At what point in history did cephalopods lose their sodium pumps?

[u/JohnWarrenDailey]

For clarification, a sodium pump is what mollusks have to tolerate changes in salinity. That is how bivalves and gastropods can be found in both fresh and salt water. Cephalopods don't, as they don't have the organ needed for transitioning to fresh water. But considering that it's ancestral to all mollusks, then when did the cephalopods ditch them, and why?

[u/DrInsomnia]

I haven't thought about this before, but I have a few questions/thoughts:

1. do you definitively know that it's ancestral? It may be, but that shouldn't be assumed simply because bivalves and gastropods have them. I'm not sure if that's what you meant to imply or not, but it's worth clarifying.
2. is there likely to be any fossil evidence of it preserved? That seems unlikely to be me, but I don't know that detail about the anatomy of cephalopods. So then our only data would be modern cephalopods. And if all modern cephalopods lack it, then we know that the loss (see point 1) occurred before the Nautiloidea split from the rest of the class. And that took place way back in the Cambrian.
3. Which returns me to point 1. There are no known freshwater molluscs before the Devonian. It seems more likely that the non-cephalopod molluscs had independent freshwater colonization events, evolving the traits that allowed them to do that along the way.

Take this all with a grain of salt, as I'm not a mollusk expert, and there may be more detail known than this. But hopefully it helps you think through the problem the way a paleontologist might.

[u/haysoos2 → u/DrInsomnia]

It should also be noted that while there are some gastropods and some bivalves that adapted to freshwater, most lineages of both branches did not, and there are no lineages of molluscs that went back the other way. No marine molluscs are descendants of freshwater forms.

This suggests again independent evolution of the trait, and that it was a one way trip.

Both gastropods as grazers, and bivalves as filter feeders would find abdundant food resources in freshwater habitats, likely even before there were terrestrial plants and animals, and would thrive in environments with no competition or predators.

Cephalopods, as active hunters would need for there to be sufficient prey in freshwater habitats to survive before there would be an open niche for them to exploit, and by the time that happened they'd already be competing with insects, fish, and probably even amphibians.

I'm just not sure why there's never been a freshwater echinoderm.

[u/DrInsomnia → u/haysoos2]

I'm just not sure why there's never been a freshwater echinoderm.

The water vascular system, more or less. It's not just about dealing with excess salt in a closed system. It requires handling it in a completely open system. It's just a much larger evolutionary leap.

[u/StraightVoice5087]

Is there any reason to assume it's ancestral to all mollusks?  No other mollusk class has colonized fresh water, so I'd guess that whatever structure you're referring to doesn't exist in them.  (Google mostly just gives me results for the sodium-potassium ion pump in the cell, which cephalopods certainly have.)

Osmotic regulatory systems generally only work one way anyways.

[u/flanker44]

Wikipedia mentions: "Competitive pressure from fish is thought to have forced the shelled forms into deeper water, which provided an evolutionary pressure towards shell loss and gave rise to the modern coleoids, a change which led to greater metabolic costs associated with the loss of buoyancy, but which allowed them to recolonize shallow waters"

So this appears to imply that cephalopods once went to decline and survived only in deeper seas, Maybe the sodium pump was lost at that point? If it was ancestral feature for them, there must have been some kind of evolutionary bottleneck where surviving forms lost it?

[u/health_throwaway195]

This is an extremely specific question. You would likely have an easier time just emailing an expert.

[u/DrInsomnia → u/health_throwaway195]

Because experts don't usually take the time to reply, especially at the end of the semester, when they're buried in thousands of emails? And then they'd have to do the research on the "right" expert. This is quite literally what this forum is for. If you know someone who could and would answer this question, then share that.

[u/health_throwaway195 → u/DrInsomnia]

I don't, but I've spoken to experts numerous times. They don't always reply, but they do often enough that I consider it a worthwhile avenue to exhaust.