An association between the caprellid amphipod Caprella subtilis Mayer, 1903 and the elpidiid holothurian Ellipinion kumai (Mitsukuri, 1912) was confirmed through in situ submersible capture at 309 m depth in Sagami Bay, central Japan, and through shipboard observations. Information on this association, including behavioural and morphological data on both species, is presented. Information on the taxonomic standing of C. subtilis and related species is also introduced. Furthermore, we record swimming behaviour in the holothurian genus Ellipinion for the first time. We also introduce other biotic associations revealed during studies using submersible vehicles operated by the Japan Agency for Marine-Earth Science and Technology (JAMSTEC), including associations between benthopelagic holothurians and mysids, as well as hydrozoan polyps.