Tissue, ontogenic and sex-related diVerences in 13C and 15N values of the oceanic squid Todarodes Wlippovae (Cephalopoda: Ommastrephidae)
Résumé
Stable isotopes are increasingly used in the study of trophic interactions of many aquatic animals and most recently cephalopods. To evaluate the application of the method to squids, it is important to assess isotopic diVerences among and within consumer tissues that may confound the resolution of ecological relationships. Interand intra-tissue isotopic variation was examined in 55 individuals of the oceanic squid Todarodes Wlippovae that were collected at the beginning of April 2000 in the southwestern Indian Ocean (between 44°S, 76°E, and Saint Paul and Amsterdam islands, 38°S, 78°E). Delipidated soft tissues (mantle, arm, buccal mass, gill and reproductive organs) showed small 13C and 15N diVerences, which were probably tissue-speciWc. A lower carbon value was observed in the digestive gland as a consequence of incomplete lipid removal. Hard tissues, such as beaks and gladii, had lower 15N values than soft tissues, which can be explained by the presence of chitin, a 15N-depleted molecule. Females (n = 38) and males (n = 17) had identical 13C values, but females showed higher 15N values than males. The diVerence was size-related rather than sex-related, however, as females were generally larger than males. A comparison of similar-sized females and males produced identical nitrogen values. These data suggest dietary shifts from lower to higher trophic levels during growth, because 15N values of large T. Wlippovae were much higher than that of small specimens. As expected, nitrogen values of lower beaks and gladii of large squids increased from the oldest to the most recently formed region, reXecting the progressive growth of chitinized tissues in parallel with dietary changes. Sequential sampling along the growth increments of squid beaks and gladii can likely be used to produce a chronological record of dietary information throughout an individual's history.