M olluscs (bivalves, gastropods, cephalopods) along Lake Timsah have received much scientific concern during past decades; however, opithobranchs (sea hares) did not receive any attention yet. This investigation represents the first study, so far dealing with the ecology of the sea hare Notarchus indicus along the lake. Sea hare usually moves in depressions or ripples marks (wave marks) of sandy bottom of Lake Timsah forming chains of 3 to 5 individuals. Their densities ranged from 15 to 20

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... viduals/ transect (ImxIOm). The size frequency distribution reveafs that its individuals grow up to 7.25cm length with a model size of 2.75 -3.50cm, representing 37% of its population. It usually feeds on organic matter and blue greens accumulated in depressions of sandy substrates and represent up to 5.2% of sediment dry weight. The water is typically saline (43%o), well aerated with normal alkaline pH. The sea hare usually secretes pink ink when attacked or disturbed. Investigation of the biological activity of the sea hare Notarchus indicus; and its ink showed that methanol extracts of large, medium and small organisms and the ink have a moderate bioactivity against three pathogenic bacterial strains, namely the Gram-positive bacterium Staphylococcus aureus NRRL B-767, and the Gram-negative bacteria Klebsiella pneumoniae NRRL B-14232 and Escherichia coli NRRL B-3704. Moreover, they have moderate bioactivity against Candida albicarts NRRL Y-12983. Thus, such ink shows a most prominent antimicrobial activity. Moreover, the aqueous extract of large organisms as well as the ink showed strong cytotoxic activity against the nauplii of the brine shrimp Artemia salina. Ecologically, sea hares usually use the ink and other bioactive substances to protect itself from predation.