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Effect of the plant Azolla pinnata on survival, growth rate, fecundity and hatchability of egg-masses of Biomphalaria alexandrina snails.


Laboratory data indicated that Azolla pinnata reduced the growth rate of Biomphalaria alexandrina, as expressed by the increase in shell diameter, following direct (snails free in aquaria containing plants) or indirect (snails in mesh cages in aquaria containing plants) exposure. In both cases, plant density was inversely related to snail growth: the higher the plant density, the lower the growth rate and vice versa. The exposure of B. alexandrina eggs to A. pinnata reduced the growth rate of the hatchlings. The results showed that direct and/or indirect exposure to an abnormally high density of plants (50 000 plants/litre) resulted in the complete kill of B. alexandrina snails after 2 weeks of continuous exposure. Snails exposed directly to A. pinnata at between 25 000 and 50 000 plants/litre failed to lay eggs. Snails exposed to 10 000 plants/litre laid few eggs and had a low reproductive rate (total egg output/snail, 57.9) compared with unexposed snails (total egg output/snail, 110.6). The same trends were was recorded with hatchability of snail eggs.