"[F]ish had their lips injected with chemicals that would produce a painful sensation....Before they received the injections, the fish showed normal behaviors. After, however, the fish began to rub their lips against the gravel and walls of their tanks....It was also found that their breathing rate nearly doubled. The researchers then treated the fish with morphine....The fish began to rub their lips much less, and their breathing rate lowered significantly as well....The same research team did further studies on fish pain, and also found that when experiencing painful stimuli, the neurons of both humans and fish brains fire in the same way."
"[In another study], one set of fish were given morphine, and another set was given saline. Then, they were exposed to hot water. All of the fish tried to escape the heat, but the ones that received the pain-relieving morphine had slower responses. After the initial exposure, the fish would avoid the heat source. 'The experiment shows that fish do not only respond to painful stimuli with reflexes, but change their behavior also after the event. Together with what we know from experiments carried out by other groups, this indicates that the fish consciously perceive the test situation as painful and switch to behaviors indicative of having been through an aversive experience.'"
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