Nucleic acid ratio as a proxy for starvation of coastal and pelagic copepods in the North Pacific Ocean

We investigated the nucleic acid ratio (RNA/DNA) as a proxy for starvation in copepods collected from coastal and pelagic sites across the North Pacific Ocean. While RNA/DNA ratios were variable among copepod species and their life stages, lower ratios were found for the copepods collected from the Kuroshio region compared to ones from the western subarctic site of the North Pacific Ocean. Log-transformed RNA/DNA ratios showed a negative correlation to log-transformed mean temperature in the sampling layer and a positive correlation to log-transformed chlorophyll a concentrations. Copepod incubation experiments demonstrated that RNA/DNA ratios significantly declined after exposure to starvation and the decline was greater for the copepods collected from the subtropical sites compared with those from the subarctic. However, no significant decline was found for some species and stages accumulating lipids. These findings suggest that copepod RNA/DNA ratios are associated with food availability but are not sensitive to starvation for some copepods accumulating lipids.