Confirming a new species of Amalthea D’Archino & W.A.Nelson (Halymeniales, Rhodophyta) from the mesophotic zone of Hawai‘i through taxogenomics

The order Halymeniales represents a significant number of species in the mesophotic red algal flora of Hawai‘i, with numerous undescribed taxa. The cultivation and restoration of species in the Halymeniales in Hawai‘i, particularly members of Grateloupia C.Agardh and Halymenia C.Agardh, have focused substantial attention on these taxa, highlighting uncertainty in the genetic diversity within this group. In this study, we adopt a taxogenomic approach integrating taxonomic, phylogenetic and phylogenomic analyses. We present the first report of the genus Amalthea D’Archino & W.A.Nelson in Hawai‘i, focusing on the species description of Amalthea mahilanii F.P.Cabrera & A.R.Sherwood, sp. nov. Our initial rbcL analysis suggested the presence of two lineages within Hawaiian Amalthea, but robust support from plastome-wide phylogenies contradicted that analysis, emphasizing the value of taxogenomics in clarifying cryptic diversity. Morphologically distinct from congeners Amalthea freemaniae D’Archino & W.A.Nelson and A. rubida H.W.Lee & M.S.Kim, A. mahilanii F.P. Cabrera & A.R.Sherwood, sp. nov. features rough, leathery blades with a unique cortical and medullary cell arrangement, including 9-10 layers of ovoid cortical cells and a dense arrangement of medullary filaments. Its blades maintain uniform thickness. The presence of larger ostiolate cystocarps (300-500 μm diameter) further distinguishes A. mahilanii F.P.Cabrera & A.R.Sherwood, sp. nov. from its Pacific conspecifics. Syntenic comparisons of published plastomes within the Halymeniales revealed remarkable similarities in gene content, with no detected rearrangements, indicating conserved plastidial evolution within the order. Overall, this study significantly advances our understanding of Halymeniales diversity in mesophotic ecosystems, shedding light on the remarkable biodiversity of macroalgae in Hawaiian mesophotic reefs. We emphasize the need for further study of Halymeniales and propose ongoing efforts in taxogenomics to enhance taxonomic resolution, particularly in mesophotic habitats.