Micro-elemental retention in rotifers and their trophic transfer to marine fish larvae: Influences of green algae enrichment

Application of green water has been widely practiced in larval fish aquaculture, whereas their impacts on micro-elemental nutrition of rotifers and marine fish larvae have been seldom investigated. In the present study, we evaluated the impacts of green water and clean water on the micro-elemental changes and profiles in rotifers (both S-and L-types), and their subsequent impacts on marine fish larvae. In clean water, the gut contents (unfinished algae) of rotifers were lost rapidly. However, the application of green water relieved the rapid loss of all microelements (especially for Mn and Co), with more profound effects observed on the S-type rotifers. Under long-term enriched conditions, Cu and Zn in rotifers were rapidly lost in clean water, and the application of green water even facilitated the loss of all three enriched micro-elements (i.e., Cu, Zn, and Se). We demonstrated that the inclusion of gut contents (algae) in rotifers provided more bioaccessible micro-elements (> 80% in the non-skeleton fraction) than the empty-gutted rotifers. Furthermore, these gut contents (algae) in rotifers also provided more bioavailable Fe source by increasing its assimilation efficiency in both 5 and 25 dph medaka larvae. In conclusion, the present study showed the positive effects of green water on the micro-elemental nutrition in rotifers and larval fish. However, their negative impacts on the enriched microelements in rotifers should also be considered during real application.