Second Discovery of an Epizoic Deep-sea Holothurian-Ascidian Association, Pseudostichopus mollis (Echinodermata: Holothuroidea) and Cnemidocarpa ochotense (Urochordata: Ascidiacea), on the Western Slope of the Chishima Trench, Northeast Japan

An association comprising a solitary ascidian attached to the dorsal surface of a holothurian was discovered in 2022 on the bathyal seafloor (ca. 2000 m depth) of the western slope of the Chishima (Kuril-Kamchatka) Trench, off the southeastern coast of Hokkaido, Japan, using a beam trawl during the KH-22-8 cruise of the R/V Hakuho Maru. This is the second taxonomically detailed description of a deep-sea epizoic holothurian-ascidian relationship. The host holothurian was identified as Pseudostichopus mollis Théel, 1886, and the associated ascidian as Cnemidocarpa ochotense Sanamyan, 1992. The association seemed stable due to the ascidian being firmly attached to the holothurian body surface by its broad tunic attachment surface and numerous elongated tunic extensions. It was difficult to determine the nature of the association, whether commensalism, mutualism, or parasitism, but it clearly seemed to be beneficial for the attached ascidian. Furthermore, the host holothurian showed no rejection of its symbiont.