It turns out crabs may be in pain
It is becoming clear that not only higher animals such as humans, dogs, and cats, but also various other creatures such as
Putative Nociceptive Responses in a Decapod Crustacean: The Shore Crab (Carcinus maenas)
https://www.mdpi.com/2079-7737/13/11/851
Brain test shows that crabs process pain | University of Gothenburg
https://www.gu.se/en/news/brain-test-shows-that-crabs-process-pain
Scientists Confirm Crabs Really Can Experience Pain After All : ScienceAlert
https://www.sciencealert.com/scientists-confirm-crabs-really-can-experience-pain-after-all
It has been known that crustaceans such as crabs, shrimp and crayfish can feel pain, but there is limited information about the presence of 'nociceptors' that respond to painful stimuli.
So a research team led by Lynn Sneddon, an animal physiologist at the University of Gothenburg in Sweden, conducted an experiment using European green crabs (Carcinus maenas) captured live in Portugal's Ria Formosa Natural Park.
In the experiment, the researchers first immersed the crabs in seawater so they could breathe, attached them to a rock with rubber bands, drilled holes in their shells, and attached microelectrodes to the exposed central nervous system. They also injected neuromuscular blockers to keep the crabs still. The neuromuscular blockers immobilized the crabs' claws and legs, but allowed their mouths to move.
The researchers then applied chemical and mechanical stimuli to about 30 areas of the mouse, including the eyes, antennae, and the soft tissue between the claws. The chemical stimuli were acetic acid in various concentrations, while the mechanical stimuli were von Frey hairs, which are fibers used to test pain sensation.
The results showed that stimulating soft tissues such as the eyes produced a significant response in the crabs' central nervous system.
The graph below shows the crab's reaction when stimulated with seawater, von Frey hairs, and acetic acid from top to bottom, with the moment of stimulation indicated by the red vertical line. The results showed that while there was no particular reaction to seawater, there was a definite reaction to vinegar and von Frey hairs.
by Kasiouras et al., Biology, 2024
In an experiment in which acetic acid was applied to the eye, the stronger the reaction, the more concentrated the acetic acid (left). On the other hand, in an experiment in which von Frey hairs were applied to the eye, the reaction did not differ much depending on the hardness of the hairs (right).
by Kasiouras et al., Biology, 2024
The research team points out that crabs' eyes are compound eyes made up of hundreds of thousands of light sensors, which are delicate and easily damaged, so they may have a low threshold for mechanical stimuli.
In experiments on the walking legs (a and c) and the intersegmental membrane of the nail (b and d), it was observed that the response increased in proportion to the pressure of the mechanical stimulus (bottom row) applied to the tissue. On the other hand, when the concentration of acetic acid (top row) was high, the response decreased. This is thought to be because the receptors were damaged when the concentration of vinegar was too high, slowing down the response.
by Kasiouras et al., Biology, 2024
The overall trend was that mechanical stimulation evoked shorter and more intense neural activity compared to acetic acid.
For an animal to be defined as feeling pain, several criteria must be met, including more than just a receptor response; the response must be processed in specific areas of the brain, the animal must try to avoid the stimulus, show a negative reaction, or learn to protect itself from the stimulus.
Therefore, it has not yet been proven that crabs feel pain, but this research is considered to be the first step in research showing that crabs have organs that are thought to be nociceptors and that these are linked to a central nervous system response.
The study also raises the possibility that shellfish may need to be included in animal welfare measures in the future.
'If we want to continue eating crustaceans, we need to find less painful ways to do it, because there is scientific evidence that crustaceans feel and respond to pain,' Sneddon said.
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