Lipid-ovary indices in food-limited Daphnia

The amount of lipid carried by a zooplankton individual is highly variable. Various lines of evidence have suggested that the nutritional condition of animals could be ascertained via the determination of lipid content. However, few studies have been conducted where the lipid levels of animals of known condition have been measured. Here, we report on the lipid index and combined lipid-ovary index values of Daphnia obrusa reared on foods made up of varying quantity and quality of a unialgal (Scenedesmus acutus) diet. Lipid levels differed in different food types. However, in no cases were these indices positively correlated to growth rate, such as would be necessary if one were to use them to diagnose food limitation in the field. Thus, we urge caution in the utilization of either the lipid or lipid-ovary index until more is known about the role of fatty acids in the diets of these animals.