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Dietary Preference and Digestive Enzyme Activities as Indicators of Trophic Resource Utilization by Six Species of Crab

School of Aquaculture, Tasmanian Aquaculture and Fisheries Institute, University of Tasmania, Launceston, Tasmania, Australia 7250

^* To whom correspondence should be addressed, at WA Marine Research Laboratories, PO Box 20, North Beach, Western Australia 6920, Australia. E-mail: djohnston{at}fish.wa.gov.au

The digestive physiology and stomach contents of six crab species from a variety of habitats were investigated to provide an indication of their digestive capability and dietary preferences. Stomach contents varied between species, but the key enzymes present were generally consistent with the types of dietary material being ingested. Nectocarcinus integrifons (red rock crab) consumed large quantities of seagrass and had high cellulase activity (0.02 ± 0.004 units mg^–1) to digest the constituent cellulose. Petrolisthes elongatus (porcelain crab) ingested brown and green phytoplankton and algae and had considerable laminarinase (0.35 ± 0.08 units mg^–1) and ß-glucosidase (0.025 ± 0.005 units mg^–1) activities to digest the laminarin in its diet. Leptograpsus variegatus (omnivorous swift-footed shore crab) had high activities of protease (1.2 ± 0.02 units mg^–1), -glucosidase, and -amylase and appeared well equipped to utilize both dietary protein and carbohydrate. Stomach contents in Nectocarcinus tuberculosus (velvet crab) and Carcinus maenas (green crab) also suggest that these species are omnivorous. N. tuberculosus had high cellulase and chitinase for digesting the cellulose in plants and the chitin in invertebrate shells respectively. C. maenas had intermediate digestive enzyme levels and may employ more of a generalist feeding strategy than other species. Plagusia chabrus (speedy crab) is carnivorous, consuming encrusting bryozoans, hydroids, crustaceans, and fish. It has high protease activity, particularly trypsin (0.73 ± 0.12 units mg^–1), to digest the protein in its animal prey. Each species of crab studied had a complex suite of digestive enzymes, the relative activities of which reflected individual and very different species-specific dietary niches.