Unveiling coralline diversity of mesophotic rhodoliths in subtropical Japan, including new species of Sporolithon , emended genera Orientalilithon and Rose

Abstract The mesophotic zone off Tanegashima Island (approximately 30–40 m deep) is a marine macroalgal hotspot in subtropical Japan; however, the taxonomic diversity of calcifying red algal community remains poorly understood. Our phylogenetic analyses based on psb A, rbc L, and COI‐5P gene markers disclosed the presence of at least 12 rhodolith‐forming non‐geniculate coralline algae from five genera across three orders. The confirmation of previously described Lithophyllum neo ‐ okamurae , L . okamurae (Corallinales), and Orientalilithon confluens (Hapalidiales) herein extended their vertical distributional range. The remaining nine species represent novelty, with four newly described and five unnamed taxa: Crustaphytum sp., Roseolithon aggregatum sp. nov., R. sparsituberculatum sp. nov., Orientalilithon compactum sp. nov., Orientalilithon sp. 2, Hapalidiales sp. (Hapalidiales), Sporolithon variotuberculatum sp. nov., Sporolithon sp. 1, and Sporolithon sp. 2 (Sporolithales). Roseolithon aggregatum barely differs from R. sparsituberculatum by external growth form and length of protuberances. These two mesophotic species can be separated from the shallow‐water R . littorale and R . sabulosum (1–6 m depth) in Japan based on depth‐stratified distribution. Orientalilithon compactum and O . confluens can, at present, be identified by DNA sequences and to some degree by spatial distribution. Sporolithon variotuberculatum is a genetically verified Japanese species of Sporolithon and appears to be related to S . molle from Egypt. They morpho‐anatomically differed by the number of cells in paraphyses, the diameter of sporangial pore openings, and the size of stalk cells subtending tetra‐/bisporangia. This study underscores the remarkable species diversity of Japanese rhodoliths in subtropical mesophotic ecosystems that deserve further investigation.