[Comparative morphofunctional analysis of the gastropod-trematode interactions]

A morphofunctional aspect of pathogenesis in various mollusc--trematode combinations is very different. To evaluate the level of antagonism between the host and parasite, it is reasonable to use a summarized parameter of general physiological state of infected individuals. The somatic growth of the host could be used for that purpose. Analysis of original and reference data has shown, that the gastropod growth response to the trematode infection depends on the longevity of molluscs. The growth acceleration (gigantism) in infected individuals is common for snails with the intermediate longevity (3-4 yr), such as Hydrobia, Onobia and Bithynia (Prosobranchia: Rissoacea). As an exception, the gigantism is observed in the short-living species, which do not change the growth rate when they are parasitized: Biomphalaria, Bulinus, Helisoma and Lymnaea (Pulmonata: Basomathophora). Earlier, it was assumed that the single trend manifested in the infected long-living species (6-27 yr), such as Littorina (Prosobranchia: Littorinacea) and Cerithidea (Prosobranchia: Cerithiacea), in the decrease of the growth rate. The only case of the growth acceleration within the long-living molluscs was detected in the Littorina littorea from the White Sea infected with low pathogenic partenits of Cryptocotyle lingua (Heterophyidae). Based on the groups of snails with different longevity and being phylogenetically distant from each other, we can presume that the growth response to the infection correlates with the morphological peculiarities of hosts. The morphofunctional distinction in the system formed by trematodes and molluscs of different taxa are expressed in the parthenit localization in the host body and in the degree of digestive gland and/or gonad destriction. From the viewpoint of general physiological status of the host organism, the results of disfunctions in these organs are not similar. The destruction of digestive system ("energy power station" of organisms) is more fatal for infected individuals than a parasitic castration. Therefore, the localization of parasites in the mollusc gonad can be recognized as a parasitic adaptation to decrease the probability of a premature host's death and respectively to prevent the elimination of parasites. Phylogenetically distant gastropods with different life cycles use a diverse reproduction strategy, which implies a different proportion of gonad and digestive gland volume. Thus, snail species from different taxa are not similar in relation to the life space, which could be "granted" to the parasite without any risk to reduce a viability of the host. The space lack forces the parthenits parasitizing in the molluscs with the longer life span to occupy the digestive gland tissues, that increases their pathogenicity and decreases the host's growth rate.