Re: Do fish and shellfish have pain receptors?

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.