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.

I find it amusing to jump straight to the "because some species lack watches." It isn't like humans started with them. Kids aren't even really able to use a clock for quite a few years.

Article is still neat, mind. I am curious why it is not more compelling to think in terms of reserves and duty cycles. Build up enough energy to get you through periods of needed energy and you will settle on a cycle that matches when energy is available. At least, if you want to minimize complete depletion. Which is about the only thing I would expect evolution to fully avoid. Or, at least, the ones that didn't will have died off.

I don't understand how this works on a 20 hour cycle.

  Normal day: Spawn 10 hours after sunrise
  Early rising sun: Spawn 10 hours after sunrise
  Constant sun: Spawn every 20 hours

Does it reset back to zero after a cue of darkness? That's the only thing I can think of.

Very difficult to read, and think anything but, “This is AI slop”…