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 Behaviour 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.

"The goldfish that did not get morphine experienced this painful, stressful event. Then two hours later, they turned that pain into fear like we do," Garner said. "To me, it sounds an awful lot like how we experience pain."

Then again, scientist don't fully understand pain in humans. It is felt when electrical signals are sent from nerve endings to your brain, which in turn can release painkillers called endorphins and generate physical and emotional reactions. The details remain unclear, which his why so many people suffer chronic pain with no relief.