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Feeding by marine invertebrates affects dietary lipids as they pass through the gut (Volkmanet al, 1980s; Tanoueet al, 1982; Prahlet al, 1984a, b,1985; Nealet 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.

A laboratory study simulating herbivorous feeding was carried out with the marine crustacean Neomysis integer (Leach) and the dinoflagellate Scrippsiella trochoidea (Stein). Analyses of the total fatty acids, sterols and fatty alcohols in the food and faecal material, and in the animal tissue, have allowed the detailed changes in the dietary lipids during feeding to be characterised.The results show this feeding leads to a net decrease in total lipid in the material passing through the gut of the animal, particularly due to the bioassimilation of fatty acids. All fatty acid saturation classes are assimilated but the mono-unsaturated and particularly polyunsaturated fatty acids are preferentially assimilated over others. Herbivorous feeding does, however, lead to the quantitative and relative increase in ‘bacterial’-type odd C number branched-chain fatty acids in the faecal material.

Changes in dietary lipids (fatty acids, sterols and fatty alcohols) during herbivory and coprophagy by the annelid worm Hediste (Nereis) diversicolor (O.F. Muller) were modelled in laboratory feeding experiments. The dinoflagellate Scrippsiella trochoidea (Stein) was used as the food in herbivory; faeces from the crustacean Neomysis integer (Leach) after feeding on this same alga, were used as the food in coprophagy.Nereis is extremely efficient in its assimilation of dietary lipids and produces faeces with very low fatty acid:sterol (FAST) ratios in both herbivory and coprophagy. The net decrease in total lipid in both modes of feeding with this species suggests that annelids, where present, are as important as other invertebrate groups in affecting the flux of lipids through marine food chains.Unlike species of crustaceans and molluscs studied to date, Nereis assimilates all fatty acid to a high degree, though herbivorous and particularly coprophagous feeding leads to relatively high abundances of 'bacterial' odd carbon-number normal and branched fatty acids in the faeces. As such, annelids are likely to be responsible for part of the microbial element of sedimentary lipid distributions.The quantity of cholesterol in the diet affects the manner in which Nereis changes the sterol distribution of the digested material. With a cholesterol-poor diet, as in herbivory, this sterol is significantly contributed to the faeces while A 804) sterols appear to be transformed to A 5 sterols to compensate for the loss of A 5 sterol. With cholesterol-rich diets, as in coprophagy, cholesterol is taken up directly from the diet and no A 8U4> conversion is observed.Overall, Nereis has little quantitative effect upon the dietary 4-methyl sterols and hence the use of these compounds as quantitative dinoflagellate markers is further strengthened. However, the quantity of 4-desmethyl in the digested material depends upon the feeding mode: accordingly, any assessment of dinoflagellate input to sediments based on the relative quantities of 4-methyl to 4-desmethyl sterols, must be treated with caution.