|MadSci Network: Zoology|
I'll try to address your question in two parts: 1. The "philosophical" response: The simplest answer to whether any animal can feel pain is this - only the animal knows. People have the tendency to project their own experiences onto others, whether the "others" are other people or other species. An organism may try to escape from an undesirable ("painful"?) situation or search for a more desirable one, whether the animal is a human, a jellyfish, or a plant. But while we might assert that humans use a very complex set of cognitive processes to carry out those types of actions, it would be large leap of faith to assert that the jellyfish or the plant are capable of those types of cognition. All life forms try to find the most desirable environment and leave the undesirable ones, and this trend presumably preceded any neural response we might call "pain". My point is that "pain" is a concept humans use to describe a mental reaction to physical injury; and once you leave the context of human experience, the existence or absence of "pain" in other organisms is an assertion and a projection. Only the organism experiencing the physical injury knows whether it feels "pain". In the case of fish and shellfish, they cannot tell us when it hurts, so there is no way to know whether they feel "pain". Without their feedback on this topic, it is up to us to use our own individual ethic to decide. 2. The "scientific" response: The simplest answer is that we still have a lot more to learn about "pain" on a biological basis, but that the neurobiology of "pain" is probably so different between humans and fish/ shellfish that we might never be able to say whether fish/shellfish experience "pain" like humans. First, there is no such thing as a definitive "pain receptor". There have been studies that suggest that several receptors are somehow involved in the process, but none that are solely responsible for pain or that are similarly utilized in fish/ shellfish. While I am not an expert in the neurobiology of pain, the only pertinent evidence I am aware of suggests that fish and shellfish probably do not feel "pain" the way we do (especially shellfish!). For instance, the area of our brain called the cerebrum (a part of the forebrain) has been suggested to be responsible for processing "pain" in humans. In fish, the forebrain is greatly reduced in size and complexity compared to humans, and they lack any region that resembles a cerebrum. Additionally, the area of the brain suggested to be responsible for processing escape responses to "painful" situations in fish is found in the hindbrain. Since fish do not possess the part of the human brain responsible for pain processing and use a completely different part of the brain to mediate their own reactions to similar stimuli, I think it is fairly safe to assume that "pain" as we conceive of it and experience it is qualitatively different from how fish might experience it. As for shellfish, they are much more evolutionarily distant in their relation to humans than fish, and have a very different nervous system from fish or humans. Therefore, I think there is no way to know or assert whether shellfish feel "pain", but is very unlikely that they experience it the same way we do. I'm sorry to not have a more clear answer for you, but I hope this helps a little. I think the bottom line is that there is no way to know whether they feel pain like we do, but you have to be careful how you project human mental qualities onto other animals just because they exhibit similar physical behaviors.
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