Prey capture behavior and kinematics of the Atlantic cownose ray, Rhinoptera bonasus

Abstract

The structurally reinforced jaws of the cownose ray, Rhinoptera bonasus testify to this species’ durophagous diet of mollusks, but seem ill-suited to the behaviors necessary for excavating such prey. This study explores this discordance by investigating the prey excavation and capture kinematics of R. bonasus. Based on the basal suction feeding mechanism in this group of fishes, we hypothesized a hydraulic method of excavation. As expected, prey capture kinematics of R. bonasus show marked differences relative to other elasmobranchs, relating to prey excavation and use of the cephalic lobes (modified anterior pectoral fin extensions unique to derived myliobatiform rays). Prey are excavated by repeated opening and closing of the jaws to fluidize surrounding sand. The food item is then enclosed laterally by the depressed cephalic lobes, which transport it toward the mouth for ingestion by inertial suction. Unlike in most sharks, upper jaw protrusion and mandibular depression are simultaneous. During food capture, the ray's spiracle, mouth, and gill slit movements are timed such that water enters only the mouth (e.g., the spiracle closes prior to prey capture and reopens immediately following). Indigestible parts are then hydraulically winnowed from edible prey portions, by mouth movements similar to those used in excavation, and ejected through the mouth. The unique sensory/manipulatory capabilities of the cephalic lobes, as well as the cownose ray's hydraulic excavation/winnowing behaviors and suction feeding, make this species an effective benthic predator, despite its epibenthic lifestyle.