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The marine basidiomycete Nia vibrissa has been regarded as a species complex, possibly including several species, because morphological variations in fruitbody, spore, and spore appendage have been observed in materials from worldwide collections. Using more than 50 monosporic isolates of N. vibrissa–like fungi mainly obtained from Japanese beach coasts, we investigated their molecular phylogeny, morphological characteristics, mating compatibility, nuclear behavior during spore formation, and life cycles. Molecular phylogenetic analyses separated the examined strains into seven clades. Each clade of fungi exhibited distinctive characteristics in fruitbodies and spores produced by culturing monokaryotic strains and mated dikaryotic strains; these characteristics included the color of fruitbodies, apical structure of peridial hair hyphae, spore shape, and apical structure of spore appendages. Mating tests of monokaryotic strains demonstrated mating compatibility between strains within a clade and incompatibility among clades. Therefore, each clade of fungi was phylogenetically, morphologically, and biologically recognized as a different Nia species. Observation of the type specimen of N. vibrissa revealed a tiny T-shaped apical structure of spore appendages—not mentioned in the original description—that is unique to the species. This finding, together with the original description, suggests that our studied strains include N. aff. vibrissa, whose morphology is mostly identical to N. vibrissa sensu stricto, and three new species. Thus, we describe three new Nia species and propose emendation of the descriptions of the genus Nia. Culture-based studies have demonstrated that Nia species have both sexual and asexual morphs that produce morphologically similar fruitbodies (basidiomata and conidiomata) and spores (basidiospores and conidia). Because it has both morphs forming appendaged waterborne basidiospores and conidia, Nia must be the most well-adapted marine basidiomycete, ensuring the continuation of new generations by two morphs, while distributing in and inhabiting numerous marine environments. [ABSTRACT FROM AUTHOR]