Single-celled marine algae called Alexandrium minutum, which are responsible for nearly half of the photosynthesis on Earth, sometimes grow with abandon, producing neurotoxic alkaloid compounds that accumulate in food chains and harm commercial fisheries. What motivates these phytoplankton to produce the poisons has been a long-standing question. According to a team of researchers led by Erik Selander of the University of Gothenburg, in Sweden, the algae’s zooplankton predators are to blame (Proc. Natl. Acad. Sci. USA 2015, DOI: 10.1073/pnas.1420154112). The researchers discovered that phytoplankton-eating zooplankton produce a family of polar lipids called copepodamides that likely help them digest their algae prey. However, when the algae detect the copepodamides, they start producing the alkaloids, such as saxitoxin, as a defense strategy. Just pico- to nanomolar levels of copepodamides can increase production of the neurotoxic alkaloids by a factor of 20, the researchers say.