Moon jellyfish undergo a bizarre transformation during their lifetime. They start out living on the sea floor as polyps—immobile stalks with a tentacle-ringed mouth—but in early spring they bud into segments, with each one becoming a single adult jellyfish (pictured, showing the polyp, top left, transitioning into an adult, bottom center and right). What triggers this unusual change? To answer this question, researchers studied the behavior of the moon jelly (Aurelia aurita) under various temperature shifts. An 8° drop in water temperature (to 10°C), as might be felt over winter, began the change in about 3 weeks. The researchers discovered that the temperature drop causes the jellyfish to secrete large amounts of a special protein, the precursor to a hormone that causes them to metamorphose. The protein acts like a timer, gradually accumulating until a critical concentration is reached, triggering change in the spring. This buildup process enables the jellyfish to distinguish between the onset of winter and normal temperature fluctuations, the team reports online today in Current Biology. Armed with this knowledge, the researchers were able to synthesize the metamorphosis hormone themselves and force the polyp-to-jellyfish transformation in just 48 hours. The findings could help manage local populations of moon jellyfish, large blooms of which clog fishing nets, or worse—as demonstrated by last October’s forced shutdown of the Oskarshamn nuclear power station, in Sweden, following the buildup of jellyfish in the plant’s coolant intake pipes. By inducing blooms early, in winter—when food is scarce—wildlife managers could limit the number of jellyfish in the water.