A WATERY ARMS RACE: ATTACK AND DEFENCE MECHANISMS IN THE MARINE PLANKTONVictor Smetacek,Alfred Wegener Institute for Polar and Marine Research, Am Handelshafen 12,27570 Bremerhaven, Germany,E-mail: vsmetacek@awi-bremerhaven.deThe seasonal cycle of plankton is characterised by the wax and wane of a myriad species which replace each other in the surface layer at time scales of a few weeks. Although this universal phenomenon has been known since about 150 years, a causal understanding of the factors driving this species succession has yet to be developed. This is partly due to the fact that plankton ecology has been traditionally based on the bottom-up or food-chain approach in which the structure of the ecosystem is described in terms of summary parameters that lump species together on the basis of their functional role in the system. This approach can not explain why populations of the various species appear when and where they do and why some species, but not others, dominate the assemblage at certain times of the year. A mechanistic understanding of the factors shaping the species composition of the plankton is, however, a prerequisite to modelling food-web structure and biogeochemical cycles because these are strongly influenced by the properties of the dominant or key species.In contrast to terrestrial ecosystems, species-specific interactions amongst plankton organisms based on attack and defence systems between predators, parasitoids and pathogens and their potential prey or hosts have not received much attention so far. Indeed, only a few examples of such interactions are reported in the literature 1, 2. Since most organisms in the plankton are eventually eaten, one can expect a strong selection for mechanisms that reduce mortality within species populations3. Defence mechanisms can range from mechanical and behavioural to various types of chemical warfare and dedicated studies of the arms race in the plankton are likely to reveal many types of previously unsuspected defence strategies involving chemical compounds that will be of interest to the natural products scientific community. Mainstream plankton ecology will also profit greatly from increasing emphasis on chemical aspects of species-specific, organism interactions.1. Wolfe, G.V. Biol. Bull. 2000, 198, 225-2442. Ianora, A. Nature 2004, 429, 403-407.3. Smetacek, V. Nature 2001, 411, 745.