The effect of Echinostoma friedi experimental infection on longevity, growth and fecundity of two susceptible first intermediate host snails, Radix peregra and Biomphalaria glabrata, was studied to contrast the level of compatibility. 120 R. peregra and 150 B. glabrata snails were used exposed to one, three or five miracidia and divided in three categories: INF (snails exposed and infected); ENI (exposed but not infected) and C (control or not miracidial-exposed snails). R. peregra INF snails' death process starts sooner, but in a prolonged extension, while B. glabrata INF snails have a much shorter life span. The infection and the miracidial exposure are able to reduce R. peregra normal development (stunting). B. glabrata INF snails' growth exceeds that of C snails (gigantism). E. friedi produces a total parasitic castration of R. peregra and B. glabrata INF snails. R. peregra would be considered as the required snail host, while B. glabrata only as an adequate snail host.

An experimental infection with echinostomatid miracidia in sympatric or 'local' vs. allopatric or 'away' snail combinations, as a model to examine parasite compatibility, was carried out. We employed Euparyphium albuferensis miracidia to infect Gyraulus chinensis snails, from three different natural parks: Albufera (Valencia, Spain); the Ebro Delta (Tarragona, Spain) and Coto de Doñana (Huelva, Spain). Insignificant differences between the three snail strains were noted for the infection rate and the rhythm of daily cercarial production. However, a significantly higher total cercarial production per snail, patent period and life span were observed in local snails. The different infection characteristics in the three G. chinensis strains considered reveal that E. albuferensis miracidia demonstrate local adaptation.

Abstract. Euparyphium albuferensis and Echinostoma friedi cercarial infectivity to four species of sympatric snails was examined under single-or multiple-choice laboratory conditions to show the level of parasite-snail host compatibility. Radix peregra, Lymnaea fuscus, Physella acuta and Gyraulus chinensis act as second intermediate hosts of both parasite species although different cercarial transmission success (CTS) was observed. In single-host experiments, R. peregra and P. acuta showed a high degree of compatibility with E. albuferensis, while only P. acuta in the case of E. friedi. In two-choice snail communities, a snail with high CTS increased the values of another with low compatibility, in both parasite species. In multiple-choice snail communities, high CTS of some hosts decreased, while low CTS of other hosts increased. The degree of parasite-host compatibility of each snail species could be determined by the presence of other snails in the community.