(Disclaimer: although it's still under development, my current moral system is based on satisfying the preferences as many entities as possible.)

I believe it is possible that any of the above experience suffering. But I recognize that attempting to consider every possible source of suffering is impossible. So I'm using my best judgment based on how I know that I personally experience suffering, and how other creatures are likely to be similar to me. I am assuming that suffering requires a fairly complex nervous system, and that it is unlikely to have developed in lifeforms that don't respond much to stimuli.

I eat clams, because as far as I can tell there is no reason for them to have developed the ability to suffer. They also (as far as I know, although it is possible I am wrong here) usually farmed in a manner that's most independent of the rest of the ecosystem. (Whereas I'd be okay with eating shrimp, but they are harvested from the ocean, not only catching other animals in the process but disrupting the food chain of creatures that eat them).