Energetic costs of mounting an immune response in a coral reef damselfish (Pomacentrus amboinensis)

While immune responses can be energetically costly, quantifying these costs is challenging. We tested the metabolic costs of immune activation in damselfish (Pomacentrus amboinensis Bleeker, 1868) following a mass-adjusted injection of lipopolysaccharide (LPS) endotoxin. Fish were divided into eight treatments: two controls (handling and saline injection) and six LPS groups with concentrations ranging from 3 to 100mg kg-1. We used intermittent flow respirometry to measure differences in oxygen uptake (deltaṀO2) 20h before versus 20h after LPS injection and changes in metabolic traits (lowest, routine and peak metabolic rates) as proxies of the aerobic costs of metabolism. Spleen somatic index (SSI) and gene expression in spleens were measured to assess immune activation. We found no difference in metabolic traits or SSI but observed different non-linear patterns of deltaṀO2 in fish exposed to 50 and 100mg kg-1 LPS compared to lower doses and controls. Fish exposed to high doses of LPS also had lower residual aerobic scope compared to controls and lower LPS doses. Fish exposed to doses of 3, 50, and 100mg kg-1 showed altered gene expression compared to the handling control. Overall, our results suggest that immune activation has measurable effects on metabolic traits that are both dose and time-dependent.