Mysticladia gen. nov. (Cladophorales, Ulvophyceae): A new genus encompassing two long‐recognized Cladophora species from the South Atlantic Ocean

The genus Cladophora (Cladophorales, Ulvophyceae) has a long history of taxonomic challenges due to morphological plasticity and incongruence between traditional classifications and molecular phylogenies. Here, we propose Mysticladia gen. nov. to accommodate species previously assigned to Cladophora but phylogenetically placed within the Siphonocladus clade. Analyses of concatenated SSU + LSU rDNA gene sequences from specimens collected along the Brazilian coast revealed that populations morphologically identified as C. prolifera and C. aokii form a distinct and phylogenetically distant clade from Cladophora sensu stricto. Genetic divergence and morphological comparisons supported the recognition of two new cryptic species: Mysticladia nigra sp. nov. and M. edisonii sp. nov. Both seemingly restricted to the southwestern Atlantic. These species are morphologically indistinguishable from their closest relatives but distinguishable by DNA sequences. Additionally, we transfer C. prolifera, C. aokii, and C. coelothrix to Mysticladia. Our findings broaden the widespread cryptic diversity within the Cladophorales and emphasize the critical need for molecular analyses across populations in this group, including those already and correctly identified based on morphology alone. Such efforts are essential for elucidating cryptic lineages and species boundaries in simple filamentous green algae, where phenotypic plasticity often disguises true evolutionary relationships.