Jean said:There is a theory that it has to do with tides! That somehow these animals can detect the comparatively minute changes in water pressure!
I didn't knew that! but for intertidal burrowing bivalves such as cockles, a transversal cut through the shell reveals tidal bands associated with reduced growth when the tide is out and thus cease filter feeding. So they show more than 1 band a day and show two small band and 2 larger ones a day. It is also possible to determine bands caused by disturbance such as when dredges hit them which sometimes confuses the band counting for age determination!
It is interesting to note that down at deep sea hydrothermal vents, there are strong tidal periodicity in the outflow of hydrothermal fluid. This is because the difference in pressure caused by the tide affects the oceanic crust. This cycle of fluid emmanation in turn may affect the activity of the animals, who cease their activity when the flow is high thus more toxic.
I guess that for cephalopods the daily growth rings in beaks and statoliths are associated with their activity cycle and considering their fast growth rates, these should be more evident than in slow growing fish or bivalves. But do not know more...worth exporing!
As for deep and midwater cephs that do not perform vertical migrations, maybe they developed a daily activity rhythm due to their prey's vert migration? Or as Monty said, the daily activity rhythm is a shallow water ceph trait that was somehow not detrimental (even beneficial?) and thus kept in those cephs that colonised deep waters...I THINK it has been suggested for some deep water animals but which?? don't remember now...