I've always wondered what the mechanism is for there being a daily change in ceph beaks/statoliths/... causing the rings. In a tree, it makes sense that there are seasonal changes that show up in the growth patterns, and I could even buy daily changes in fast-growing plants in that the new material is different depending on whether light or dark photosynthesis is happening or based on temperature or something.
I can maybe imagine that in shore and shallow water cephs, they can see the sunlight, but I don't understand why that would change the biochemistry of how they grow their statoliths. I guess it's plausible that an octopus might grow more when it's hiding in its den than when it's out and about actively hunting, and perhaps that might be extended to squids actively hunting versus lazily swimming. However, for deep sea cephs, I wonder how they even know what time of day it is, anyway-- it's dark and pretty much uniformly cold once you get below a few hundred meters or so, right? I know Humboldts and Nautilus have vertical migration from day to night, but aren't these rings present even in cephs that never get close enough to the surface to tell from light what time it is?
As I'm typing, I'm wondering if awareness of daytime is an evolutionary throwback, since I've read that it's believed that cephs evolved near the shore but many were driven to the depths by modern predators...
It's also pretty interesting that the growth curves appear to be so different in male and females; it'll be interesting to see how broadly this applies...
