Species assignation amongst morphologically cryptic larval Digenea isolated from New Zealand topshells (Gastropoda: Trochidae).

In New Zealand, a single morphotype, comprising three genetically distinct opecoelid species, infects four sympatric species of trochid snails. Two species (a and b) are specific to Diloma subrostrata while the third (c) is more general, capable of infecting three species, most commonly D. aethiops but never D. subrostrata. We sampled three D. subrostrata and D. aethiops populations, in which infection levels ranged from 0 to 29.2%, and attempted species assignation based on host information, restriction fragment length polymorphism (RFLP) analysis and morphology. Host information allowed reliable separation of species a and b from species c. Restriction mapping of ribosomal DNA internal transcriber spacer 2 (ITS2) demonstrated that the restriction enzyme SfuI only digested ITS2 from species b and c. Thus, restriction digests + host species information allowed reliable species assignation. Morphological measurements were taken for both sporocysts and cercariae dissected from 83 infected snails. Substantial overlap existed between measurements for the three species, and discriminant analysis showed that parasites could not be unequivocally assigned to a species--error rates ranged from 9 to 58%--despite statistically significant differences among several means. Amongst this group of digeneans, host information + RFLP provide a rapid, unambiguous method of species assignation that host information + morphological measurements cannot.