(Copied from Wiki) Though it has been argued that most invertebrates do not feel pain,
[21][22][23] there is some evidence that invertebrates, especially the decapod
crustaceans (e.g. crabs and lobsters) and
cephalopods (e.g. octopuses), exhibit behavioural and physiological reactions indicating they may have the capacity for this experience.
[5][6][24]Nociceptors have been found in
nematodes,
annelids and
molluscs.
[25] Most insects do not possess nociceptors,
[26][27][28] one known exception being the
fruit fly.
[29] In
vertebrates, endogenous
opioids are neurochemicals that moderate pain by interacting with opiate receptors. Opioid peptides and opiate receptors occur naturally in nematodes,
[30][31] molluscs,
[32][33] insects
[34][35] and crustaceans.
[36][37] The presence of opioids in crustaceans has been interpreted as an indication that lobsters may be able to experience pain,
[38] although it has been claimed "at present no certain conclusion can be drawn".
[38]
One suggested reason for rejecting a pain experience in invertebrates is that invertebrate brains are too small. However, brain size does not necessarily equate to complexity of function.
[39] Moreover, weight for body-weight, the
cephalopod brain is in the same size bracket as the vertebrate brain, smaller than that of birds and mammals, but as big as or bigger than most fish brains.
[40][41]