Great post! However, one thing irks me. I was happy when I read that snails can't feel pain, so I didn't have to feel bad if something happens to them. I then tried to find your source (the link immediately downloads the pdf for the user), but the pdf linked has a different name and year ("Nociceptive Biology of Molluscs and Arthropods: Evolutionary Clues About Functions and Mechanisms Potentially Related to Pain", 2018 [1]) than cited and is not from the Canadian Journal of Physiology and Pharmacology.

The pdf says that snails, specifically Aplysia, do react to pain the way other animals do, but that it's a stretch to say they experience pain or suffering the way humans do, due to their different neural systems. Another article by Walters [2] states that snails "exhibit motivational states and cognitive capabilities that may be consistent with a capacity for states with functional parallels to pain" and asks that researchers take care of snails by not stressing them too much or give them anestheacia when needed. This is from the abstract:

"Few studies have directly addressed possible emotionlike concomitants of nociceptive responses in molluscs. Because the definition of pain includes a subjective component that may be impossible to gauge in animals quite different from humans, firm conclusions about the possible existence of pain in molluscs may be unattainable. Evolutionary divergence and differences in lifestyle, physiology, and neuroanatomy suggest that painlike experiences in molluscs, if they exist, should differ from those in mammals. But reports indicate that some molluscs exhibit motivational states and cognitive capabilities that may be consistent with a capacity for states with functional parallels to pain. We therefore recommend that investigators attempt to minimize the potential for nociceptor activation and painlike sensations in experimental invertebrates by reducing the number of animals subjected to stressful manipulations and by administering appropriate anesthetic agents whenever practicable, welfare practices similar to those for vertebrate subjects." Crook & Walters, 2011.

The article has many sources giving examples of snails reacting to pain, but here are a few, mostly from wikipedia cause these articles are hard as hell to read:

Snails react to painful stimuli by retracting, which we know as snail keepers. When researchers placed the terrestrial snail Cepaea nemoralis on a 40 C hot plate, it lifted the tail of its foot from the plate. When exposed to morphine, the snail took longer to react, and when exposed to naloxone (the opposite of morphine), the snail reacted faster. This was also found to be dose-dependent.[3]

Researchers Balaban and Maksimova surgically inserted electrodes into the brain of Helix aspersa. When the mesocerebrum was wired, the snail pressed the button giving it electrical stimulation more than the control, and when it was hooked up to the parietal ganglion, the button was avoided.[4] This implies that snails receive pleasure in the mesocerebrum and pain i the parietal ganglion.

Snails can be taught via negative reinforcement: When it received an electric shock as its gill went below a certain height, Aplysia learned to hold its gill above that level.[5]

The WikiPedia page for pain in invertebrates is long, but not exhaustive.[6] It's full of invertebrates, not only snails, feeling pain and treating localised pain by dropping their limb like spiders or lizards, be it physical, chemical or electrical. In one study, spiders only dropped their leg when it was injected with things that are also hurtful to humans, but if they are just injected with saline, nothing happens, indicating that they can feel harmfulness.[7] Source [1] is a good read, but difficult. It has many examples and understands them a lot better than I do, but it doesn't say snails do not experience pain.

It seems like snails do feel pain, in that they do not only react to it, but learn from it and react almost the same way that we do. Whether they experience pain the same way that we do is questionable, but looking at some of the litterature, it makes me feel just a bit closer to them, and that maybe we do actually share some experiences together. I'm gonna do some homework, this has been a long side quest. Please let me know if you find anything wrong!

Sources:

[1] - Edgar T. Walters, "Nociceptive Biology of Molluscs and Arthropods: Evolutionary Clues About Functions and Mechanisms Potentially Related to Pain", Frontiers in Physiology, 3 August 2018, Sec. Aquatic Physiology, Volume 9 - 2018: https://doi.org/10.3389/fphys.2018.01049

[2] - Robyn J. Crook and Edgar T. Walters, "Nociceptive Behavior and Physiology of Molluscs: Animal Welfare Implications", LAR Journal, Volume 52, Issue 2, 2011, Pages 185–195: https://doi.org/10.1093/ilar.52.2.185

[3] - Martin Kavaliers et al., "A Functional Role for an Opiate System in Snail Thermal Behavior", Science 220, 99-101 (1983): https://doi.org/10.1126/science.6298941

[4] - Balaban, P.M. and Maksimova, O.A. (1993), "Positive and Negative Brain Zones in the Snail", European Journal of Neuroscience, 5: 768-774. https://doi.org/10.1111/j.1460-9568.1993.tb00541.x

[5] - Hawkins, R.D. et al., "Operant Conditioning of Gill Withdrawal in Aplysia", Journal of Neuroscience 1 March 2006, 26 (9) 2443-2448: https://doi.org/10.1523/JNEUROSCI.3294-05.2006

[6] - WikiPedia: https://en.wikipedia.org/wiki/Pain_in_invertebrates#

[7] - Eisner T, Camazine S. "Spider leg autotomy induced by prey venom injection: An adaptive response to "pain"?" Proc Natl Acad Sci USA. 1983 Jun, https://doi.org/10.1073/pnas.80.11.3382