Differences in the trophic niche along the gladius of the squids Illex argentinus and Doryteuthis gahi based on their isotopic values

The squids Doryteuthis gahi and Illex argentinus coexist in the Patagonian shelf and other oceanic waters of the South Atlantic Ocean. Both species perform movements to and from shallow waters related to their ontogenetic growth. To examine changes in the trophic habitats in both species and across the ontogeny, we analyzed the stable isotopic values of δ¹³C and δ¹⁵N in their gladii. Based on sections of the gladii, we compared the trophic niche in three stages (adult [the newest section], juvenile [middle section], and a near-birth section [near the conus]). δ¹³C and δ¹⁵N values differed significantly between the two species and between gladius segments, revealing interspecific and ontogenetic differences in the trophic niche width. In particular, differences between segments of the gladius concurred with the ontogenetic and spawning migrations of these squids, in the Patagonian Shelf.