A New Record of <em>Antithamnion hubbsii</em> (Ceramiales, Rhodophyta) from the Korean Coast: Invasive Species Interactions with Native and Non-Native Communities

Taxonomic clarity within the genus Antithamnion is important in the molecular phylogeny of red algae. Antithamnion hubbsii is reported for the first time from the Korean coast, previously misidentified as A. nipponicum due to morphological similarities. Specimens collected from Gangneung were analyzed using plastid-encoded rbcL and psaA genes, confirming their identity as A. hubbsii. Korean specimens exhibit indeterminate lateral axes, oppositely arranged pinnae and pinnules, and distinctive gland cells on the adaxial surface of pinnules. Molecular analyses revealed minimal genetic divergence between the two species (1-3 base pair differences in rbcL, none in psaA). While PTP analysis differentiated them as distinct species, ASAP and ABGD analyses grouped them as a single species. Despite these results, our phylogenetic analyses showed Korean A. hubbsii forming a distinct clade with strong bootstrap support. However, further research is needed to clarify the taxonomic boundaries between A. hubbsii and A. nipponicum. Our ecological observations revealed that this North American invasive species competes with both native red algal communities and established non-natives, apparently outcompeting A. nipponicum in shared habitats. This suggests potential shifts in community composition and ecological functions along the Korean coast as this invader spreads.