Just months after a study made a splash in the media that proved crabs experience pain, a new study of goldfish shows that not only do these fish also feel pain, but it changes their future behavior. Published in Applied Animal Behaviour Science the study tested goldfish by slowly raising the temperature in their tank. In the warming tank, researchers administered one group of fish morphine and the other saline.
“There has been an effort by some to argue that a fish’s response to a noxious stimuli is merely a reflexive action, but that it didn’t really feel pain,” Joseph Garner of Purdue said, one of the paper’s authors. “We wanted to see if fish responded to potentially painful stimuli in a reflexive way or a more clever way.”
While the researchers believed that the fish given morphine would take longer to react to higher temperature—due to the drug’s pain-killing capacity—they found that both groups started wiggling with discomfort at the same time. At first, they thought this was proof that fish do not feel pain as discomfort like humans do, but rather only reflexively react to noxious stimuli. However, later when the temperature was returned to normal, the two fish groups showed contrasting behavior.
“The fish given the morphine acted like they always had: swimming and being fish,” Garner said. “The fish that had gotten saline – even though they responded the same in the test – later acted different, though. They acted with defensive behaviors, indicating wariness, or fear and anxiety.”
According to the researchers it is likely that the morphine blocked the experience of pain in the fish, but not their reflexive responses to the hotter water. They hypothesize that the fish were still able to detect the troubling stimulus without actually experiencing pain.
“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 explains.
In contrast the group administered with just saline not only felt pain, but that painful experience later changed their behavior, causing them to display extreme anxiety.
“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.”
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