Walters, Edgar T. (2016) Pain-capable neural substrates may be widely available in the animal kingdom Animal Sentience 2016.063
Brian Key, Why fish do not feel pain
Neural and behavioral evidence from diverse species indicates that some forms of pain may be generated by coordinated activity in networks far smaller than the cortical pain matrix in mammals. Studies on responses to injury in squid suggest that simplification of the circuitry necessary for conscious pain might be achieved by restricting awareness to very limited information about a noxious event, possibly only to the fact that injury has occurred, ignoring information that is much less important for survival, such as the location of the injury. Some of the neural properties proposed to be critical for conscious pain in mammals are also found in the nervous systems of numerous species, including invertebrates. These considerations suggest that simple forms of pain might be present in some animals that lack large brains and complex behaviors.