Cownose Ray (Rhinoptera Bonasus) Predation Relative To Bivalve Ontogeny

The purpose of this study was to determine the ability of the cownose ray, Rhinoptera bonasus (Mitchill, 1815), to manipulate oysters and clams, to test for relative prey preference, and to investigate whether susceptibility to cownose ray predation changes with bivalve ontogeny. We investigated patterns of predation for captive adult and young-of-year cownose rays on 4 species of bivalves, including Crassostrea virginica (Gmelin, 1791), Crassostrea ariakensis (Fujita, 1913), Mercenaria mercenaria (Linnaeus, 1758), and Mya arenaria Linnaeus, 1758. In oyster (C. virginica) trials, predation probabilities by adult rays were highest at shell heights of 30-70 mm and shell depths of 8-22 mm. The rates of predation by adult rays in trials in which same-size oysters were used were higher than rates in most comingled trials. Adult rays showed no differences in predation between native oysters (C. virginica) and nonnative oysters (C. ariakensis; P > 0.05). Adult rays selected hard- and soft-shell clams (Manly-Chesson index M. mercenaria, alpha = 0.736 +/- 0.002, electivity = 0.473 +/- 0.007; M. arenaria, alpha = 0.742 +/- 0.003, electivity = 0.485 +/- 0.013) over oysters (C. virginica, alpha = 0.263 +/- 0.002, electivity = -0.473 +/- 0.007; alpha = 0.257 +/- 0.003, electivity = -0.485 +/- 0.003). In young-of-year feeding trials, oysters with a shell height of 10-35 mm and a shell depth of 3-12 mm had the highest probability of predation. Native oyster and hard clam peak force or load crush tests resulted in forces of 200-1,500 N and 400-1,400 N across shell depths of 10-35 mm and 21-34 mm, respectively, before valve failure. The results of this study indicate that cownose ray predation on shellfish is limited by shell size and is likely related to ray jaw gape and bite force.