Signals of Struggle: Biomarkers for Stress and Resilience in Mytilus edulis

Abstract

Coastal mussels face multiple stressors in nature, yet biomarker interpretations of mussel health are typically based on single-stressor studies. This proof-of-concept study tested whether three commonly used biomarkers: hemocyte count, superoxide dismutase activity (SOD) and catalase activity (CAT) retain their interpretative value when Mytilus edulis are exposed to hypoxia and mechanical disturbance simultaneously. Mussels were held in either open or closed respiration chambers with continuous dissolved oxygen logging. After a 42-hour normoxic or hypoxic exposure, selected mussels were exposed to a standardised mechanical disturbance. Both hemolymph and gill tissue samples were extracted and analysed for hemocyte counts and SOD and CAT activity. Mechanical disturbance appeared to increase hemocyte counts and SOD activity, while combined exposure produced lower hemocyte counts but higher CAT values. Although no statistically significant difference between treatments were found, effect sizes for hemocyte count and CAT activity were large, indicating biologically relevant responses. These patterns are consistent with reallocation of energy in which hypoxia limits aerobic ATP production and shifts physiological priorities from hemocyte mobilisation to antioxidant response. Although these results should be interpreted with caution due to limited replication and single end-point sampling, these results imply that biomarker responses are not uniform, but rather context dependent. Reliable monitoring requires more research, particularly focussing on stressor interaction and increases in temporal resolution.