Effects of different natural or prepared diets on growth and survival of juvenile spider crabs, Maja brachydactyla (Balss, 1922)

The effects of different diets (natural or pellets) on growth, survival, and moulting interval of juvenile spider crabs, weighing between 0.011-1.56 g and up to 17.6 mm in carapace length, were tested over a period of 90 days. During experiment I, five diets were tested: (1) frozen shrimp-Paleomonetes sp., (2) fresh mussels-Mytilus sp., (3) white fish fillets-Merlucius merlucius, (4) blue fish fillets-Sardina pilchardus, and (5) commercial crustacean pellets. Spider crabs fed fresh mussels grew larger (0.98 +/- 0.69 g) and had higher growth rates (4.0 +/- 0.7 %BWd(-1)) compared to the other four diets. The crabs fed shrimp pellet and frozen shrimp grew to intermediate sizes and were smaller than the ones fed fresh mussels, but they were larger than spider crabs fed either blue or white fish fillets (0.46 +/- 0.63 and 0.26 +/- 0.13 g, respectively) compared to the ones fed white fish fillets (0.12 +/- 0.04) and blue fish fillets (0.04 +/- 0.02 g). The spider crabs fed blue fish fillets only lasted until day 60 of the experiment, after this day none of the 20 fed this diet were left. During experiment II, two diets were tested: (1) white and blue fish fillets and (2) commercial fish pellet. There were no differences in growth both in weight or carapace length (2.9 +/- 1.8 and 2.1 +/- 1.5 g in weight, and 18.9 +/- 5.0 and 17.7 +/- 3.3 mm, respectively) at the end of the experiment. Similarly, there were no differences in growth rates in weight between the two diets (1.2 +/- 0.4 and 0.9 +/- 0.3 %BWd(-1), respectively) or in carapace length (0.4 +/- 0.1 and 0.4 +/- 0.2 %BWd(-1), respectively). Fresh mussel appears to be a very good diet to culture the early stages of this species, while shrimp pellets also deliver acceptable results. On the contrary, frozen shrimp, fish fillets either from blue or white species (much higher lipid content in the blue species), and fish pellets were found to be bad diets for the culture of the early stages of M. brachydactyla.