Feeding by marine invertebrates affects dietary lipids as they pass through the gut (Volkman et al, 1980s; Tanoue et al, 1982; Prahl et al, 1984a, b,1985; Neal et al, 1986; Harvey et ah, 1987,1989). Not only do animals appear to alter the dietary lipids but they also contribute their own lipids to the egested material. Faecal pellets are thus likely to have a lipid composition which has contributions from the ingested food material, the animal itself and the microbial populations residing in the animal's alimentary system.