EFFECTS ON DIETARY LIPIDS OF THE MARINE BIVALVE SCROBICULARIA-PLANA FEEDING IN DIFFERENT MODES

Changes in dietary lipids were investigated in laboratory feeding experiments simulating herbivorous and coprophagous modes of feeding in the bivalve mollusc Scrobicularia plana (da Costa). The dinoflagellate Scrippsiella trochoidea (Stein) was used as the food in herbivory experiments while faeces from the crustacean Neomysis integer (Leach) feeding on Scrippsiella, were used as the food in coprophagy experiments. Changes in dietary total fatty acids, sterols and fatty alcohols were characterised by analyses of the food, faeces and animal tissues using gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS). There is a net decrease in the total lipid of the digested material during both herbivory and coprophagy. However, while fatty acids are assimilated, sterols are contributed to the faeces, leading to a decrease in the fatty acid:sterol (FAST) ratio of the digested material. Coprophagy decreases the ratio still further, such that faeces have a FAST ratio of < 1. Scrobicularia preferentially assimilates dietary polyunsaturated fatty acids (PUFAs). Reworking of sedimentary material (as in coprophagy) will lead to PUFA-deficient sedimentary fatty acid distributions. Both herbivory and coprophagy lead to relative increases in 'bacterial' odd carbon-number normal and branched fatty acids in the digested material, though not the 'bacterial' marker 18:1 DELTA-11. Benthic molluscan feeding, particularly coprophagy, contributes partly to the 'bacterial' fatty acid content of the sediments. Scrobicularia contributes its own sterols to the faeces, especially cholesterol. Such contributions are dependent on the dietary sterols present. With a cholesterol-poor diet (herbivory), DELTA-5 4-desmethyl sterols are contributed to the faeces and dietary DELTA-8(14) sterols decrease, suggesting a DELTA-8(14) --> DELTA-5 conversion may occur. With a cholesterol-rich diet (coprophagy), the DELTA-5 sterol is taken up from the diet. Although the contributed 4-desmethyl sterols diminish the relative importance of the dinoflagellate 4-methyl marker sterols, the latter sterols are not significantly changed in quantity in either feeding mode, supporting the use of these sterols as quantitative dinoflagellate markers. The results indicate that benthic molluscan feeding can have effects upon the lipids in a marine food chain.