Helisoma duryi is a planorbid snail that has been proposed as biological control agent against intermediate hosts of schistosomiasis. When the population density of this species is increased from 200 to 25 ml per snail in containers of constant volume (i.e. 200 ml) growth decreases. Whereas the single snails (i.e. 200 ml/ snail) laid no egg masses, egg-laying in the other groups decreased with increasing density. When four snails were kept in various volumes of water giving the same population densities as above both growth and egg-laying decreased with increasing density.Cross-breeding experiments between an albino and a pigmented strain of H. duryi showed that the albino character is inherited as a recessive autosomal character, cross-fertilization appears to be the only mode of reproduction in H. duryi, stored foreign sperm remains viable for up to five months and stored sperm can survive when snails are aestivating or starving.The implications of the present findings for the use of H. duryi in biological control are discussed.
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