A new mesophotic species Platoma piiahiluum sp. nov. (Schizymeniaceae) from the Papahānaumokuākea Marine National Monument, Hawai'i, USA Phycologia

Mesophotic coral ecosystems of the Papahānaumokuākea Marine National Monument have been recognized as a major source of endemic Hawaiian assemblages and general taxonomic diversity. Specimens of the nemastomatalean genus Platoma were found to form a new, distinct clade in phylogenetic analyses of both plastid (rbcL) and mitochondrial (COI) markers. Additionally, the mitochondrial and chloroplast genomes of the new Platoma taxon were sequenced and used in phylogenomic reconstructions with related taxa. Structural investigation of the genomes showed the presence of an unusual feature – a stem-loop and inversion within the mitogenome. Morphological analyses showcasing their transparent non-branching thalli with small cortical cells and thinner medullary filaments compared to other species further confirmed these specimens represent a newly discovered and presumed endemic species from the Hawaiian mesophotic zone, which is proposed as Platoma piiahiluum sp. nov. At this point, two species of Platoma are known from Hawaiʻi: Platoma piiahiluum sp. nov. known only from mesophotic habitats in the Northwestern Hawaiian Islands, and P. ardreanum, known from shallow waters throughout the Archipelago. This study provides further evidence of the high proportion of endemic species in Hawaiian mesophotic coral ecosystems, and highlights the novel diversity of the uninhabited islands and atolls of the Papahānaumokuākea Marine National Monument.