Chitinase and apparent digestibility of chitin in the digestive tract of juvenile cobia, Rachycentron canadum

A study was designed to determine the presence of chitinolytic enzymes in cobia. Additionally, the source of the chitinolytic enzymes (bacterial or endogenous) and apparent digestibility of chitinous waste meals were investigated to determine the viability of crustacean processing waste products in juvenile cobia diets. Antibiotics were used to eliminate potential chitinolytic gut flora and chitinolytic enzyme levels were compared to control fish fed an identical diet without antibiotic. Chitinase and chitobiase were measured in the stomach, pyloric caeca, and anterior intestine but were only detected in the stomach. Control stomach chitinase and chitobiase activities were 3075 +/- 709 (mean +/- SD) and 2076 +/- 208 mu g N-acetylglucosamine (NAG) g(-1) h(-1), respectively, and not significantly different from antibiotic treatments. This suggests substantial endogenous production and that the activity, if any, from chitinolytic bacteria is not significant. Additionally, high chitobiase levels indicate breakdown of chitin to potentially nutritive NAG. To determine the apparent digestibility coefficient (ADC) of shrimp processing waste meal (shrimp meal) and crab processing waste meal (crab meal) in vivo, diets containing a 70:30 mixture of a reference diet and test ingredient with the inclusion of the non-digestible marker yttrium oxide were fed. Chitin ADC for both crab (mean = 66.8 +/- 4.5%SD) and shrimp (78.2 +/- 8.0%) meal was high and not significantly different. Organic matter digestion was significantly higher in crab meal than either shrimp meal or the reference diet. However, crude protein, lipid, and gross energy digestion for crab and shrimp meal were not significantly different from the reference diet. Both enzymatic and in vivo results indicate the capability for juvenile cobia to digest chitin through strong endogenous chitinolytic enzymes. The addition of crustacean processing wastes in cobia diets has the potential to reduce the cost of feed production and increase the sustainability of cobia aquaculture by reducing inclusion rates of fish meal and other more expensive ingredients. (C) 2010 Elsevier B.V. All rights reserved.