How do ocean animals that live below the depth to which moonlight penetrates migrate in phase with the moon?
Hans van Haren of the Royal Netherlands Institute for Sea Research in Den Burg used sonar to track zooplankton movement over 18 months. They migrate up and down the water column on daily and seasonal cycles, and van Haren also identified a strong monthly movement that coincided with lunar cycles (). But the zooplankton remain at or below 800 metres, while moonlight penetrates no deeper than 150 metres. 鈥淭his has nothing to do with physical processes,鈥� say van Haren, 鈥渂ut a biochemical clock could explain it.鈥�
have shown that zooplankton kept in the dark can retain light-induced biorhythms for several weeks. Van Haren鈥檚 work suggests that biorhythms can be preserved for far longer.
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