Feeding habits of juvenile Japanese common squid Todarodes pacificus: Relationship between dietary shift and allometric growth

In this study, ontogenetic dietary shifts in relation to morphological changes were examined in juvenile Japanese common squid Todarodes pacificus (<160 mm mantle length; ML) collected in the Sea of Japan. Ten morphometric characters were measured for 147 specimens of Japanese common squid. The allometric growth pattern of each character in relation to ML was studied by using the logarithmic form of the allometric growth model with piecewise regression analysis. The allometric growth pattern indicated that the body shape of T. pacificus changed from a rounded to a more streamlined shape, and only growth of the fin base length and fin width were positively allometric. Three morphological characters had an inflection point at 53–72 mm ML, indicating the transformation to an adult morphology. The digestive tract contents of 135 squids were examined for dietary shifts. T. pacificus smaller than 50 mm ML fed exclusively on crustaceans such as copepods, amphipods and euphausiids, and then shifted to a mixed diet of crustaceans and fish at 50–99 mm ML; after reaching 100 mm ML, fish were the most important prey items and crustaceans became less important. These suggest that the swimming performance of T. pacificus changes markedly at around inflection points, the size at which T. pacificus starts to feed on fish. Therefore, the dietary shift is suggested to be closely related to the ontogenetic change in swimming performance.