Morphology of the Feeding Apparatus of Cancer novaezelandiae in Relation to Diet and Predatory Behavior

Morphology of the mouthparts, gastric mill, and chelae of the New Zealand cancer crab, Cancer novaezelandiae Jacquinot, 1853, was investigated in relation to dietary composition and predatory behavior. Mouthparts and gastric mill were typical of those of other large, predatory brachyurans, with similar structure for male and female, small (60-70 mm) and large (120-130 mm carapace width) crabs. The third maxilliped had large crista dentata, and the inner margin of the mandible was rounded, with a sharp, cutting edge. The large, robust chelae were homeochelous with respect to structure and dental pattern. A large diastema was present and both chela exhibited high mechanical advantage (0.36 and 0.37 for left and right chela, respectively). Relative growth of the propodus was positively allometric, which remained constant throughout crab growth. Morphological features of the feeding apparatus suggested adaptations for macerating coarse, particulate material. This was supported by foregut analysis showing a predominance of sessile and slow-moving macroinvertebrates in the diet. Bivalve and gastropod molluscs followed by crustaceans dominated the diet; fish, sponges, coelenterates, and plant matter occurred less frequently. Little variation in dietary composition was evident with crab sex, size, or season. Cancer novaezelandiae adopted five distinct techniques to open bivalve shells and three techniques to open gastropod shells. These include direct, umbone and posterior crushing, umbone splitting, posterior chipping, and aperture breakage and spire removal. The success of these techniques was dependent upon crab size and prey size and shape. Large crabs were able to use direct crushing over a wider size range of prey than smaller crabs. The structural and behavioral adaptations permit C. novaezelandiae to specialize on mollusc prey and may explain its migrations into areas dominated by molluscan species