Unexpected shifts in fatty acid composition in response to diet in a common littoral amphipod

To determine whether fatty acid (FA) profiles are a useful biomarker to trace the flow of material in a coastal food web, we fed the sandhopper Bellor - chestia quoyana specific seaweed diets, each with a contrasting FA profile including Durvillaea antarctica (Phaeophyta), Ecklonia radiata (Phaeophyta) or Ulva sp. (Chlorophyta). We then compared changes in FA composition in relation to diet for this sandhopper. After 12 d, sandhoppers from each treatment had distinct FA profiles, particularly with respect to polyunsaturated FAs (PUFAs); however, increases in specific FAs did not relate to those FAs that were abundant in their diet. For example, sandhoppers fed PUFA-deficient Ulva sp. exhibited a relative increase in PUFAs. The E. radiata and Ulva sp. diets both caused significant shifts in sandhopper FA composition over the course of the experiment. In order to follow the assimilation of carbon and FAs, sandhoppers were fed natural or 13C-enhanced E. radiata or Ulva sp., and changes to the δ13C of individual FAs were measured over time. Turnover of the most abundant FAs, 16:0 and 18:1ω9, was higher for sandhoppers fed E. radiata than for those fed Ulva sp. Comparisons between bulk tissue δ13C and δ13C of individual FAs were consistent with sandhoppers modifying the turnover rate of FA in response to diet. These findings suggest that there is no consistent relationship between the FA compositions of green and brown seaweeds and that of the sandhopper B. quoyana. We caution that community-level application of FAs as a dietary biomarker tool must be accompanied by controlled experiments incorporating key species of relevance.