Stable isotopes document the trophic structure of a deep-sea cephalopod assemblage including giant octopod and giant squid.

Although deep-sea cephalopods are key marine organims, their feeding ecology remains essentially unknown. Here, we report for the first time the trophic structure of an assemblage of these animals (19 species) by measuring the isotopic signature of wings of their lower beaks, which accumulated in stomachs of stranded sperm whales. Overall, the species encompassed a narrow range in delta(13)C values (1.7 per thousand), indicating that they lived in closely related and overlapping habitats. delta(13)C values can be interpreted in terms of distribution with the more (13)C-depleted species (e.g. Stigmatoteuthis arcturi, Vampyroteuthis infernalis) having a more pelagic habitat than the more (13)C-enriched, bathyal species (e.g. Todarodes sagittatus and the giant squid Architeuthis dux). The cephalopods sampled had delta(15)N values ranging 4.6 per thousand, which is consistent with the species spanning approximately 1.5 trophic levels. Neither the giant octopod (Haliphron atlanticus) nor the giant squid reached the highest trophic position. Species delta(15)N was independent of body size, with large squids having both the highest (Taningia danae) and lowest (Lepidoteuthis grimaldii) delta(15)N values. Their trophic position indicates that some species share the top of the food web, together with other megacarnivores such as the sperm whale.