Utility of the cellular energy allocation model for assessing food limitation stress in freshwater mussels

Freshwater mussel populations are declining worldwide, but the causes and mechanisms of these declines are poorly understood. Biomarkers that reflect the health or fitness of individual mussels are needed for understanding causes of mussel declines, but existing approaches each have weaknesses. We conducted two laboratory experiments to examine the utility of the cellular energy allocation (CEA) model for assessing juvenile mussel responses to stress induced by food limitation. The CEA assesses the energetic status of an organism as CEA = <i>E</i> _a/<i>E</i> _c, where <i>E</i> _a is available energy reserves (total carbohydrates, protein, and lipids) and <i>E</i> _c is energy consumption, estimated using electron transport system (ETS) activity as a proxy for respiration rate and metabolic demands. Experiment 1 evaluated the effects of food abundance (fed and unfed) on CEA and its component biomarkers at a single temperature (mean = 26.8°C) over 23 days. Experiment 2 evaluated the response of ETS activity to food abundance (unfed, low food, high food) in relation to temperature (20, 25, 30°C) over 27 days. In Experiment 1, most constituent biomarkers were lower in unfed mussels, but CEA did not differ between treatments because <i>E</i> _a and <i>E</i> _c declined by similar magnitudes. In Experiment 2, ETS declined with decreasing food abundance, but only at 25 and 30°C, and ETS was affected by temperature only in the unfed treatment. The ETS enzyme assay can be an informative biomarker of stress, but it requires accounting for confounding factors such as food, temperature, and species identity, as well as the lag time in response of ETS relative to respiration rate. Despite its value as a robust, holistic stress biomarker in other organisms, CEA may have limited usefulness for bivalves because of their tendency to reduce feeding and energy consumption under stress, which results in a simultaneous decline in <i>E</i> _a and <i>E</i> _c.