Anthopleura nigrescens (Verrill 1928) is a common sea anemone typically encountered in the rocky crevices of shallow-water intertidal zones in the West Pacific. Despite its ubiquity, little is known about the biology of this species, with research on them hindered due to difficulties in distinguishing Indo-Pacific Anthopleura species. The original description of A. nigrescens was sparse, and while there have been two focused redescriptions of this species, data presented in those accounts were conflicting (e.g., acrorhagi appearance and cnidom), thus the species cannot be identified accurately. In this study, we report the unusual occurrence of A. nigrescens beside hydrothermal vents in Taiwan and conducted transcriptomic analyses to better understand the adaptive biology of this species. We also performed a contemporary redescription of A. nigrescens, integrating morphological and molecular evidence from both museum specimens and fresh material collected from Taiwan and Singapore, while also incorporating data from earlier descriptions of the species. Transcriptomic analyses revealed a unique A. nigrescens adaptive strategy, characterized by the enrichment of metal ion binding genes and the activation of thermal resistance pathways, enabling adaptation to the hydrothermal vent environment compared to individuals inhabiting intertidal shores. Transcriptomic data also provided additional insights to the systematics of Anthopleura Duchassaing & Michelotti, 1860, concurring to previous observations that biogeographic patterns are a good predictor of its phylogeny than the current state of the genus' taxonomy. Overall, while we found that taxonomic characters of material we had examined to agreed, we also found intra-specific morphological deviations particularly concerning occurrence of cnidae documented for the species. Collectively, our findings refined our understanding of the species' taxonomy and support its placement within a biogeographically coherent clade, contributing to a broader understanding of its evolutionary and ecological context.
Actinioidea, Anthozoa, Indo-Pacific, Hydrothermal vent, Shallow intertidal, Transcriptome
