Survival, development and growth response of mud crab, Scylla serrata, megalopae fed semi-purified diets containing various fish oil:corn oil ratios

This study was conducted to determine the optimum fish oil:corn oil ratio in semi-purified diets formulated for mud crab, Scylla serrata, megalopae. Six iso-energetic microbound diets (MBD) containing 6% total lipid were formulated to contain fish oil and corn oil either singly or in various ratios (0:1, 1:2, 2:1, 3:1, 1:0, 1:1), and each dietary treatment consisted of 20 individually reared megalopae. Survival, growth, development time to the first crab stage and signs of molting death syndrome (MDS) were recorded on a daily basis, and carapace width and dry weight of newly molted crabs were measured immediately after molt. Megalopae from all dietary treatments successfully molted into C1; however, best survival (70%) was achieved by megalopae fed MBD containing a fish oil:corn oil ratio of 1:1. Megalopae fed MBD containing fish oil:corn oil ratios of 3:1 and 1:0 showed survival of 65%, while survival of megalopae fed either live Artemia or MBD containing a fish oil:corn oil ratio of 2:1 was 60%. Lower survival (55% and 35%) was recorded for megalopae fed MBD with fish oil:corn oil ratios of 1:2 and 0:1, respectively. Significantly greater mean carapace width (3.51 ± 0.03 mm) and significantly higher mean dry weight (2.14 ± 0.14 mg) were recorded for crabs molting from megalopae fed live Artemia compared to those resulting from megalopae fed MBD. Megalopa fed Artemia also had the fastest mean development time (6.8 ± 0.5 days), but this was not significantly greater than development times for megalopae from any of the MBD treatments. Results from this study indicate that corn oil can be used to partially substitute fish oil in MBD formulated for mud crab megalopae, and that the optimal fish oil:corn oil ratio is around 1:1 when oil is supplied at a level of 6% of total diet dry weight. The study further showed that complete replacement of fish oil with corn oil in the formulated diet resulted in a high occurrence of MDS-related mortality, indicating an essential dietary requirement for > C18 highly unsaturated fatty acids (HUFA), and a link between HUFA availability and the occurrence of MDS.