Unveiling coralline diversity of mesophotic rhodoliths in subtropical Japan, including new species of <i>Sporolithon</i> , emended genera <i>Orientalilithon</i> and <i>Roseolithon</i> (Corallinophycidae, Rhodophyta)

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 psbA, rbcL, 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.