Hard prey specialists or opportunistic generalists?: An examination of the diet of the cownose ray, Rhinoptera bonasus

Atlantic cownose rays (Rhinoptera bonasus) are benthic, suction feeders whose foraging activities have been implicated in severe damage to commercial shellfish industries. With jaws highly modified for durophagy, it has been assumed that R. bonasus are crushing specialists that feed primarily upon hard molluscan prey. Stomach contents from R. bonasus caught within Florida's Charlotte Harbor estuary between July 2003 and July 2004 were analysed using the index of relative importance (IRI) to determine most important prey types. Prey items were identified from 38 families and fell into nine distinctive groups. The three most dominant prey groups were crustaceans (%IRI = 55.31), polychaetes (%IRI = 25.20) and bivalves (%IRI = 12.58). Shoalmates had more similar diets than non-shoalmates, suggesting group feeding. Most small or softer-bodied prey consumed were relatively intact, indicating lack of prey processing and capture through suction feeding. All larger, harder-bodied prey showed evidence of crushing (fractured and broken shells). Although R. bonasus has been characterised as a hard prey specialist, these results suggest it may behave as an opportunistic generalist, modifying feeding behaviour to consume readily available prey.