a total of 67 discopyge tschudii was captured. individuals were sexed, measured and their stomach content analysed. siphon tips of the bivalve amiantis purpurata from each stomach were individually counted, weighed and their length and width measured. the size–frequency distribution of the local population of a. purpurata was surveyed for two years. siphons of a. purpurata were the main food item in 90% of individuals having stomach contents. the clam population was markedly bimodal, with 24 mm and 46 mm mean shell length for the younger and older size-classes, respectively. grazing by d. tschudii on a. purpurata varied with size and sex of the fish. small rays mainly ingested small siphons and large rays large siphons. nipped siphon tips were longer in male than in female rays.