Estimating food limitation in cladoceran populations1

Abstract

Cladocerans store energy for reproduction and survival in the form of triglyceride droplets. The amount of storage in adults is lowest just after egg production and increases during the intermolt period. The bright orange‐yellow of the lipid droplets allows rapid visual indexing of an individual’s body reserves. This lipid index is an extremely sensitive indicator of the feeding success of individual cladocerans. Procedures are presented for using the lipid index to estimate food limitation in natural populations of cladocerans. The relationship between lipid levels, body size, and reproduction was investigated for three species of Daphnia in three lakes of different trophic status. In adults, lipid storage is positively correlated to clutch size. All species provided their neonates with a lipid reserve (maternal investment) but the amount was different for each species. Juvenile lipid reserves were generally lower than adult reserves, but differences in allocation strategy (between juvenile and adult) and neonate maternal reserves precluded a direct relationship between lipid content and body size.