When you hook a fish, does it hurt? Yes, a new study suggests. Some researchers have previously concluded that fish react to painful stimuli without actually feeling pain in the conscious way humans do.

In the new study, researchers gave morphine to one group of fish, and injected the other group with a placebo (saline). Then the fish were treated to burning sensations that were expected to be painful but which did not damage any fish tissue.
Both groups reacted the same, by wriggling.
However, the fish that had been on morphine later went on about business as if nothing had happened. The fish that had gotten the saline were wary after the test.
“They acted with defensive behaviors, indicating wariness, or fear and anxiety,” said Joseph Garner, an assistant professor at Purdue University.
“The experiment shows that fish do not only respond to painful stimuli with reflexes, but change their behavior also after the event,” said Janicke
Nordgreen, a doctoral student in the Norwegian School of Veterinary Science.
“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.”
A study last month indicated that crabs feel pain, too. Garner and Nordgreen published their results in the online version of the journal Applied Animal Behavior Science.
Garner figures the morphine blocked the experience of pain, but not behavioral responses to the heat stimulus itself, either because the responses were reflexive or because the morphine blocked the experience of pain, but not the experience of an unusual stimulus.
“If you think back to when you have had a headache and taken a painkiller, the pain may go away, but you can still feel the presence or discomfort of the headache,” Garner said.
(Source: LiveScience)