[AnDaltan]

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.

[gausswho]

Sounds like nature's take on building a jalopy out of whatever it can scrounge. What it makes me wonder is perhaps this cobbling together creates a resilience that the other clocking architecture is vulnerable to. Could these outliers serve as a kind of important reservoir? Against calamities Earth periodically goes through that blot out the sun for longer than an individual life cycle? Or perhaps even more resilient than that - buffering against variations in planet/moon rotation speed or distances.

Jellyfish == System Recovery Mode.

[Tagbert]

That kind of thing is more standard procedure for evolution than an exception. Since evolution doesn't plan but just makes use of what it has with slide variations, changes play out along paths that are easier rather than optimal. That's how you get a thumb on the panda evolved from a projection on the wrist.