Predation on myctophids by the squid Moroteuthis ingens around Macquarie and Heard Islands: Stomach contents and fatty acid analyses

Abstract

The diet of the sub-Antarctic onychoteuthid squid Moroteuthis ingens was assessed using stomach contents analyses and fatty acids as dietary tracers. The contents of 54 stomachs (50 from squid collected near Macquarie Island and 4 from squid collected near Heard Island) were examined visually, and prey remains were identified to species level where possible. Myctophid fish were the most common prey item in the stomach contents of M. ingens and were identified in 59% of stomachs. In total, teleost fish remains were found in 96% of stomachs. The lipid class and fatty acid profiles of the digestive gland and mantle tissue were analysed for 5 to 6 squid from each area, in addition to 4 stomach fluid samples taken from Heard Island animals. Mantle tissue was low in lipid, and contained high concentrations of phospholipids and polyunsaturated fatty acids. Digestive gland tissue had a high lipid content, with a mean value of 26.8 ± 12.9% wet mass in Macquarie Island squid and 41.7 ± 8.5% wet mass in Heard Island squid, and was rich in triacylglycerol (TAG) and monounsaturated fatty acids (MUFA). Stomach fluid generally contained high concentrations of TAG, although 1 sample was high in wax ester. Stomach fluid was also characterised by high MUFA concentrations. The digestive gland of M. ingens grouped with the fatty acid profile of stomach fluid and some myctophid species in cluster analysis and multidimensional scaling, and appears to be a source of fatty acid dietary tracers. Thus the fatty acid profile of the digestive gland supports findings from stomach contents analyses that myctophids, particularly TAG-rich species, are an important prey group of M. ingens at Macquarie and Heard Islands. This combination of techniques has a potential to increase our knowledge of the feeding ecology of squids in the Southern Ocean.