Capability of Artemia to digest Chlorella vulgaris and Nannochloropsis oculata under stagnant culture conditions

The microalgae genera Chlorella and Nannochloropsis are commonly used as foods for rotifer culture and its nutritional enrichment materials. However, Artemia nauplii and early metanauplii cannot easily digest these genera because of their thick cell walls, and the conditions under which Artemia can digest them remain unclear. We investigated the size at which Artemia is capable of digesting these microalgae and the effects of Chlorella supply on Artemia fatty acid composition. Decreased microalgae cell densities in culture water were observed after Artemia reached 1.6 mm in total length for Chlorella and 1.1 mm for Nannochloropsis, but the rate of decrease was slow and excrement from Artemia was filled with intact microalgae cells, suggesting that Artemia can ingest microalgae but cannot digest them. The density of residual microalgae decreased linearly after Artemia reached 3.2 mm for Chlorella and 2.0 mm for Nannochloropsis, and the excrement contained broken cells, indicating true digestive performance. The absorption of fatty acids of Chlorella by the Artemia was detected after Artemia reached 3.5 mm in total length. These results suggest that Artemia acquires the ability to consume microalgae in a stepwise manner depending on the growth and species of microalgae.