Identification of digestible carbohydrate sources for inclusion in formulated diets for juvenile spiny lobsters, Jasus edwardsii

Carbohydrates are a key ingredient in crustacean formulated diets because of their potential to greatly improve production efficiency. For the culture of spiny lobsters where daily food intake is limited, carbohydrates have the potential for delivering a low cost source of energy that could spare protein for growth. Therefore, the digestibility of different carbohydrate sources including refined sugars, mussel glycogen, algal polysaccharides, carboxymethyl cellulose (CMC) and starches (i.e., native, dextrinised, gelatinised) were assessed in juvenile spiny lobster (30–60 g). This was done by measuring the rate of carbohydrate hydrolysis in vitro using enzyme homogenates and postprandial haemolymph glucose concentrations following ingestion of semi-purified diets containing different carbohydrate levels and sources. Fresh mussel gonads and a practical formulated diet were also included for comparison. Storage polysaccharides (i.e., gelatinised starches, dextrin, mussel glycogen) and the structural polysaccharide CMC were the best digested carbohydrate sources in vitro. CMC was more digestible than the algal polysaccharides, agar and alginate, and therefore may have potential as a binding agent in formulated diets for J. edwardsii. The poor hydrolysis of sucrose and trehalose suggests that their use as an energy source might be limited in J. edwardsii. Native wheat starch was the best digested among the various plant starches tested. Gelatinisation of starches markedly improved their digestibility suggesting that pre-treatment of the dietary starch source would have a beneficial influence on the digestibility of diets for J. edwardsii. Consumption of the semi-purified diets (i.e., 1% BW) containing the digestible starch sources (i.e., 27% dry weight), as well as the practical diet, resulted in high haemolymph glucose concentrations (N5 mmol l−1) and a prolonged hyperglycaemic response (N24 h) suggesting that these carbohydrate sources are well digested and absorbed, but possibly poorly utilised. In contrast, the fresh mussel gonad diet (i.e., 27% glycogen by dry weight) appeared to be better utilised (reduced glycaemia after 12 h). A lower inclusion level (i.e., 7%) of gelatinised maize starch reduced the peak (1.87 mmol l−1) and extent (12 h) of the glycaemic response. Using lower inclusion levels (b27%) of the rapidly digested starches (i.e., gelatinised, dextrinised) identified in this study, or incorporating digestible carbohydrate sources resulting in slower appearance of haemolymph glucose (i.e., native wheat starch, CMC), in formulated diets may have the potential to improve their utilisation for growth of juvenile spiny lobsters.