Abstract
Pain is defined as an unpleasant sensory and emotional experience associated with, or resembling, actual or potential tissue damage. It is a complex process mediated not only by physiological mechanisms, but also by psychological and social factors, all of which can influence its intensity and quality at the sensory, cognitive, and emotional level. Although there is substantial evidence available for pain in animals, their ability to feel pain rather than sense pain still elicits some confusion, probably due to the difficulty to clearly distinguish nociception (i.e. the neural detection of noxious stimuli), from pain as a conscious, affective experience. Despite all its negative connotations, pain is a critical ability for the survival of organisms in that it drives immediate responses, attention, and adaptive learning to avoid pain and promote healing. In fish, studies directly addressing pain are limited to a few species. In the present study, we evaluated the behavioural effects of acetic acid (AA) subcutaneously injected into the lips of gilthead seabream (Sparus aurata), an important aquaculture species. Two experiments were carried out: In Experiment 1, fish received sham handling (needle insertion), PBS (control solution), 1, 5 or 10% AA. In Experiment 2, fish received PBS, 10% AA, or 10% AA combined with lidocaine at 1 or 2 mg mL-1. In both experiments, behaviour was recorded 1 h before treatment (Time 0) and at 0.5, 1.5, 3 and 6 h post-injection (p.i.). Qualitative inspection of the lips showed that 10% AA was associated with visible swelling and mouth opening movements, which were not observed in controls. 10% AA increased freezing, with greater use of the lower zone of the tank at 0.5, 1.5 and 3 h p.i. Co-administration of lidocaine reversed these behavioural changes at both doses tested. These results are in line with the responses found in other species to painful stimuli and suggest that 1) AA elicits a typical pain-like response in gilthead seabream and 2) the physiological sensing component shares the same conserved mechanisms of reception and transmission of painful signals as other vertebrates. These results add to the mounting evidence of pain in fishes and have ethical implications on the treatment of this (and other) fish species under a welfare perspective.
Acknowledgment
JCCS received funding from Margarita Salas grant R-1593/2022 and Juan de la Cierva postdoctoral fellowship JDC2023-052846-I. JLS received funding from FCT – Foundation for Science and Technology through projects UIDB/04326/2020 (DOI:10.54499/UIDB/04326/2020) and LA/P/0101/2020 (DOI:10.54499/LA/P/0101/2020). This study received funding from project EUPAHW (HORIZON-CL6-2023-FARM2FORK-01-2 / 101136346)