Widely different sex ratios in certain New Zealand populations of the hydrobiid gastropod Potamopyrgus antipodarum (Gray) were generally found to vary little though in one stream a high-male/low-male boundary moved steadily upstream over 20 years. Laboratory breeding and chromosome studies indicated that New Zealand high-male populations are diploid with 2n = 34 and reproduce sexually, while New Zealand low-male populations and all those from Europe and Australia have 2n = 52 or 46 and reproduction is largely parthenogenetic.
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C Wallace (1992) studied this question.