The Sphagnum cuspidatum complex: phylogeny, species delimitation, and morphology.

The article focuses on species delimitation within the Sphagnum cuspidatum complex, a group of aquatic peat mosses that dominate peatland hollows and exhibit high morphological variation. Using high-resolution population genomic data (RADseq) combined with detailed morphological analyses, the study supports recognizing Sphagnum cuspidatum, S. fitzgeraldii, S. mississippiense, and S. trinitense as genetically and morphologically distinct species, while finding that S. viride is not genetically distinct from S. cuspidatum despite morphological differences. Key morphological characters distinguishing these species include leaf shape, leaf serration, and tubularity, particularly in branch leaves, with some traits showing stability despite environmental variation. The findings highlight the importance of integrating genomic and morphological data for species delimitation in aquatic bryophytes and provide a revised taxonomic key for the complex.