Biological surveys reveal unexpectedly high faunal diversity at Nankai Trough methane seeps

Abstract Cold seeps are chemosynthesis‐based ecosystems powered by microbial primary production that support diverse and specialized faunal assemblages in the deep sea. Despite Nankai Trough in Japan being a geologically active margin hosting numerous seeps, much of the faunal diversity remains undocumented. Here, we present results from the first coordinated biological survey across five methane seep sites in the Nankai Trough with different water depths: Daini Tenryu Knoll (600 m), Ryuyo Canyon (1000 m), Oomine Ridge (2000 m), Yukie Ridge (2500 m), and the Off Cape Muroto 4600 m Site. The seep sites were imaged in situ using a combination of high‐definition video cameras and still cameras, and biological samples were taken using suction and scoop sampling in addition to grab samples collected directly using the submersible's manipulator. Our results, together with existing records, together reveal a total of 80 seep‐associated macro‐ and megafaunal animal species in these five Nankai Trough seeps (33 molluscs, 23 annelids, 11 arthropods, five nemerteans, four echinoderms, three cnidarians, and a bryozoan), over a fivefold increase compared to the previous number (14). The species diversity at each site also increased from 1–6 to 15–30. These findings include numerous range extensions, new records at seeps, and previously undocumented associations. Our results shed light on the exceptional and underappreciated diversity of seep fauna along the Nankai Trough. In light of growing interest in methane hydrate exploitation in this region, our findings provide essential biodiversity baselines and highlight the need for site‐specific conservation measures to protect endemic, depth‐segregated communities from anthropogenic disturbance.