Can fish detect what harms them?

Fish have nerve receptors, similar to those found in amphibians, birds, and mammals, which detect painful events. In one experiment, twenty-three pain receptors were found on the face of rainbow trout.

pain receptors in face of fish

A nociceptor is a sensory receptor that reacts to potentially damaging stimuli by sending nerve signals to the spinal cord and brain. This process, called nociception, usually causes the perception of pain.

In one experiment, neural activity was recorded from single cells in the face following the application of stimuli to the head of the fish. Stimulation was:

  • mechanical - a glass mechanical probe was applied to the skin
  • thermal - a thermal stimulator was placed above the skin,
    but did not burn it
  • chemical - a drop of 1% acetic acid was placed onto the receptor field
A drop of water was also placed onto the face to act as a control stimulus. None of the receptors responded to this.

Scientists in this research proved that rainbow trout can detect painful events.

58 nerve endings were found on the head of the trout - 22 of these were pain receptors.

The nerve endings had resemblance to those found in birds, amphibians, humans, and other mammals. They had similar widths, and were as tender to pressure and heat.

However, the fish nerve endings on the face were actually more sensitive, when pressure was applied, than those in humans - as sensitive as our eyes.

Do Fishes Have Nociceptors? Evidence for the Evolution of a Vertebrate Sensory System, Sneddon, L.U., Braithwaite, V.A., Gentle,M.J., Proc. R. Soc. Lond. B 270, 1115-1121, 2003, 1. Roslin Institute, Midlothian, 2. University of Edinburgh
22 Pain Receptors Found on Face and Head
"We located 58 receptors on the face and head of the rainbow trout. Twenty-two of these receptors could be classified as nociceptors."
"The polymodal nociceptors found here in the trout have similar properties to those found in amphibians, birds and mammals including humans."

"This study demonstrates that the rainbow trout is capable of nociception."

"Receptor diameter, thermal thresholds and mechanical responses are similar to those measured in higher vertebrate groups."
"Mechanical thresholds were lower than those found in humans: at least 0.6g is required for noxious stimulation in human skin but many of the nociceptors in the fish skin were stimulated by 0.1g. This may be a consequence of the more easily damaged nature of the fish skin requiring the nociceptors to have lower thresholds. Similar thresholds were found in mammalian eye nociceptors."

Fish Pain