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

<jats:title>Abstract</jats:title><jats:p>Guerra, Á., Rodríguez-Navarro, A. B., González, Á. F., Romanek, C. S., Álvarez-Lloret, P., and Pierce, G. J. 2010. Life-history traits of the giant squid Architeuthis dux revealed from stable isotope signatures recorded in beaks. – ICES Journal of Marine Science, 67: 1425–1431. 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 δ15N ranged from 5.5 to 13.4‰ and of δ13C from −14.4 to −17.8‰. Nitrogen isotope profiles differed significantly in shape among the four animals analysed, but δ15N increased along each profile, with lowest values around the rostral tip. The difference between the lowest and the highest δ15N values was ∼5.8‰, slightly less than a two-level difference between the trophic position of small and large A. dux. The increase in δ13C values was not as pronounced as for the δ15N 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 δ13C 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 δ13C 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.</jats:p>