An extensive field survey of 12 duck species yielded two morphologically distinct populations of Typhlocoelum (Digenea: Cyclocoelidae). Studies on the variability of these two populations in conjunction with experimental cross infections provided conclusive evidence that they must be considered as subspecies of Typhlocoelum cucumerinum. A laboratory comparison of their survival strategies revealed that, in the definitive host, Typhlocoelum cucumerinum cucumerinum develops slowly, survives for an extended time and has a high fecundity, whereas Typhlocoelum cucumerinum sisowi develops more quickly, has a shorter longevity and a lower fecundity. In the intermediate host, T. c. cucumerinum develops more slowly than T. c. sisowi. The almost complete segregation of these two subspecies into dabbling and diving ducks in the field may be maintained by an ecological separation of the ducks as well as intrinsic differences between the two host-parasite associations.

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