Pyrosomas are colonial tunicates capable of forming dense aggregations. Their trophic function in the ocean, as well as their ecology and physiology in general, are extremely poorly known. During the ANTARES-4 survey (January and February 1999) their feeding dynamics were investigated in the south Indian Ocean. Results show that their in situ clearance rates may be among the highest recorded in any pelagic grazer, with up to 35 1 h(-1) per colony (length: 17.9 +/- 4.3 [SD] cm). Gut pigment destruction rates, estimated for the first time in this tunicate group, are higher than those previously measured in salps and appendicularians, ranging from 54 to virtually 100% (mean: 79.7 +/- 19.8 %) of total pigment ingested. Although individual colony ingestion rates were high (39.6 +/- 17.3 [SD] mu g pigment d(-1)), the total impact on the phytoplankton biomass and production in the Agulhas Front was relatively low, 0.01 to 4.91% and 0.02 to 5.74% respectively, as a result of the low abundance of colonies. Colonies showed higher retention efficiency for particles larger than 10 mu m. Low levels of lipids and percentages of triacylglycerols and free fatty acids were found in zooids. Markers for diatoms (C16 PUFA), dinoflagellates (22:6n-3, 18:5n-3) and prymnesiophytes (18:1n-9, 18:4n-3) were observed. Stepwise discriminant analysis of published data on phytoplankton fatty acid showed a strong similarity between the composition of the 2 neutral lipid classes found in P. atlanticum and that of both dinoflagellates and prymnesiophytes. The sterol composition confirmed this result with the contribution of 24-methylcholest-5,22-dien-3 beta-ol, cholesterol, 24-methylenecholesterol and 24-ethylcholesterol. Colonies exhibited large numbers of ciliate protozoans (possibly Strombidium sp.). It is not clear whether this constituted a mere opportunistic intrusion, or rather a more stable association between the 2 organisms.