Sagami Bay is among the most surveyed areas in Japan and the northwestern Pacific for marine biodiversity, partly owing to collecting efforts by the Showa Emperor, Hirohito. It was also here that the first chemosynthesis-based community was found in Japan, in the Off Hatsushima hydrocarbon seep site dominated by large vesicomyid clams. Nevertheless, new discoveries continue to be made in the deep. Here, we report observations of rare bathyal gastropods from unusual habitats. From the Off Hatsushima seep, the calliostomatid snail Otukaia kiheiziebisu was found living on tubeworm, the second record of the family from seeps. The cocculinid limpet Cocculina tenuitesta, previously only known from Suruga Bay, was found on a piece of branch-like plant debris inside the actively seeping clam field. This supports the step-wise evolutionary scenario from organic falls to hot vents recently suggested for this family. The predatory naticid snail Cryptonatica affinis represents the second record for the family at seeps, and we also show the first evidence for naticids feeding on chemosymbiotic clams—a phenomenon formerly only known from fossil seeps. We also found live individuals of Provanna stephanos, hitherto known from just two empty shells. Outside the seep area, the lepetid true limpet Sagamilepeta sagamiensis was found living on plastic debris, highlighting the function of plastic in the deep sea as settlement substrates among the muddy expanse. Our observations here have ecological and evolutionary implications, underscoring how much remains to be learned from the deep sea, even in well-studied areas like Sagami Bay.

相模湾是日本和西北太平洋海洋生物多样性调查最多的地区之一，部分原因是昭和天皇裕仁的收集工作。也是在这里，在日本发现了第一个基于化学合成的群落，位于以大型蛤蜊为主的初岛附近的碳氢化合物渗漏点。尽管如此，新的发现仍在继续进行。在这里，我们报告了对来自不寻常栖息地的稀有深海腹足动物的观察。从初岛附近的渗流中，发现了蛔虫Otukaia kiheiziebisu以管虫为食，这是该家族的第二个记录。Cocculinid 帽贝Cocculina tenuitesta以前仅在骏河湾已知，是在活跃渗出的蛤蜊田内的一块树枝状植物碎片上发现的。这支持了最近为这个家庭建议的从有机瀑布到热喷口的逐步进化方案。掠食性 naticid 蜗牛Cryptonatica affinis代表了该家族在渗漏处的第二项记录，我们还展示了 naticids 以化学共生蛤为食的第一个证据——这种现象以前只能从化石渗漏中得知。我们还发现了Provanna stephanos 的活个体，迄今为止仅从两个空壳中得知。在渗漏区外，lepetid true limpet Sagamilepeta sagamiensis被发现生活在塑料碎片上，突出了塑料在深海中作为泥泞中的沉降基质的功能。我们在这里的观察具有生态和进化意义，强调了还有多少需要从深海中学习，即使在像相模湾这样的研究充分的地区也是如此。