Lipid Accumulation and Allocation in Daphniid Cladocera

The purpose of this study is to identify constraints on the accumulation and allocation of lipids by daphniids. A comparison of fatty acid synthesis and accumulation rates indicates that less than 2% of their accumulated lipids are synthesized de novo by daphniids, over 98% are obtained from the diet. Accumulation rates of storage lipids parallel the rate of increase of adult body mass or growth rate (r ² = 0.987). Smaller taxa have higher specific-growth rates and their mass-specific lipid accumulation rates are higher than those of larger animals. The amount of lipid stored as a proportion of body mass varies substantially among taxa, independent of body mass. The total amount of lipid present at the end of an instar is most closely related to the amount of triglyceride present in the body at the beginning of the instar. When given abundant food, daphniids accumulate lipids differentially, relative to other dietary compounds, so that the proportion of lipid mass increases relative to the proportion of non-lipid mass. This suggests that daphniids may selectively assimilate lipids, or eliminate some non-lipid dietary compounds either by non-absorption and egestion, or by catabolism as a metabolic energy source. Because this process would lead to the excretion of non-catabolized end-products, ammonia will be released when protein is catabolized. Thus, lipid accumulation may account in part for high regeneration rates of nutrients by daphniids.