The first conclusive evidence of pain perception in fish is said to have been found by UK scientists.

This complements earlier findings that both birds and mammals can feel pain, and challenges assertions that fish are impervious to it.

The scientists found sites in the heads of rainbow trout that responded to damaging stimuli.
They also found the fish showed marked reactions when exposed to harmful substances.

The research, by a team from the Roslin Institute and the University of Edinburgh, is published in Proceedings B of the Royal Society, the UK's national academy of science.

The researchers, led by Dr Lynne Sneddon, say the "profound behavioural and physiological changes" shown by the trout after exposure to noxious substances are comparable to those seen in higher mammals.

They investigated the fish for the presence of nociceptors, sites that respond to tissue-damaging stimuli.

The researchers applied mechanical, thermal and chemical stimuli to the heads of anaesthetised fish and recorded their neural activity.

Dr Sneddon said: "We found 58 receptors located on the face and head of the trout that responded to at least one of the stimuli.

"Of these, 22 could be classified as nociceptors in that they responded to mechanical pressure and were stimulated when heated above 40 Celsius.

"Eighteen receptors also responded to chemical stimulation and can be defined as polymodal nociceptors."

These polymodal receptors are the first to be found in fish, and resemble those in amphibians, birds and mammals, including humans.

But mechanical thresholds were lower than those found in human skin, for example, perhaps because fish skin is relatively easily damaged.