Microzooplankton are important consumers of phytoplankton primary production, but phytoplankton defenses against these predators are not well understood despite their expected importance. We tested for inducible defenses in the coccolithophore Emiliania huxleyi, an abundant and cosmopolitan bloom-forming species with a heteromorphic haploid-diploid life cycle, against the ciliate predator Strombidinopsis acuminatum. We hypothesized that the two life-cycle phases (calcifying diploid and motile noncalcifying haploid) of E. huxleyi are differently defended against predation. Using short-term (30 min) ingestion rate assays, we compared predation on E. huxleyi that had been previously exposed to predators for 24 h to predation on naïve (unexposed) E. huxleyi. Prey were considered to have a defense response when ingestion rates on naïve cells were higher than ingestion rates on predator-exposed cells. Haploid E. huxleyi had a strong defense response, with a 25-43% reduction in ingestion rate due to predator exposure, whereas diploid E. huxleyi had no defense response. Haploid E. huxleyi grown with reduced nutrient availability showed no defense response when initially offered to ciliates (although there was evidence of a delayed defense response). Additionally, the presence of defended haploid E. huxleyi did not reduce ciliate ingestion on another prey species (Heterocapsa triquetra). This defense system points to complex predator-prey interactions as key factors controlling the structure and function of marine planktonic communities.

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