The stress–immunity axis in shellfish

It is a difficult task to describe what constitutes a 'healthy' shellfish (e.g., crustacean, bivalve). Visible defects such as discolouration, missing limbs or spines, fouling, lesions, and exoskeletal fractures can be indicative of underlying issues, senescence, or a 'stressed' animal. The absence of such symptoms is not evidence of a disease-free or a stress-free state. Now, more than ever, aquatic invertebrates must cope with acute and chronic environmental perturbations, such as, heatwaves and cold shocks, xenobiotic contaminants, intoxication events, and promiscuous pathogens expanding their host and geographic ranges. With that in mind, how does one determine the extent to which shellfish become stressed in situ (natural) or in cultured (artificial) settings to enhance disease susceptibility? Many biomarkers - predominantly biochemical and cellular measures of shellfish blood (haemolymph) - are considered to gauge immunosuppression and immunocompetence. Such measures range from immune cell (haemocyte) counts to enzymic activities and metabolite quantitation. Stressed invertebrates often reflect degraded conditions of their ecosystems, referred to as environmental indicators. We audit briefly the broad immune functions of shellfish, how they are modulated by known and emerging stressors, and discuss these concepts with respect to neuroendocrinology and immunotoxicology. We assert that chronic stress, alone or in combination with microbial, chemical and abiotic factors, increases the risk of infectious disease in shellfish, exacerbates idiopathic morbidity, and reduces the likelihood of recovery. Acute stress events can lead to immunomodulation, but these effects are largely transient. Enhancing our understanding of shellfish health and immunity is imperative for tackling losses at each stage of the aquatic food cycle and disease outbreaks in the wild.