Protecting Complex Biodiversity Systems: Ciliophora Endosymbionts of the Threatened Freshwater Mussel Species Unio crassus

Abstract

Infections with symbiotic single-celled organisms classified as Ciliophora, are commonly reported in various species of invertebrates. Freshwater mussels also play the role of a host for them, being responsible for their biodiversity. Knowledge about these relationships is insufficient. Freshwater mussels are among the most endangered groups of animals. Ciliate endosymbionts in mussels may be responsible for the diseases and increasingly frequent cases of mass mussel deaths and are particularly dangerous for commercially farmed mussels and for restocking of mussels as part of active conservation measures. On the other hand the Ciliophora parasites and commensals living in freshwater mussels is undescribed. Many of these species are likely to become extinct before they are known to science. Recently, two genera of Ciliophora, Conchophthirus sp. and Trichodina sp., have been described in the mantle cavity of U. crassus - an endangered and protected species in the EU. The basis of extensive research on the impact of endosymbiotic ciliates on U. crassus populations is the knowledge of their species composition, proportion of infected individuals, level of infection intensity and distribution in different types of rivers. Such studies were carried out in the mantle cavity of U. crassus from three rivers in three seasons. Cloning, sequencing and functional analysis of the genetic material of Ciliophora was carried out using NGS (Next-Generation Sequencing) analysis based on the hypervariable V4 and V9 regions of the 18SrRNA gene, which enables the identification of taxonomic groups, including genera and species. The most numerous OTUs are common and cosmopolitan species. But some commensals and potentially parasites were found too. Ciliophora associated with mantle cavity form an interesting, hierarchical biocomplex but their interactions with mussels need further studies.