bio

Circalunar clock

An endogenous clock running on a roughly monthly cycle instead of a daily one, synchronised to the phase of the moon - and a distinct oscillator, not the circadian rhythm slowed down. The most striking thing about it is its light sensitivity: the marine worm *Platynereis dumerilii* reads moonlight at just a few tenths of a lux, and a few nights of exposure to that sub-lux light is enough to reset its monthly clock and pull a whole population into one synchronous spawning night. It is the plainest evidence that biology reads light levels we would dismiss as negligible.