Nociceptors are sensory neurons that are tuned to tissue damage. In many species, nociceptors are often stimulated by noxious extreme temperatures and by chemical agonists that do not damage tissue (e.g., capsaicin and isothiocyanate). We test whether crustaceans have nociceptors by examining nociceptive behaviours and neurophysiological responses to extreme temperatures and potentially nocigenic chemicals. Crayfish (Procambarus clarkii) respond quickly and strongly to high temperatures, and neurons in the antenna show increased responses to transient high temperature stimuli. Crayfish showed no difference in behavioural response to low temperature stimuli. Crayfish also showed no significant changes in behaviour when stimulated with capsaicin or isothiocyanate compared to controls, and neurons in the antenna did not change their firing rate following application of capsaicin or isothiocyanate. Noxious high temperatures appear to be a potentially ecologically relevant noxious stimulus for crayfish that can be detected by sensory neurons, which may be specialized nociceptors.

BackgroundNociception is the physiological detection of noxious stimuli. Because of its obvious importance, nociception is expected to be widespread across animal taxa and to trigger robust behaviours reliably. Nociception in invertebrates, such as crustaceans, is poorly studied.Methodology/Principal FindingsThree decapod crustacean species were tested for nociceptive behaviour: Louisiana red swamp crayfish (Procambarus clarkii), white shrimp (Litopenaeus setiferus), and grass shrimp (Palaemonetes sp.). Applying sodium hydroxide, hydrochloric acid, or benzocaine to the antennae caused no change in behaviour in the three species compared to controls. Animals did not groom the stimulated antenna, and there was no difference in movement of treated individuals and controls. Extracellular recordings of antennal nerves in P. clarkii revealed continual spontaneous activity, but no neurons that were reliably excited by the application of concentrated sodium hydroxide or hydrochloric acid.Conclusions/SignificancePreviously reported responses to extreme pH are either not consistently evoked across species or were mischaracterized as nociception. There was no behavioural or physiological evidence that the antennae contained specialized nociceptors that responded to pH.

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