Invertebrates are the most common animals on earth, composing 97% of known species (“Articles 16 September 1988,” n.d.), and have complex behavior and nervous systems. While it may be impossible to tell conclusively whether a non-human species can feel pain or not, there are concrete factors that may increase the chance that a given species feels something analogous to pain in humans. There are six such factors that are experimentally verifiable, and can be studied in species both related and distant from humans: identified pain-related neurons and brain structures, the presence of natural opioids, behavioral responses to damaging stimuli (either general responses or altered response to the damaged body part), evolutionary similarity to humans, and a wide repertoire of behaviors.
Social ants and bees have been demonstrated to be sophisticated learners, as are fruit flies and crickets (Deneubourg, Aron, Goss, & Pasteels, 1987; Entler, Cannon, & Seid, 2016; Hammer, 1993). A variety of insects including bees, ants, crickets, fruit flies, termites, butterflies, moths, wasps, and others display a nociceptive response (Giurfa, 2013). Insects are not known to pay special attention to injured body parts, such as grooming an injured limb or limping on a damaged leg (Eisemann et al., 1984). Honeybees make more pessimistic evaluations of decisions after exposure to unpleasant stimuli (Mendl, Paul, & Chittka, 2011), and may enact complex grooming protocols when infested by mites (Peng, Fang, Xu, & Ge, 1987). Some but not all insects display lessened pain responses in the presence of morphine, including praying mantises, honeybees (Smith, 1991), and ants, the latter of which will dose themselves with morphine to the point of addiction (Entler et al., 2016). Many insects continue with normal behaviors despite grave tissue damage (Eisemann et al., 1984).
Some theories suggest that social insects are more likely to have a pain response, as a visible pain response can call allies to assist an injured individual. For instance, bees and ants use alarm pheromones to alert their colonies to danger, and bees that cannot remove their own mites will perform a dance that calls colony-mates to come and groom them (Peng et al., 1987).
At the same time, some exclusively social insects (including honeybees, termites, ants, and wasps) perform self-sacrificing behaviors, voluntarily causing either death or enormous tissue damage. These range from sting autonomy (a sting that leaves internal organs attached to the embedded stinger, and carries a 50% death rate for the stinging animal), to abandoning the colony when sick or near death, to autothysis (a defensive strategy where an insect contracts its abdominal muscles until it explodes, showering nearby enemies in toxins) (Shorter & Rueppell, 2012).
Prawns will groom antenna that are crushed or exposed to noxious chemicals (STUART BARR, PETER R. LAMING, JAIMIE T. A. DICK & ROBERT W. ELWOOD, n.d.). Crabs and hermit crabs learn from electric shocks (Appel & Elwood, 2009; Denti, Dimant, & Maldonado, 1988), and hermit crabs make decisions based on relative values when deciding when to leave a shelter or abandon the shell they currently reside in (Appel & Elwood, 2009). In crabs, negative reinforcement is reduced by morphine (Lozada, Romano, & Maldonado, 1988). Some crustaceans do not have a nociception response to certain stimuli, including isothiocyanate, capsaicin, and cold temperatures (Puri & Faulkes, 2015).
Several gastropods have identified nociceptor cells, despite having simple brains (11 ganglia with a total of about 20,000 neurons), including land snails and two types of sea slug (Crook & Walters, 2011). Walter Snails have a lower threshold for withdrawing into their shells for several days after being injured (Crook & Walters, 2011), and sea hares are more sensitive to touch on an injured body part (Walters, 1987). Reinforcement learning has been demonstrated in a variety of gastropods (Crook & Walters, 2011). Land snails will preferentially stimulate electrodes in the mesocerebral region of their cerebral ganglia, which includes neurons responsible for sexual behavior. Responses to noxious stimuli do not appear to be mediated by morphine (Crook & Walters, 2011).
Clams, oysters, scallops, and their relatives have simple nervous systems consisting of two pairs of nerve cords, and three pairs of ganglia. They show nociceptive responses such as withdrawing their siphons when prodded, but are not especially sensitive at injured parts of their bodies, nor display general changes in behavior when injured (Crook & Walters, 2011).
Cephalopods include the most intelligent known invertebrates, and are extremely competent problem-solvers (Crook & Walters, 2011). The cephalopod brain is not structured like the human brain, and the brain region responsible for pain or nociception is not known (Crook & Walters, 2011). Octopuses and squids do exhibit nociception, however, and octopuses have decreased thresholds for triggering escape responses when they are injured (Alupay, Hadjisolomou, & Crook, 2014). Squids do not give special attention such as grooming to injured tentacles, but are generally more sensitive to touch over a long period when injured. Like gastropods, cephalopods do not seem to respond to morphine (Crook, Lewis, Hanlon, & Walters, 2011).
Annelids have identified nociception nerves (Deneubourg et al., 1987). They show nociceptive responses to stabbing, pinching, and exposure to caustic chemicals, such as contraction and escape attempts. These responses are mediated by morphine in leeches and earthworms (Kavaliers, 1988). Annelid’s brains are relatively simple, but some are still capable of associative learning in response to pain (Deneubourg et al., 1987).
Nematodes have identified nociception nerves, and their nociceptive responses are mediated by morphine (Nieto-Fernandez et al., 2009). They are capable of simple reinforcement learning (Rankin, 2004).
Echinoderms have unusually-structured nervous systems, with clusters of neurons diffused into each limb. There is a great deal of diversity in neural structure: neuron clusters in different sub-groups are variously located in the ectodermal, mesodermal, or endodermal tissue layers (Moroz, 2009). In practice, little research on nociception in echinoderms has been done. Sea cucumbers are observed to contract and retreat from danger; similarly, when sun starfish are nicked or exposed to a caustic chemical, they will quickly retreat in the opposite direction. On a perpendicular surface, they will also detach from the surface and fall to the bottom of the water. Starfish use pedicellariae1Small jaw-shaped grasping appendages found on the surface of an echinoderm’s skin. to clear materials off of their surfaces, and crawl towards light and away from darkness (Brain: A Journal of Neurology, 1883). Some starfish can learn to associate food with a light stimulus (Landenberger, 1966). The response of echinoderms to morphine, or any other capacity for associative learning, has either not been studied or published.
Cnidarians, other multicellular phyla, protists
Behavior of these creatures is simple and not well studied. Cnidarians have extremely simple nervous systems, without central congregations of nerves. Protists are single-celled organisms and have no nervous system at all. Paramecium poked by a needle react evasively (Smith, 1991). Amoebas indicate some ability to memorize patterns in their environment, which has been explained as the result of oscillating currents in the amoebic protoplasm. Amoebas do not show signs of associative learning (Pershin, La Fontaine, & Di Ventra, 2009).
Biases and gaps in current research
Studies on which invertebrates have pain-like responses, or behaviors that may indicate such, are often conducted on model laboratory organisms. Nociceptive responses of the world’s most common wild invertebrates, such as krill (Chappell, n.d.) and Collembola (springtails) (Fleming, n.d.), have barely been studied at all. In addition, meta-studies have found that negative experimental results are under-published in science literature (Fanelli, 2012), so research into which invertebrates do not display pain-like responses is less likely to to be available, or to have been studied.
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