exerted on the surface of prey items are as powerful as one-third of an atmosphere below ambient. Prey is often further winnowed from ingested sediment by repeated, often asymmetrical protrusions of the jaws while sand is expelled from the spiracles, gills and mouth. Initiation of this protrusion is similar to that proposed for other batoids, in that the swing of the distal ends of the hyomandibulae is transmitted to the mandible. In Narcine, the tight associations between the lateral edges of the jaws and extremely flexible symphyses allow medial compression of the entire jaw complex (i.e. shortening the distance between the right and left corners of the jaws). As a result, a more acute symphyseal angle is formed and the jaws re-orient from the animal�s transverse to the mid-saggital axis. Motion of the skeletal elements involved in this extreme jaw protrusion is convergent with that described for the wobbegong Orectolobus maculatus. The euhyostylic jaw suspension of Narcine, however, allows a degree of ventral protrusion impossible for the Orectolobidae, creating great versatility and maneuverability for retrieving prey items from the benthos.