| Summary: | Shallow water coral reefs are the most diverse marine ecosystems, but there is an immense gap in knowledge when it comes to understanding the diversity of the vast majority of marine biota in these ecosystems. This is especially true when it comes to understudied small and cryptic coral reef taxa in understudied ecosystems, such as mesophotic coral reef ecosystems (MCEs). MCEs were reported in Japan almost fifty years ago, although only in recent years has there been an increase in research concerning the diversity of these reefs. In this study we describe the first stoloniferous octocoral from MCEs, <i>Hadaka nudidomus</i> gen. nov. et sp. nov., from Iriomote and Okinawa Islands in the southern Ryukyus Islands. The species is zooxanthellate; both specimens host <i>Cladocopium</i> LaJeunesse & H.J.Jeong, 2018 (formerly <i>Symbiodinium</i> ‘Clade C’) and were collected from depths of ~33 to 40 m. Additionally, <i>H. nudidomus</i> gen. nov. et sp. nov. is both sclerite-free and lacks free pinnules, and both of these characteristics are typically diagnostic for octocorals. The discovery and morphology of <i>H. nudidomus</i> gen. nov. et sp. nov. indicate that we still know very little about stoloniferous octocoral diversity in MCEs, their genetic relationships with shallower reef species, and octocoral–symbiont associations. Continued research on these subjects will improve our understanding of octocoral diversity in both shallow and deeper reefs.
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