A Lunar Cycle in Zooplankton

A cycle of zooplankton density that fluctuated in phase with the moon was observed throughout 1982—1983 in Cahora Bassa Reservoir on the lower Zambezi, in southeastern Africa. Despite constant birth rates, densities of four cladoceran and two copepod species, as determined from vertically hauled plankton net samples taken every 2—6 d, fluctuated over one order of magnitude. The pattern followed by each species included an exponential increase in population density from the last quarter of the moon through the new moon and the first quarter, till the full moon, then a sudden decrease resulting in lowest numbers during the moon's last quarter. The cycle was shown to be induced by predation. Much higher death rates between the full moon and the last quarter were caused by the abundant Tanganyikan sardine Limnothrissa miodon. As seen from an examination of gut contents, sardines crop zooplankton most efficiently on nights when the full or nearly full moon rises after sunset, i.e., when zooplankton approach the surface during darkness and become suddenly vulnerable in the first light of the rising moon. After the last quarter, zooplankton density is low, the moon gives little light, the fish shift to alternate food resources, and zooplankton populations grow exponentially again. I suggest that the moon phase cycle in zooplankton is a global phenomenon, but, previously uninterpreted, has been seen only as distracting "random" variations in seasonal density patterns. I also suggest that similar prey—predator interactions might have been responsible for selecting for and fixing intrinsic monthly rhythms in behavior and physiology of animals with long life—spans.