| Summary: | Poster.-- Cephalopod International Advisory Council Symposium. Vigo (Spain), 3-11 September 2009 Three upper beaks of the giant squid Architeuthis dux from Asturias (North Iberian Peninsula) and one from Namibian waters were used to investigate whether variations in the stable isotope composition of nitrogen and carbon along chitin-protein complex growth layers in the hood of a single beak reflect differences in ontogenetic dietary shifts in animals from different regions. δ15 N values ranged from 6.3 to 7.1‰ and δ13C from -10.2 to -11.4‰. δ15 N profiles differed significantly between the four animals analyzed. These profiles differed in shape but all of them decreased along each profile with a sharp downturn around 20 mm from the rostral tip of the beak hood. This trend was clear but it was not linear in most cases. The δ 13 C profiles also differed in shape and decreased along each profile, although the decrease in value was not as pronounced than in the δ15 N profiles. Both 15N/ 14 N and 13 C/12 C isotopes ratios suggested the existence of an ontogenetic diet shift during the early life of the giant squid from smaller preys of relatively low trophic status to larger preys higher in the food web. Fluctuations in δ 13 C profiles observed near the rostral tip might be associated to the greater intrinsic variability in the composition of relatively small prey and/or may be attributed to a migratory behaviour. The relative stability in δ 13C profiles shown afterwards suggests that adult giant squids probably stay in a relative small and well defined productive area where the food resources have a relatively constant carbon isotope composition. Both δ15N and δ13 C values segregated the giant squids from North Iberian Peninsula, Namibia and those from Kerguelen Islands, which could indicate that these giant squids grew in different marine ecosystems. The stable isotope signature of only one beak of Architeuthis revealed interesting aspects of its life history traits and is a powerful tool in particular to ...
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