Pain in invertebrates

This is the ability to detect noxious stimuli which evokes a reflex response that moves the entire animal, or the affected part of its body, away from the source of the stimulus.

[12] However, it is possible that different structures may be involved in the pain experience of other animals in the way that, for example, crustacean decapods have vision despite lacking a human visual cortex.

[13] Two groups of invertebrates have notably complex brains: arthropods (insects, crustaceans, arachnids, and others) and modern cephalopods (octopuses, squid, cuttlefish) and other molluscs.

Emerging results suggest that a convergent evolutionary process has led to the selection of vertebrate-like neural organization and activity-dependent long-term synaptic plasticity in these invertebrates.

[17] Later studies on the responses of leech neurones to mechanical, chemical and thermal stimulation motivated researchers to write "These properties are typical of mammalian polymodal nociceptors".

Rather than a simple withdrawal reflex, the flatworm, Notoplana aticola, displays a locomotory escape behaviour following pin pricks to the posterior end.

[29] Touching the larvae of fruit flies, Drosophila melanogaster, with a probe causes them to pause and move away from the stimulus; however, stronger mechanical stimulation evokes a more complex corkscrew-like rolling behaviour, i.e. the response is plastic.

A set of defensive behavior patterns in larval Manduca sexta is described and shown to undergo sensitization following noxious mechanical stimulation.

The striking response is a rapid bending that accurately propels the head towards sharply poking or pinching stimuli applied to most abdominal segments.

More intense noxious stimuli evoke faster, larger strikes and may also elicit thrashing, which consists of large, cyclic, side-to-side movements that are not directed at any target.

[33] Over 200 species of invertebrates are capable of using autotomy (self amputation) as an avoidance or protective behaviour[34][35] including: These animals can voluntarily shed appendages when necessary for survival.

Autotomy serves either to improve the chances of escape or to reduce further damage occurring to the remainder of the animal such as the spread of a chemical toxin after being stung, but the 'decision' to shed a limb or part of a body and the considerable costs incurred by this suggests a pain response rather than simply a nociceptive reflex.

[30] In contrast, cold stimuli (≤14 °C or 57.2 °F) primarily elicit a bilateral full-body contraction along the head-to-tail axis; larvae might also respond by lifting their head and/or tail, but these responses occur less frequently with decreasing temperatures.

[46] The prawn Palaemon elegans shows protective motor reactions when their antennae are treated with the irritants acetic acid or sodium hydroxide.

[47] The prawns specifically groom the treated antennae and rub them against the tank, showing they are aware of the location of the noxious stimulus on their body rather than exhibiting a generalised response to stimulation.

In Carcinus maenas, the common shore crab, acetic acid induces a number of behavioral changes, including movement of the mouthparts, rubbing with the claws, and increased attempts to escape from an enclosure.

[53] Adult flies find menthol,[54] AITC,[55] capsaicin,[56] and a number of other chemicals to be aversive, affecting both the proboscis extension reflex and egg-lay site preference.

[57][58][59] The sea-slug, Tritonia diomedia, possesses a group of sensory cells, "S-cells", situated in the pleural ganglia, which initiate escape swimming if stimulated by electric shock.

Many invertebrate species learn to withdraw from, or alter their behaviour in response to, a conditioned stimulus when this has been previously paired with an electric shock—cited by Sherwin[1]—and include snails, leeches, locusts, bees and various marine molluscs.

If vertebrate species are used in studies on protective or motor behaviour and they respond in similar ways to those described above, it is usually assumed that the learning process is based on the animal experiencing a sensation of pain or discomfort from the stimulus, e.g. an electric shock.

[28] In the crab Neohelice granulata,[Note 1] electric shocks delivered via small holes in the carapace elicited a defensive threat display.

[47] Another local anaesthetic, xylocaine, reduced the stress of eyestalk ablation in female whiteleg shrimps, Litopenaeus vannamei, as indicated by levels of feeding and swimming.

Extracellular recordings of antennal nerves in the Louisiana red swamp crayfish revealed continual spontaneous activity, but no neurons that were reliably excited by the application of sodium hydroxide or hydrochloric acid.

In one study, shore crabs, Carcinus maenas received electric shocks in a preferred dark shelter but not if they remained in an unpreferred light area.

Robert Elwood and Mirjam Appel at the Queen's University of Belfast argue that pain may be inferred when the responses to a noxious stimulus are not reflexive but are traded off against other motivational requirements, the experience is remembered and the situation is avoided in the future.

This demonstrates the experience of the electric shock altered future behaviour in a manner consistent with a marked shift in motivation to get a new shell to replace the one previously occupied.

He suggested that worms appear to "have the power of acquiring some notion, however crude, of the shape of an object and of their burrows" and if so, "they deserve to be called intelligent; for they then act in nearly the same manner as would a man under similar circumstances.

With respect to the small size of the cerebral ganglia, we should remember what a mass of inherited knowledge, with some power of adapting means to an end, is crowded into the minute brain of a worker-ant.

For instance, a spider can eat a fly held in front of it by an experimenter, bypassing the usual step of moving toward an insect caught on its web.

[107] Social behavior is widespread in invertebrates, including cockroaches, termites, aphids, thrips, ants, bees, Passalidae, Acari, spiders, and more.