system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Do fish and small sea creatures feel pain , how does it work ?

I just watched a vid of a crab eating a fish on a nature sub and it just kinda sits there not moving while it’s being eaten (in water)
gpt
In general vertebrates are expected to feel pain. Fish have a spinal column and are a vertebrate. The fish is not moving due to shock, blood loss, or death.

The ability to feel pain is usually defined as being able to learn to avoid a stimulus. For example, crabs have a natural desire to hide under rock shelves. If you shock the crab every time it goes under a rock shelf it will stop hiding under rock shelves, telling you two things:

1. You can train a crab.
2. Crabs must feel bad when you shock them and want to avoid feeling bad.

Other organisms, like bacteria, will avoid hazards that will lead to their death, but have no ability to associate phenomena with those stimuli and only act in direct response to a stimulus.