Male diamondback terrapins, Malademys terrapin (190–240 g; 101–117 mm carapace length; 11–8 ±0–6 mm maximum jaw gape) selected small littorinids (<12 mm shell height) and mussels (<30 mm shell length) when offered prey of a range of sizes (at 26°C and 34‰. Jaw gape appeared to be the main constraint on feeding on molluscs. Responses to crabs were complex. Small crabs (10–25 mm carapace width) were eaten whole. Medium-sized crabs (30–50 mm carapace width) were ‘cropped’ (i.e. walking legs were eaten without killing the crabs). Large crabs (52–75 mm carapace width) were usually avoided, though terrapins sometimes took legs from pairs 3 or 4. Experiments with medium-sized crabs showed that diamondbacks evaluated crabs visually before attacking and preferred to eat crabs without chelipeds, rather than crabs with one or two chelipeds. Terrapins carried out attacks on medium-sized crabs from the side or rear and preferentially cropped leg pair 4 (the furthest from the chelipeds), even though leg pair 4 was significantly less valuable energetically than other leg pairs. Evidence is presented to show that cropping behaviour allows terrapins to exploit crabs which are too large to eat whole because of the constraints of jaw gape and the hard, smooth surface of the crab cephalothorax. Limb cropping is as energetically profitable as eating small crabs whole, but the threat posed by chelipeds causes terrapins to attack crab prey in the order small>medium>large.