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.

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.

  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.