Protists are probably the most species-rich eukaryotes, yet their systematics are inaccurate, leading to an underestimation of their actual diversity. Arcellinida (= lobose testate amoebae) are amoebozoans that build a test (a hard shell) whose shape and composition are taxonomically informative. One of the most successful groups is Arcellidae, a family found worldwide in many freshwater and terrestrial environments where they are indicators of environmental quality. However, the systematics of the family is based on works published nearly a century ago. We re-evaluated the systematics based on single-cell barcoding, morphological and ecological data. Overall, test shape appears to be more related to environmental characteristics than to the species’ phylogenetic position. We show several convergences in organisms with similar ecology, some traditionally described species being paraphyletic. Based on conservative traits, we review the synapomorphies of the infraorder Sphaerothecina, compile a list of synonyms and describe a new genus Galeripora, with five new combinations. Seven new species: Arcella guadarramensis sp. nov., Galeripora balari sp. nov., Galeripora bufonipellita sp. nov., Galeripora galeriformis sp. nov., Galeripora naiadis sp. nov., Galeripora sitiens sp. nov. andGaleripora succelli sp. nov. are also described here.

The salinity barrier that separates marine and freshwater biomes is probably the most important division in biodiversity on Earth. Those organisms that successfully performed this transition had access to new ecosystems while undergoing changes in selective pressure, which often led to major shifts in diversification rates. While these transitions have been extensively investigated in animals, the tempo, mode, and outcome of crossing the salinity barrier have been scarcely studied in other eukaryotes. Here, we reconstructed the evolutionary history of the species complex Cyphoderia ampulla (Euglyphida: Cercozoa: Rhizaria) based on DNA sequences from the nuclear SSU rRNA gene and the mitochondrial cytochrome oxidase subunit I gene, obtained from publicly available environmental DNA data (GeneBank, EukBank) and isolated organisms. A tree calibrated with euglyphid fossils showed that four independent transitions towards freshwater systems occurred from the mid‐Miocene onwards, coincident with important fluctuations in sea level. Ancestral trait reconstructions indicated that the whole family Cyphoderiidae had a marine origin and suggest that ancestors of the freshwater forms were euryhaline and lived in environments with fluctuating salinity. Diversification rates did not show any obvious increase concomitant with ecological transitions, but morphometric analyses indicated that species increased in size and homogenized their morphology after colonizing the new environments. This suggests adaptation to changes in selective pressure exerted by life in freshwater sediments.

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Accurate species delimitation based on an integrative approach is indispensable for biodiversity evaluation. Accordingly, species that were formerly only described morphologically often need to be split into several evolutionary independent units. However, taxonomic actions often lag behind, even when the required data are already available. As a result, invalid species names are carried over the years, with negative implications on ecology, biogeography, and conservation; we designate these entities as ‘shadow species’. This is particularly common in protists, due to the lack of specialized taxonomists and the difficulties of working with microscopic organisms. Here, we resolve the case of the testate amoeba shadow species Cyphoderia ampulla (Rhizaria: Cercozoa: Euglyphida: Cyphoderiidae), a known polyphyletic taxon. Purposely, we integrated the current ecological, genetic, and morphological data on the family Cyphoderiidae with its described evolutionary history. Subsequently, we took the required taxonomic actions to resolve the taxonomy of the family, erecting four new genera (Psammoderia gen nov., Knarr gen. nov., Ichthyosquama gen. nov., and Oleiformis gen. nov.), emending Cyphoderia and describing four new species (Ichthyosquama sanabriensis sp. nov., Ichthyosquama catoirensis sp. nov., Ichthyosquama loricaria sp. nov., and Oleiformis carmelae sp. nov.). Finally, we discuss the extension and relevance of the ‘shadow species’ issue in eukaryotic taxa, and differentiate it from species complex concept.