Research on crustacean nociceptors missing useful evidence

A new paper by Kasiouras and colleagues attempts to provide evidence for nociceptive neurons in a decapod crustacean, a shore crab. 

The authors took crabs, poked them and dropped some acid on them, and recorded the neural responses. Neurons did indeed respond, in dose-and intensity specific ways. From this, the authors conclude these are potential nociceptive responses.

I am unconvinced. There is a big difference between showing that neurons can respond to a possibly noxious stimulus and showing that those neurons are responding as nociceptors.

First:

The same category of stimuli can have detected by by nociceptors and by “regular” sensory neurons that are not nociceptors. For example, an increase in temperature can stimulate both nociceptors and thermoreceptive sensory neurons. Mechanical pressure can stimulate both mechanoreceptors and nociceptors. That there are two neural pathways are probably why we distinguish a burn (painful) as different than heat, or a pinch from a strong touch.

The results would be more convincing if the authors showed that neurons responded in ways that are typical of other nociceptors. Many (though not all) nociceptors have a few common properties.

1. They respond to several different types of stimuli. For example, they react to high temperature and acid and mechanical pressure and a chemical like capsaicin. (The technical term is that they are polymodal or  multimodal sensory neurons.)
   
2. They respond to repeated stimulation with increased activity. Most sensory neurons “get used to” the same sensory stimuli over time, but many nociceptors do the opposite. (The technical term is that they show sensitization.)

The authors couldn’t do either of these, because they are recording from the whole nerve cord and they did not pick out the activity of single neurons. Sometimes it is possible to recognize the activity of single neurons in this sort of record with spike sorting techniques, but that is done in this paper.

Second:

Species respond to potentially noxious stimuli in different ways. Some species respond to capsaicin with nociceptive behaviours, but others do not. No stimulus is guaranteed trigger nociceptors in all cases. 

This paper used mechanical touch and acetic acid, but because the paper did no behavioural experiments, it’s not clear if the crabs perform nociceptive behaviour in response to the level of stimuli presented.

Another paper used acetic acid as a potentially noxious stimulus with shore crabs, and crabs do respond to it (Elwood et al. 2017), but that paper was criticized (Diggles 2018; not cited in the current study) for not considering the possibility that acetic acid caused a feeding (gustatory) response, not a nociceptive response. Acetic acid is the technical name for vinegar, after all.

The results would be more convincing if the electophysiological recordings of neurons were connected back to the crab’s behaviour. For example, the authors could tried an experiment to shown that a touch of low intensity caused one sort of behaviour, but a touch of higher intensity caused a different behaviour that looks like nociceptive behaviour. Then they could have seen what the differences in neural activity to those two kinds of tactile stimulation were.

I am glad to see more labs trying to establish the presence or absence of nociceptors in crustaceans. There is still more work to demonstrate their existence and characterize their physiology if they exist.

References

Diggles BK. 2018. Review of some scientific issues related to crustacean welfare. ICES Journal of Marine Science: fsy058. http://dx.doi.org/10.1093/icesjms/fsy058
 

Elwood RW, Dalton N, Riddell G. 2017. Aversive responses by shore crabs to acetic acid but not to capsaicin. Behavioural Processes 140: 1-5. https://doi.org/10.1016/j.beproc.2017.03.022

Kasiouras E, Hubbard PC, Gräns A, Sneddon LU. 2024. Putative nociceptive responses in a decapod crustacean: The shore crab (Carcinus maenas). Biology 13(11): 851. https://doi.org/10.3390/biology13110851
 

Picture by Dunnock_D on Flickr; used under a Creative Commons license.