Dietary fatty acid analyses of the squid Idioteuthis cordiformis: further evidence for predation on deepwater sharks

Abstract

Idioteuthis cordiformis is the largest deepwater mastigoteuthid squid in the southern Pacific Ocean. Signature fatty acid (FA) and lipid class analysis was carried out on the digestive gland, fins and caecum oil of 18 individuals of I. cordiformis caught in the waters off southern Australia during late 2004. Lipid classes varied between the tissues and oil samples, with sex not being an important factor. The presence of hydrocarbons within the digestive gland and caecum was noteworthy, as high proportions of this lipid class are generally only common in the livers of many deepwater sharks. Monounsaturated FAs dominated the digestive gland and caecum oil, while the fin had high values of both saturated and polyunsaturated FAs. FA profiles of I. cordiformis were compared to profiles of potential prey species (sharks, small and large fish, crustaceans and squid) using Bray-Curtis similarity coefficients. Analysis of the digestive gland and caecum oil FA profiles revealed a close match with the following prey: myctophids (Lampanyctodes australis, Electrona paucirastra, Symbolophorus barnardi), the dragon fish Stomias boa, the smooth oreo Pseudocyttus maculatus and the deepwater sharks Etmopterus baxter, Dalatias licha, Centroselachus crepidater, Centroscymnus coelopsis and Centrophorus zeehaani. The fin FA profile did not match closely to any potential prey and was most similar to other squid mantle tissue. Based on the results of this study, I. cordiformis has a broad diet spectrum of teleost fish and deepwater sharks and occupies a high trophic position.