On temperate and tropical shores around the world, a guild of sacoglossan opisthobranchs associates with filamentous green algae. This guild of four genera in three different families (Hermaeidae: Aplysiopsis; Limapontiidae: Ercolania, Limapontia; Plakobranchidae: Elysia) has been well studied in many geographical regions. Despite detailed information for Hong Kong, there is comparatively little information for Japanese shores. From 2000 to 2010, this guild of sacoglossans was investigated in two regions of Japan. On the Pacific shores of Honshu, Japan, at least ten species occupy this guild including Aplysiopsis minor, A. toyamana, A. nigra, A. orientalis, Ercolania boodleae, E. subviridis, Elysia hamatanii and E. abei. Two described species (Elysia nigrocapitata and E. hirasei) were not recorded but also occupy this temperate guild in Japan. On the west-coast shores of Okinawa (East China Sea), at least three species occur in this guild: Ercolania subviridis and two undescribed or unrecognized species of Ercolania. We quantified the size–frequency and abundance of specimens, characterized their spawn masses, and conducted feeding preference experiments. Many of the sacoglossans were seasonally frequent to abundant, their feeding activities were concentrated on uniseriate filaments of green algae (Order Cladophorales: Family Cladophoraceae and/or Order Siphonocladales: Family Boodleaceae), and their larvae were typically planktotrophic; yet, many of these species are regional endemics. Our results complement the wealth of ecological information available for other, more depauperate geographical regions.