Digestive Enzyme Activities in the Alimentary Tract of Redclaw Crayfish, Cherax Quadricarinatus (Decapoda: Parastacidae)

The redclaw crayfish, Cherax quadricarinatus, is of commercially important interest, but its digestive capabilities are not yet completely understood. The understanding of how its digestive enzymes operate can help in the formulation of better diets for this species. A range of digestive enzymes (proteases, carbohydrases, and lipase) was found in the midgut gland and gastric fluid, with activities generally higher in the gastric fluid. Total protease showed maximal activity at pH 7.0–7.5 in both midgut gland and gastric fluid and was inhibited by |$1.5\ {\rm mM}\ {\rm CuCl}_{2}$| and 10 mM phenylmethylsulfonyl fluoride (PMSF). On the other hand, protease was activated by |$1.5\ {\rm mM}\ {\rm CaCl}_{2}$|⁠. Trypsin-like activity showed pH optima at 7.0 in both the midgut gland and the gastric fluid and was activated by |$1.5\ {\rm mM}\ {\rm CaCl}_{2}$|⁠, |${\rm FeSO}_{4}$|⁠, and |${\rm Na}_{2}{\rm SO}_{4}$|⁠. Chymotrypsin-like activity and carboxypeptidase A-like activity showed higher activities in the gastric fluid. Carboxypeptidase B-like activity showed optima at pH 6.0 and 7.0 in both the midgut gland extract and the gastric fluid, respectively. Leucine aminopeptidase was detected at low concentration. The |$\alpha$|-amylase (optimum pH 6.0) was inhibited by |$1.5\ {\rm mM}\ {\rm CuCl}_{2}$| and 5 mM citric acid. A significant increase in the |$\alpha$|-amylase activity was observed when it was pre-incubated in the presence of |$15\ {\rm mM}\ {\rm CaCl}_{2}$|⁠. Cellulase and laminarinase activities were readily detected. Other |$p$|-nitrophenyl glycosidases were also detected in the gastric fluid and the midgut gland. Lipase was detected only in the gastric fluid. The presence of these enzymes is an indication of the ability of this species to digest a wide range of food components.