ИСТИНА

Bivalves of the subfamily Pliocardiinae (Vesicomyidae) are highly specialized chemosymbiotic molluscs containing sulfide-oxidising bacteria in their gills. All known pliocardiins are mainly infaunal animals. In spite that the pliocardiins are not depended on food from the water as majority of other infaunal bivalves, the siphons lay an important role in functioning of these molluscs being the key structures in the water exchange between mantle cavity and environment. Characters of siphons an be responsible for narrow ecological niches suggested for pliocardiin species. Altogether data on siphon structure were obtained for representatives of presumably 7 genera. Additionally to the study or preserved material by means of light and scanning electronic microscopy, siphons were also observed at images of living specimens of 10 species. All investigated pliocardiins have following siphonal features in common: completely united inhalant and exhalant siphons; the presence of an inner siphonal sleeve in the exhalant siphon and the presence of inner valve in the proximal aperture of the inhalant siphon. In other respects siphonal morphology varies greatly among genera. There are differences in the arrangement of siphonal retractor muscles and development of musculature of siphonal tubes, relative length of siphons, structure of the inner valve of inhalant siphon, number and distribution of tentacles, and also in distribution of ciliated sensory organs on the inner surface of the inhalant siphon. Functions of different siphonal structures are discussed. For the pliocardiins occurring habitats with intense suspended sediment it is important to prevent unwanted particles from entering the mantle cavity. Various approaches to resolve this problem in different genera are described. Utilities of siphonal characters for systematics at different taxonomic levels are considered.