CARBOHYDRATE DIGESTION IN THE EUROPEAN LOBSTER HOMARUS-GAMMARUS (L)

The ability of lobsters to utilize dietary carbohydrates is dependant upon the nature and amount of carbohydrases present in these crustaceans. Since this is a potential species for culture, a knowledge of its ability to digest carbohydrates could usefully be applied in the formulation of inert diets. The aim of this study was to identify the activities present in the gut and to assess the digestibility of physically modified starches using midgut-gland extracts. A range of carbohydrase activities including alpha-amylase (EC 3.2.1.1), maltase (EC 3.2.1.20), alpha-glucosidase (EC 3.2.1.20), beta-glucanase (EC 3.2.1.39), beta-glucosidase (EC 3.2.1.21), alpha- and beta-galactosidases (EC 3.2.1.22 and EC 3.2.1.23) were detected in the midgut gland and in the gastric juice of the European lobster Homarus gammarus. Optimal activity for the above enzyme activities was in the pH range of 4.5-5.5 and highest levels were generally located in the gastric contents. alpha-Glucans were more readily digested than beta-glucans; hydrolysis of pentosans was minimal and chitin was poorly hydrolyzed. Polysaccharides and disaccharides were well digested by gastric extracts, whereas the predominant digestive capacity of the midgut gland was toward disaccharides. Molecular weights of the midgut gland alpha-amylase and maltase activities were estimated by gel-filtration to be approximately 21 kDaltons and 63 kDaltons, respectively. Physical modification of starch granules enhanced their susceptibility to digestion by midgut-gland extracts and therefore pretreatment of dietary starches may be considered in order to increase their nutritional value in inert aquaculture diets.