First records of <i>Boreolithothamnion astragaloi</i> and <i>Boreolithothamnion soriferum</i> ( <scp>Hapalidiales, Rhodophyta</scp> ) from <scp>Japan</scp> with the characterization of <i>Boreolithothamnion japonicum</i> from <scp>Hokkaido</scp> base

SUMMARY Boreolithothamnion is a non‐geniculate coralline red algal genus distributed in cold to warm temperate regions of the Northern Hemisphere. Since the establishment of the genus in 2023, following the redefinition of the genus Lithothamnion , its species diversity and distribution of Boreolithothamnion in Japan needed to be reassessed. Our molecular phylogenetic analyses using psb A, rbc L, and SSU rDNA gene sequences revealed the presence of at least four Boreolithothamnion species in Japan, viz. Boreolithothamnion astragaloi, Boreolithothamnion japonicum, and Boreolithothamnion soriferum , as well as an unknown Boreolithothamnion species. Based on our detailed morpho‐anatomical characterization, the Japanese B . astragaloi and B . soriferum exhibit reduced thallus growth forms (encrusting to warty) compared to their conspecific taxa (encrusting, warty, lumpy to fruticose) in the northeastern Pacific and Arctic‐North Atlantic Oceans, respectively. The Japanese B. japonicum shares gross morphology with the Korean specimens and is currently limited to the northwestern Pacific region. Species from Japan can be separated from one another based on morpho‐anatomical characters and distributional status. Boreolithothamnion astragaloi differs from B . japonicum and B . soriferum by its relatively smoother thallus surface. Boreolithothamnion japonicum differs from B . astragaloi and B . soriferum in having multiple growth forms (encrusting, warty, lumpy to fruticose) and somewhat differs from the former two species by the number of epithallial cell layers (1–2 vs. 1). At the local scale, B . astragaloi and B . soriferum are restricted to the eastern Pacific coast of Hokkaido, whereas B . japonicum showed wider distribution, spanning from the eastern and southern Pacific coast of Hokkaido through the Japan Sea coast of central Honshu. This study documents the first records of B . astragaloi and B . soriferum in Japan and extends the southward distribution of these two taxa in the Pacific Ocean.