A new freshwater monothalamid (Rhizaria, Foraminifera) from the Pyrenees branching within a marine clade

Monothalamous (single-chambered) foraminifera are widespread in marine benthic environments and are also a common part of freshwater and soil microbial communities. Based on molecular and morphological characteristics, seven non-marine families are currently recognized, branching either as sisters to marine clades or independently within the paraphyletic class Monothalamida. In this study, we describe a new monothalamous freshwater foraminifera sampled from a Pyrenean pond near the French town of Cauterets. We erect the novel genus Poseidonella, with its type species Poseidonella transaquatica sp. nov. The new species branches within the marine clade E, which includes the genera Psammophaga, Vellaria, Niveus, and Nellya. This represents the first evidence of a mixed clade comprising both marine and freshwater monothalamids, highlighting an ongoing transition from coastal marine environments to freshwater habitats.