Mass mortality during metamorphosis to megalops in the seed production of mud crab Scylla serrata (Crustacea, Decapoda, Portunidae)

The phenomenon of mass mortality of larval mud crab Scylla serrata is described during the metamorphosis to megalops due to abnormal molting by morphologically advanced last stage zoeas (fifth stage) in the seed production. Seven seed production trials were carried out using 100 kL concrete tanks. The larvae were reared on rotifers and after the third or fourth zoeal stage with Artemia nauplii. Minced mysid was also given from the megalopal stage. The seven trials were based on the addition of different concentrations of the marine phytoplankton Nannochloropsis or of the freshwater Chlorella to the larval rearing water. The survival rate, morphological features of the fifth stage zoeas, and frequency of abnormal molting during the metamorphosis to megalops were examined. The mass mortality occurred during the metamorphosis to megalops in tanks containing high concentrations of Nannochloropsis. The fifth stage zoeas in these tanks were equipped with some advanced morphological features similar to those of megalops, such as, long endopods of the second antennae, large chelipeds and pleopods with plumose setae. The frequency of abnormal molting tended to increase depending on the degree of megalopal features in the fifth zoeal stage. We propose that zoeal morphogenesis is accelerated by the nutritional conditions of zoeas due to the supplementary effect of Nannochloropsis, which contains eicosapentaenoic acid; an essential fatty acid for larval mud crab.