[Comparison of resistance to environmental factors of the molluscs Hydrobia ulvae infected with trematoda parthenitae and free from infection].

The influence of trematode infection, mainly with Microphallus claviformis, onto the resistance of mudsnails Hydrobia ulvae to fresh water, desiccation and extremely high temperature has been investigated. It was found out in all variants of experiments that the intensity of mortality in infected individuals is reliably higher than in individuals free of infection. It is suggested, that the negative influence of parasites on the resistance of hosts is related to the disturbance of molluscs' capability to isolate themselves from extremal condition by shutting up the shell with operculum. It is proved by the high rate of salt loss in the infected molluscs in a comparison to non-infected individuals. Our hypothesis based on results obtained and reference data suggests that the rate of trematode parthenites' influence onto the resistance of molluscs depends upon the character of interrelationships in hostparasite systems. Normally, the trematode species having the active cercaria stage in the life cycle show more negative impact onto the resistance of infected molluscs, than those species, larval stages of which develop to metacercaria inside the parthenites.