Expanding Knowledge of Sea Pen (Octocorallia: Pennatuloidea) Diversity and Distribution Through Integrative Taxonomy: Insights From Hong Kong's Coastal Waters

Sea pens (Octocorallia: Scleralcyonacea: Pennatuloidea) are widespread but remain under-documented in the Northwest Pacific. Using a combined molecular (MutS-ND2-28S rRNA) and morphological approach, we analysed sea pen samples collected from Hong Kong's urban waters. MutS sequences at a 0.3% divergence threshold and ASAP automatic delimitation resolved nine species across three families and four genera for Hong Kong specimens, consistent with phylogenies inferred from concatenated datasets (MutS-ND2-28S rRNA), which recovered nine species in three well-supported clades within Pennatuloidea. Morphological examinations (colony form, zooid arrangement, axis shape, sclerite shape and size) corroborated species boundaries. We described three new species (Cavernularia solaris sp. nov., Lituaria triscleromorpha sp. nov., and Virgularia exilis sp. nov.) and updated the morphological descriptions of four previously known species. Additionally, we compared these species with sea pens reported from other regions, particularly Japan and Palau. By discovering new species and new distribution ranges, providing new DNA sequences, and clarifying phylogenetic placements for Hong Kong's sea pens, this work augments species inventories and contributes to ongoing revisions of Pennatuloidea systematics and biogeography in the Northwest Pacific.