Biomarkers (lipids, fatty acids and stable isotopes) to examine diet of the sea urchin Evechinus chloroticus.

Abstract

This thesis describes the use of biomarkers as tools to better understand the diet preferences and trophic position of Evechinus chloroticus, the dominant grazer of New Zealand rocky reefs. Biomarkers, such as lipids, fatty acids and stable isotopes, were combined to improve the knowledge of feeding habits of wild sea urchins from north-eastern New Zealand, revealing a high contribution from Ecklonia radiata to E. chloroticus diets, but also a contribution from Carpophyllum plumosum in the Hauraki Gulf. The use of lipid and fatty acid composition of E. chloroticus gonads collected from three locations within the Hauraki Gulf revealed a digestive pathway from the brown seaweed, especially E. radiata, to gut contents, gut and gonads, confirming the storage role for nutrients played by gonads in sea urchins. Biosynthesis and selective retention of essential fatty acids seen in the gonads confirm their dual function, as a reproductive and storage organ. This pattern was consistent at the three locations. However, lipid and fatty composition showed a variation between different populations of E. chloroticus within the Hauraki Gulf, outer (Great Barrier Island), intermediate (Matheson’s Bay) and inner gulf (Rakino and Rangitoto Islands), suggesting that environmental conditions, such as water temperature, light, turbidity, salinity and nutrients affect the lipid and fatty acid composition of seaweed. Additionally, artificial diets also had an effect on the fatty acid composition of E. chloroticus, indicating that this sea urchin species was able to biosynthesise and selectively retain essential fatty acids when they are present in high amounts in the diets. Seasonal lipid variation was observed in three of the dominant seaweed species E. radiata, C. plumosum and Cystophora torulosa, from the Hauraki Gulf, with higher levels of lipids during winter and spring, when E. chloroticus is accumulating nutrients to prepare for gametogenesis. Application of a mixing model to the stable isotope data allowed quantification of the contribution of these potential food items to E. chloroticus diet. Almost 40% of the diet is derived from E. radiata and 40% from C. plumosum, making them the principal contributors to urchins feeding in Matheson’s Bay. A very low contribution to its diet was made by C. maschalocarpum and C. torulosa. Additionally, a trophic level of 1.5 indicated that E. chloroticus is also feeding on microorganisms living on the seaweeds or possibly has nitrogen-fixing bacteria resident in the gut. A comparison of the fatty acid composition of two different species of sea urchins, E. chloroticus and A. dufresnii, from different geographical locations revealed that not only the diet, but also the environmental conditions and the reproductive stage of the animals affect their fatty acid composition. Both sea urchin species showed similar fatty acid composition to the dominant brown seaweeds of those environments, indicating that E. chloroticus feeds on E. radiata and A. dufresnii on Undaria pinnatifida. The information presented in this thesis is valuable not only for ecological purposes but for future application in aquaculture as E. chloroticus is the only sea urchin commercially harvested in New Zealand.