Greenwater, but not live feed enrichment, promotes development, survival, and growth of larval Portunus armatus

Aquaculture of portunid crabs is hindered by high mortality during larval rearing, which is often attributed to inadequate nutrition. Live feed enrichment and greenwater are techniques used to improve the nutritional quality of feeds for larval fish and some invertebrates, but their usefulness for crab larvae is unclear. This study investigated the effects of live feed enrichment and three different microalgae as greenwaters on the development, growth, and survival of Portunus armatus from hatching to the megalopa stage. P. armatus reared in greenwater had superior development and survival compared to crabs reared without greenwater. Survival and development rates were better when larvae were reared in Nannochloropsis oculata or Proteomonas sulcata greenwaters compared to Tisochrysis lutea greenwater and the crab larvae. P. armatus were also largest when reared in P. sulcata greenwater. The carapace length of megalopa was positively correlated with the biologically important eicosapentaenoic acid (20:5n-3, EPA), but negatively correlated with oleic acid (18:1n-9, OA). This suggests the benefits of using greenwater to rear larval crabs may be tied to live feeds improving their nutritional value by consuming microalgae. Interestingly, enriching live feeds with a commercial lipid emulsion did not affect P. armatus larval development, survival, or growth, despite increasing levels of docosahexaenoic acid (22:6n-3, DHA) in megalopa to a point considered harmful for the larvae of other species of crabs. This study demonstrates the benefits of greenwater when rearing larval crabs as opposed to live feed enrichment, and highlights the potential effectiveness of cryptomonad microalgae as a greenwater media.