The detail that makes this interesting is the two-layer mechanism. There’s a 20-hour free-running oscillator that doesn’t meet the usual definition of circadian because it’s temperature-sensitive and then on top of that a separate countdown triggered by sunrise that governs the spawning event. Two imprecise systems combining into precise, synchronised behaviour.

Also worth noting that the hydrozoan lineage lost the CLOCK/BMAL1/CRY genes associated with circadian rhythms in most other animals. So whatever this timing system is, it seems to have evolved independently. Rosato’s question in the commentary is a good one: how many other unconventional clocks are out there that people have missed because they were looking only for the usual genetic components? There’s something very neat about evolution backing into a precise clock this way because the reproductive timing pressure is doing so much work.

Would love to see this kickstart research into more unconventional time-keeping processes that might be out there.