From host-parasite systems to parasitic systems: Interactions of littoral mollusks of the genus Littorina with their trematode parasites.

The idea of parasitic systems, formulated by V.N. Beklemishev 70 years ago, represents a conceptual tool for the analysis of populational and biocoenotic roles of parasites. The questions concerning longterm stable persistence of host-parasite systems in communities can be discussed meaningfully only within this concept. Importantly, the set of terms elaborated within the parasitic system concept is applicable not only to parasitology, but also contributes to the general knowledge of life cycles of organisms and differences in the environment. This concept provides an opportunity for comprehensive analysis of systems, based on any type of stable biocoenotic interactions in the community (predation, commensalism, competition, etc.). Trematode-based parasitic systems, involving populations of intertidal mollusks of the genus Littorina, allow demonstrating how the strong 'negative' effect of parasites on hosts at the individual level (complete parasitic castration) can be compensated at the population level. Such compensation functions as a prerequisite for maintaining long-term stable interactions between populations of parasites and their hosts within parasitic system (the ecosystem, biocoenotic level). [ABSTRACT FROM AUTHOR]