The effect of dietary chitin supplementation on the survival and immune reactivity of the shore crab, Carcinus maenas.

Adult male shore crabs (Carcinus maenas) were maintained on a fish-based diet supplemented with 0, 5 or 10% chitin for 11 weeks. Significantly greater mortality was found during this period in the control group (0% chitin) than those fed 10% chitin. Crabs fed 5 or 10% chitin had lower numbers of cultivatable bacteria in the hepatopancreas than those on the basal diet alone. The addition of chitin had no significant effect on the serum concentrations of protein and glucose, and the levels of glycogen in the hepatopancreas. The total number of circulating hemocytes in the blood was unaffected by the addition of chitin to the diet, however, at week 6 there were significantly more hyaline hemocytes in those crabs fed 10% chitin than the control group. The in vitro phagocytic activity of hemocytes was unaffected by chitin supplementation and crabs challenged with Vibrio alginolyticus showed a similar pattern of susceptibility in the three dietary groups (0, 5 or 10% chitin). Overall although crabs on a chitin-supplemented diet showed greater survival, this was not explained in terms of elevation in the cellular defences of these animals. The enhanced survival of crabs-fed chitin is probably as a result of the removal of potentially pathogenic bacteria from the hepatopancreas. Because chitin appears to 'purge' bacteria from the gut, this may prove to be a useful addition to diets on animals undergoing oral probiotic treatment.