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Cephalopods play an important role in marine food webs, but their feeding ecology has been mostly examined by traditional methods. In this study, δ 13 C and δ 15 N values were measured along the proostracum (a morphological part of the gladius) to reconstruct squid feeding variations. Gladii or the internal shell of jumbo squid Dosidicus gigas was collected at 6 sites from the eastern Pacific Ocean to evaluate trophic shifts as a function of size. The results showed that C- and N- isotopic values varied throughout proostracum length for all squid; changes that could be determined by temporal variation in prey consumption. Both δ 13 C and δ 15 N significantly increased with length and supported ontogenic shifts in diet. Detectable differences in δ 15 N values were found at all sites. This δ 15 N-oceanic variation may be associated with variations in squid assimilated diet, but may also be due to differences in the biochemical cycle among areas. Isotopic values only overlapped among the nearest sites. These results suggest that D. gigas did not migrate over large geographic areas, but rather moved within narrower latitudinal ranges. We recommend the use of stable isotopic analyses along the gladius as an effective approach to tracing foraging variations and also in geographically differentiating subpopulations.

Ontogenetic variation of δ13C and δ15N recorded in the gladius of the jumbo squid Dosidicus gigas: geographic differences