Erhaia (Gastropoda : Rissooidea): Phylogenetic relationships and the question of Paragonimus coevolution in Asia

The human lung fluke Paragonimus is transmitted by gastropod taxa of two su perfamilies: Ceritheoidea and Rissooidea. The question whether or mot Purag onimus shows the same specificity of host-parasite coevolved relationship a s the human blood fluke Schistosoma was inspired by the finding of two symp atric snail species as hosts for Paragonimus skrjabini in Fujian Province, China: Gammatricula and Erhaia. The former species can clearly be classifie d as Pomatiopsidae: Triculinae. The latter has previously been classified a s Pomatiopsidae: Pomatiopsinae. However, this classification based on anato mical characteristics is uncertain. In order to obtain a robust phylogeneti c hypothesis for Erhaia, we have studied three gene fragments from this tax on as well as from twelve related taxa. The data show that the species invo lved represent four families: Pomatiopsidae, Hydrobiidae, Cochliopidae ther e raised to family status), and Amnicolidae. Erhaia fits securely into the Amnicolidae. This indicates that P. skrjabini has not coevolved with snail lineages. However, P. skrjabini has so far only been reported from rissooid ean snails, whereas members of the Paragonimus westermani complex have only been found in ceritheoidean snails. The implication is that there is a hos t specificity on the superfamily level. However, Asian freshwater species o f the Ceritheoidea and Rissooidea usually are not sympatric and often prefe r different habitats. It is therefore possible that ecological niche partit ioning plays the primary role for Paragonimus evolution.