FOOD HABITS OF 2-YEAR-OLD CHINESE MITTEN CRAB, ERIOCHEIR SINENSIS, STOCKED IN LAKE BAO'AN

Gastric mills of 362 specimens of the 2-year-old Chinese mitten crab, Eriocheir sinensis, which contained recognizable food items, in Lake Bao'an were examined. The crab specimens sampled were preserved immediately in 8% formalin and analyzed within 20 days, except specimens in April, May and June which were observed immediately after the crabs were captured. In the laboratory, carapace width (cw) were measured using caliper, and sex recorded. The gastric mills were dissected out, and the food contents of each individual were flushed into a 5 ml plastic tube with 5% formalin and stirred lightly using tweezers to break up and then examined under a binocular microscope at 160× to 600×. All food items were identified to the lowest possible taxonomic level and later sorted into fourteem dietary categories: macrophytes, algae (Cyanophyta, Chlorophyta, Euglenophyta, Bacillariophyta, Xanthophyta), arthropods, oligochaetes, fishes, protozoan, rotifer, gastropods, detritus and the unidentified animal tissue. The results showed that the crab is omnivorous, with tendency for carnivory, feeding on members of benthic communities. The food items' percent frequency of occurrence was 87.3% (macrophytes), 82%(algae, including 47 genera), 48.2%(arthropods), 28.2%(oligochaetes), 28.7%(fish), 0.3%(protozoans), 0.6%(rotifers), 0.3%(gastropods) and 88.7%(detritus). Unidentified animal tissues were often observed, with an occurrence frequency of 46.1%. In total, the frequency of occurrence of plants (macrophytes+algae) is 87.7% and animals 89.8%. However, 5.8% of the gastric mills was filled only by animals, 5.3% by macrophytes and 0.3% by algae. T-test showed there was no significant difference in food habits between sexes (p>0.05). Correlation analysis indicated that there was no ontogenetic diet shift in the 2-year-old crab. The ratio of cell number of macrophytes to algae was about 118—219:1. Finally, some difficult questions in studying food habits of the crab were discussed.