Allocation of essential lipids in Daphnia magna during exposure to poor food quality

In nature, food conditions change temporally and force consumers into trade-offs during resource allocation. In particular, under poor food conditions e.g. during cyanobacterial blooms, herbivores have to optimise their resource allocation to maximise fitness, and face two decisions: i) an individual might attempt to allocate acquired essential resources to reproductive tissues or use them for its own maintenance, and ii) an individual might decide to optimise the chemical quality of its eggs. Since cyanobacteria feature a deficiency in some essential lipids that leads to a decline in the growth and fecundity of Daphnia, an important freshwater herbivore, we investigated Daphnia magna’s allocation of lipids such as polyunsaturated fatty acids (PUFAs) and cholesterol during an experimental non-toxic cyanobacterial bloom. Under poor conditions D. magna not only decreased the number of eggs produced but, in principal, reduced both the previously high concentrations of the particularly important omega-3 (n-3) PUFAs, in particular eicosapentaenoic acid (EPA) in eggs and in somatic tissues to a similar degree. In contrast to EPA, the concentrations of a-linolenic acid and cholesterol, although lower than EPA, were more homeostatic in eggs than in somatic tissues, in which concentrations decreased. When food quality was improved, D. magna was able to completely recover their fatty acid concentrations in their somatic tissues and eggs.