Letters to you

"Many organisms that live between the tides have intrinsic rhythms that correspond to the 15-day spring tide cycle. The brown alga, Dictyota dichotoma, sheds its reproductive cells into the waters of a high tide on a roughly 15-day cycle. A marine midge, Clunio marinus, emerges, mates, and lays its eggs on the red algae growing at the lowest limit of the lowest tides. Since the algae are exposed only once each 15 days, the midge has but two hours or so to complete its entire adult life- a life cycle with a roughly 15-day rhythm that persists even under constant conditions in the laboratory"

And so on. It's a natural cornerstone of sorts...