Optical‐digital measurements of energy reserves in Calanoid copepods: Intersegmental distribution and seasonal patterns

We present a simple optical‐digital method to measure visible energy reserves in calanoid copepods. Seasonal patterns (spring‐fall) of triacylglycerol energy reserves in Diaptomus sicilis were determined in a hypereutrophic and in a saline prairie lake. In addition to optical measurement of energy reserves, two well‐established chemical techniques (microgravimetric and Iatroscan TLC‐FID analyses) were used over the same period. Seasonal patterns of visible energy reserves measured by the optical method were positively correlated with both the Iatroscan measurements of triacylglycerol (Pearson’s r was between 0.72 and 0.87, P < 0.005) and the microgravimetric measurements of total lipid ( r was between 0.73 and 0.88, P < 0.005). Thus, the optical method can be used to investigate seasonal changes in energy reserves in calanoid copepods but at a substantially lower cost (equipment, handling time) than chemical‐based methods. Moreover, the optical method offers increased resolution because energy reserves can be measured for each animal so that individual feeding success can be assessed, provides data on the partitioning of lipids in various body regions, and is nondestructive because anesthetized animals can be measured and returned to culture. Patterns of storage lipid reabsorption within discrete body regions may predict starvation risk in culture and in natural populations of copepods.