The Ross Sea cephalopod community: insights from stable isotope analysis

Based on mass balance modeling and mixed trophic impact analysis, cephalopods have been identified as having relatively high importance in the food-web of the Ross Sea. However, information on the trophic ecology of the cephalopod assemblage of the Ross Sea region is poor. Stable isotope signatures of nitrogen (δ¹⁵N) and carbon (δ¹³C) were determined in muscle and beak tissue from a range of cephalopod taxa sampled from the Ross Sea region. Samples were acquired through scientific trawls and from stomachs of Antarctic toothfish Dissostichus mawsoni returned from the longline fishery. Most octopods were not identified to species which limits analysis. All squid samples were identified to species, but isotopic comparisons were made at the level of genus to accommodate the uncertainty in octopod identifications. Generally, squid clustered into two distinct isotopic groups, one with depleted isotope signatures typical of feeding on relatively low trophic level prey within the Ross Sea, and the second with elevated isotopic signatures typical of feeding on relatively high trophic level prey with some suggestion of either movement northwards out of the Ross Sea or of coupling with the benthic system. In contrast, octopod genera exhibited relatively elevated and diverse isotope signatures, typical of benthic feeding where nutrients tend to be recycled repeatedly. Beak isotope values were consistently depleted compared to those in muscle tissue, whereas carbon isotope values were similar in both beak and muscle tissues. Size (and hence age) was an important factor in explaining variation in both beak and muscle tissue isotope signatures in only one species – the colossal squid Mesonychoteuthis hamiltoni: both isotope signatures were positively and significantly correlated with beak size. The utility and value of applying stable isotope analyses to cephalopod taxa from the Ross Sea is discussed.