Examples of chemical defenses and their influence on predator-prey interactions abound in the terrestrial and marine benthic ecological literature. In contrast, considerably less is known about the role of chemical defenses in marine planktonic systems. In this study, we examined the potential for the phytoplankton-produced compound dimethylsulfoniopropionate (DMSP) and its cleavage products dimethyl sulfide (DMS) and acrylate to act as chemical defenses in seawater. Although added DMS and acrylate had no effect on protist grazers, added DMSP reduced grazing on the coccolithophorid Emiliania huxleyi by all four tested species (dinoflagellates Amphidinium longum, Gymnodinium sp., and Oxyrrhis marina and ciliate Coxliella sp.). A. longum and Gymnodinium were highly sensitive to 20-mol L Ϫ1 concentrations of DMSP, nearly ceasing to feed, whereas O. marina and Coxliella showed only minor reductions in grazing activity. Grazing suppression in A. longum was DMSP concentration-dependent over the range 0.05 to 1,000 mol L Ϫ1 and was also inhibited by added glycine betaine, a structural analog of DMSP. DMSP reduced A. longum grazing by similar amounts on five different algal species, some of which do not produce DMSP on their own. Thus, the efficacy of DMSP as a grazing deterrent appears to depend on grazer species, but not on algal strain or species. Because DMSP does not seem to be toxic, we hypothesize that DMSP and related compounds act as signals for the presence of potentially harmful algal cells. Should such sublethal chemical defense interactions be widespread in nature, they could be important in regulating the composition and biomass of phytoplankton communities.Chemical defenses against grazers, along with other grazing deterrence strategies, should be evolutionarily favored in marine phytoplankton. This is because most marine phytoplankton production is grazed by planktonic protists, both in coastal and in oceanic waters (Verity et al. 1993;Neuer and Cowles 1994;Landry et al. 1997). Thus, adaptations yielding even slight protection against protist grazing should confer a significant competitive advantage for a given phytoplankton strain or species (Strom in press).Some phytoplankton species are known to be toxic to planktonic protists: contact with or ingestion of these cells results in death (Hansen 1989;Uchida et al. 1995;Kamiyama and Arima 1997). Sublethal defenses, known to be widespread in other ecosystems, have been much less studied in planktonic communities. If present, sublethal grazing deterrents could play a role in shaping phytoplankton community structure. For example, it has been hypothesized that suppression of grazing is an important element in the formation and persistence of phytoplankton blooms, including blooms of ''harmful'' taxa (Turner and Tester 1997). Wolfe et al. (1997) have hypothesized that the cleavage of dimethylsulfoniopropionate (DMSP) into dimethyl sulfide (DMS) and acrylate constitutes a chemical defense against 1 Present address: Department of Biological Sciences, Ca...

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