The effects of diet on the growth energetics of postlarval lobsters (Homarus americanus)

Abstract

The growth energetics of postlarval lobsters (Homarus americanus) fed a brine shrimp diet (Artemia salina; 51% protein, protein:carbohydrate = 5.1) were compared with the energetics of lobsters fed three artificial diets. The artificial diets were pelletized shrimp meal diets, varying in both protein (16.65-23.30%) and carbohydrate content (22.85-31.27%) and the protein:carbohydrate ratio (0.5-1.0). The best growth was measured among lobsters fed the brine shrimp diet and the 23.30% protein diet, followed by the two lower protein diets. The protein efficiency ratios (g wet wt. gain/ g dry wt. protein fed) were inversely related to the protein level of each diet. All diets were assimilated at the same level (~ 9 0% ) but there were significant differences in food consumption rates, respiration rates and ammonia excretion rates among lobsters from the four experimental groups. Although all lobsters were given equal rations in grams, the artificial diets were lower in caloric content than the brine shrimp and the pellets were fragmented by the lobsters during the feeding process, resulting in significantly lower (P <0.01) food consumption rates of the artificial diets. Respiration rates measured immediately after feeding were significantly lower among lobsters fed the three artificial diets than those fed the brine shrimp diet; the increased respiration rate of the latter group of lobsters reflects an increased calorigenic effect due to the higher protein level of the brine shrimp diet. Ammonia excretion rates of lobsters from the four groups were significantly different from one another ( P <0 . 01) and were directly correlated with the protein level of each diet. The O:N ratios (atomic ratio of oxygen consumed to NH+ -N excreted) measured in the four experimental groups were inversely related to the protein level of the four diets, indicating an increased dependence on carbohydrate catabolism for energy production with low dietary protein levels. The reduced growth rates of lobsters fed the two lower protein diets were apparently a result of differences in the amounts of food consumed and not increased energy expenditures or reduced assimilation efficiencies.