Dinoflagellate bioluminescence can reduce copepod grazing of light-emitting cells. In unique footage, taken with high speed and super light sensitive cameras, we show that direct handling of bioluminescent cells by a copepod trigger a flash of light, which is followed by rejection of the light producing cell. When kept in copepod-free cultures dinoflagellates lose some of their bioluminescence capacity. This is speculated to be linked to a damaged photosynthesis in cells kept in culture for a long time. On the contrary, we show that dinoflagellates relax the bioluminescence defence in absence of copepod predators, and are fully capable of upregulating their bioluminescence when they sense the presence of copepods. Strains of the dinoflagellates Lingulodinium polyedra and Alexandrium tamarense, kept in culture for 14 and 9 years respectively, increased their total bioluminescence capacity by 30-300% in response to lipid compounds released from copepods, copepodamides. The copepodamides also affected the sensitivity of L. polyedra to mechanical stimulation. Only a few hours of exposure to copepodamides was enough to increase bioluminescence capacity. This makes bioluminescence the most flexible among the copepodamide inducible defences described so far. To our knowledge, this study provides the first evidence of changes in bioluminescence physiology as a response to chemical cues from natural enemies and emphasizes the importance of bioluminescence as an anti-grazing strategy.