Life-history traits of the giant squid Architeuthis dux revealed from stable isotope signatures recorded in beaks

Carbon and nitrogen isotope profiles constructed from the upper beaks of four giant squid Architeuthis dux from the Bay of Biscay and Namibian waters provided a time-integrated record of their diet. Values of delta N-15 ranged from 5.5 to 13.4% and of delta C-13 from 214.4 to 217.8%. Nitrogen isotope profiles differed significantly in shape among the four animals analysed, but delta N-15 increased along each profile, with lowest values around the rostral tip. The difference between the lowest and the highest delta N-15 values was similar to 5.8%, slightly less than a two-level difference between the trophic position of small and large A. dux. The increase in delta C-13 values was not as pronounced as for the delta N-15 profiles, but the changes suggest an ontogenetic shift in diet early in life from smaller prey of relatively low trophic status to larger prey of higher status. Fluctuations in delta C-13 values observed near the rostral tip may be associated with a greater intrinsic variability in the carbon isotope composition of relatively small prey, and/or transient migratory behaviour early in life. The relative stability of the delta C-13 profiles over the remainder of their lifespan is consistent with the hypothesis that adult giant squid inhabit relatively small, well-defined, and productive areas, where food resources have a constant carbon isotope composition.