That is kinda what I was hinting at. If we define pain as something that causes a defensive (or similar) reaction, then sure: Plants react to “pain” in their own way. I have never seen a jellyfish react to “pain” though.

When my chinchilla starts barking, it’s easy to assume that he is in pain or otherwise uncomfortable, but to say that he is truly in pain is impossible. However, when animals hurt their foot and start limping, it’s a good theory that they are reacting to pain. My examples only include mammals though, to your point.

Excuse my rambling. I am not disagreeing with you, but just thinking out loud.