Hydrodynamically Derived Negative Lift Generation of the Cephalic Lobes of Foraging Benthopelagic Myliobatids to Maintain Ground Contact

Abstract

The cownose ray (Rhinoptera bonasus) and spotted eagle ray (Aetobatus narinari) are benthopelagic myliobatids that forage on the ocean bottom. To sense prey under the bottom substrate, cownose rays deploy two depressible cephalic lobes, which are anterior modifications of the pectoral fins. Spotted eagle rays have a delta-shaped flattened rostrum from two fused cephalic lobes that is angled down in contact with the substrate when foraging. Geometry and orientation of the cephalic lobes of both rays, when in contact with the bottom, potentially indicate a passive hydrodynamic function. CT scans of the heads of the rays were used to construct physical models for water tunnel testing. Without cephalic lobes of the cownose ray deployed, a positive lift was generated when situated in the water column, but a negative lift was observed for a model with the cephalic lobes extended when in near contact with a solid surface. Flow visualization indicated that cephalic lobes deflected the water flow downward due to a Venturi effect from the pressure difference between fluids located externally and internally of the lobes. Likewise when angled downward and situated near a solid surface, cephalic lobes of the spotted eagle ray generated a negative lift. For both species, increased negative lift near a bottom substrate would aid in keeping the sensory surfaces of the cephalic lobes in contact with the substrate and counter any pitching motions induced by propulsive oscillations of the pectoral fins.