Ontogenetic variation of delta C-13 and delta N-15 recorded in the gladius of the jumbo squid Dosidicus gigas: geographic differences

Cephalopods play an important role in marine food webs, but their feeding ecology has been mostly examined by traditional methods. In this study, delta C-13 and delta N-15 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 delta C-13 and delta N-15 significantly increased with length and supported ontogenic shifts in diet. Detectable differences in delta N-15 values were found at all sites. This delta N-15-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.