In South America, the knowledge of trematode diversity parasitizing freshwater fishes is still scarce, as less than 5% of the freshwater fish fauna has been examined for parasites. A similar situation applies to studies on digenean life cycles, which have become increasingly rare. Among the digenean families parasitizing freshwater fishes in the region, Haploporidae is considered the richest in species diversity. However, information about the developmental stages of haploporid life cycles remains fragmentary. Particularly, in Argentina, nine cercariae attributed to the family Haploporidae have been described using morphological analysis, and only two life cycles of this family have been completely elucidated. In this study a new type of cercaria, morphologically assigned to the family Haploporidae and collected from the snail Heleobia parchappii (Cochliopidae) in Los Padres shallow lake, Buenos Aires province, was identified using morphological and molecular techniques. The molecular analysis, based on 28S and ITS2 sequences, revealed that the cercariae were 100% identical to adult specimens of Saccocoelioides nanii (Haploporidae) parasitizing the fish Prochilodus lineatus (Prochilodontidae) from Los Talas, Buenos Aires province. Our results not only provide information about the life cycle of S. nanii but also show that a molecular and morphological approach can be extremely useful in identifying the developmental stages of digeneans and elucidating their life cycles.

This paper presents a new haploporid digenean that expands the number of species of Saccoccoelioides to 27. The new species, Saccocoelioides kirchneri n. sp. was collected from the intestine of Cnesterodon decemmaculatus (Poeciliidae: Cyprinodontiformes) from Lago del Bosque, La Plata, Argentina. The new species possesses the diagnostic features for Saccocoelioides: a sac like ceca; the vitellarium confined in two irregular groups of follicles distributed between the ventral sucker and the anterior margin of the testis; and a uterus confined largely in the hind-body, but encroaching into the range of the ventral sucker. The new species is differentiated from the 26 congeners by the body size, pharynx size, ventral sucker size, posterior extent of ceca, posterior extent of uterus and egg size. S. kirchneri n. sp. also is supported by the molecular analysis.