AbstractCellulase and xylanase digestive enzyme activitieswere compared in four freshwater cray¢sh (GenusCherax) and three marine prawn (GenusPenaeus )species.TemperatureandpHpro¢lesforcellulase(en-doglucanase)werefoundtobeverysimilarinallspe-cies, with maximum activityoccurring at 601CandpH5.0.TemperatureandpHpro¢lesforxylanase(en-doxylanase)werealsoverysimilarinallcray¢shspe-cies, with maximum activity occurring at 501CandpH 5.0. Xylanase activity was not detected in thethree prawn species examined. In addition, in vitrostudiesshowedthatmostspecieswereabletoliberateglucose from carboxymethyl cellulose, indicatingthat cellulose substrates can be a source of energyforbothcray¢shandprawnspecies.Keywords: cellulase, xylanase, Cherax, Penaeus ,digestion, nutritionIntroductionImproving the digestibilityof plant-based feeds usedinaquaculturewilldependinpartonovercomingtheanti-nutritionaleiectsofnon-starchpolysaccharides(NSPs)associatedwithplantcellwalls(Francis,Mak-kar & Becker 2001). Current evidence suggests thatsoluble, high molecular weight NSPs (commonlycel-lulose and arabinoxylan polymers) increase digestaviscosity, thereby reducing digestive enzyme accessto other nutrients (Castanon, Flores & Pettersson1997; Bedford 2000; Francis et al. 2001). In ¢sh andcrustaceans this can result in poor growth and feede⁄ciency, depending on ingredient type and propor-tion, species cultured and age/size of candidate or-ganisms (Watanabe 2002).Inclusion of exogenous enzymes as additives inplant-based feeds has greatly improved feed utiliza-tion in terrestrial animals (Bedford 1995; Castanonet al. 1997; Bedford 2000). Commercial enzymeblends typically contain endoglucanase and en-doxylanase enzymes, which hydrolyse the b-1,4-bonds in cellulose and xylan polymers, respectively,thereby reducing the size of soluble NSPs withinthe digestive tract. The action of these enzymes isalso thought to aid in the release of nutrientsbound by plant cell walls. In aquaculture, additionof NSP-degrading enzymes to canola meal dietshas been shown to improve growth rates of juveniletiger prawns (Buchanan, Sarac, Poppi & Cowan1997). These studies indicate that endoglucanaseand endoxylanase enzymes could play a signi¢cantroleintheutilizationofplantmaterialsincrustaceandiets.Enzyme supplementation of plant-based feeds re-quires carefulconsideration, however, as inappropri-ate enzyme concentrations may potentially result inpoororevennegativeeiectsonanimalgrowth(Cas-tanonetal.1997).Relativelyhighenzymelevelscouldliberate excessive quantities of monosaccharides, in-ducing hyperglycaemia (Stone, Allan & Anderson2003). In contrast, low enzyme levels may elevatethe soluble NSPcomponent bysolubilizing the inso-luble NSP fraction (Castanon et al.1997).Inthisre-gard, the development of appropriate strategies forexogenous enzyme supplementation of plant-basedfeeds requires a greater understanding of endogen-ous enzyme activity within candidate crustaceanspecies.