Abstract
Recent efforts have been devoted to the link between responses to non-physical stressors and immune states in animals, mostly using human and other vertebrate models. Despite evolutionary relevance, comparatively limited work on the appraisal of predation risk and aspects of cognitive ecology and ecoimmunology has been carried out in non-chordate animals. The present study explored the capacity of holothuroid echinoderms to display an immune response to both reactive and anticipatory predatory stressors. Experimental trials and a mix of behavioural, cellular and hormonal markers were used, with a focus on coelomocytes (analogues of mammalian leukocytes), which are the main components of the echinoderm innate immunity. Findings suggest that holothuroids can not only appraise threatening cues (i.e. scent of a predator or alarm signals from injured conspecifics) but prepare themselves immunologically, presumably to cope more efficiently with potential future injuries. The responses share features with recently defined central emotional states and wane after prolonged stress in a manner akin to habituation, which are traits that have rarely been shown in non-vertebrates, and never in echinoderms. Because echinoderms sit alongside chordates in the deuterostome clade, such findings offer unique insights into the adaptive value and evolution of stress responses in animals.
Highlights
Recent efforts have been devoted to the link between responses to non-physical stressors and immune states in animals, mostly using human and other vertebrate models
All controls across treatments showed a baseline density of coelomocytes between 1.0 and 1.7 × 106 cells ml−1 from the beginning to the end of the 72-h trial
In experiment 1, representing direct physical contact with the predatory asteroid, the total density of free coelomocytes rose from baseline values at time 0 to 11.6 × 1 06 cells ml−1 after 3 h, representing an increase of about 730% (Fig. 1a)
Summary
Recent efforts have been devoted to the link between responses to non-physical stressors and immune states in animals, mostly using human and other vertebrate models. The stress response to both situations may involve a wide range of mechanisms, including changes in genetic, metabolic, energetic, immune, endocrine, neural and behavioural processes aiming to overcome and compensate for the imbalances produced by the stressor With these reactions, the animal tries to avoid dangerous situations and any threats to its survival or integrity and to reintegrate a state of balance[1]. A new framework has been proposed for studying emotions across all animal species, based on a central emotion state with properties that are expressed through cognitive, behavioural, physiological and subjective components[10] This approach disentangles emotions and their precursors from feelings (the conscious experience of emotional reactions), and moves away from comparisons with humans in favour of defining common features like scalability, persistence, valence and generalization to multiple contexts. The correlation found between escape and cardiac responses supports that the crab triggered several defensive reactions in the face of impending danger[17]
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