Marine and freshwater ecosystems are fundamentally different regarding many biotic and abiotic factors. The physiological adaptations required for an organism to pass the salinity barrier are considerable. Many eukaryotic lineages are restricted to either freshwater or marine environments. Molecular phylogenetic analyses generally demonstrate that freshwater species and marine species segregate into different sub-clades, indicating that transitions between these two environments occur only rarely in the course of evolution. It is, however, unclear if the transitions between freshwater and environments characterized by highly variable salinities, such as the marine supralittoral zone, are also infrequent. Here, we use testate amoebae within the Euglyphida to assess the phylogenetic interrelationships between marine supralittoral and freshwater taxa. Euglyphid testate amoebae are mainly present in freshwater habitats but also occur in marine supralittoral environments. Accordingly, we generated and analyzed partial SSU rRNA gene sequences from 49 new marine/supralittoral and freshwater Cyphoderiidae sequences, 20 sequences of the Paulinellidae, Trinematidae, Assulinidae, and Euglyphidae families as well as 21 GenBank sequences of unidentified taxa derived from environmental PCR surveys. Both the molecular and morphological data suggest that the diversity of Cyphoderiidae is strongly underestimated. The results of our phylogenetic analyses demonstrated that marine supralittoral and freshwater euglyphid testate amoeba species are segregated into distinct sub-clades, suggesting that transitions between these two habitats occurred only infrequently.
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