The invertebrate fauna associated with floating Sargassum horneri (Turner) C. Agardh, 1820 in the East China Sea was investigated in terms of ecology and biogeography. Floating algal rafts consisting of only S. horneri were collected at 16 stations in the East China Sea during a cruise of research vessel (R/V) Tansei-Maru in February, 2011. A total of 53 rafts were obtained for faunal investigations at 14 of the 16 stations. In addition to fish eggs, 10 invertebrate taxa were found on the

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... ng algae. Of the collected phytal animals, harpacticoid copepods were most abundant in terms of number (80%), followed by cirripedes (15%), amphipods (4%), and others (1%). The faunal diversity on the algae was correlated to the algal weight. The faunal diversity differed significantly between stations and was highest in the northernmost part of the East China Sea. Density and abundance of animals were highest in areas close to the Kuroshio Current. Considering the direction of flow of the Kuroshio Current and the density of cirripedes immigrating from the surrounding water onto the floating algae, most of the floating S. horneri possibly originated in the southern part of the East China Sea. On the other hand, the Sargassum rafts collected in the northernmost part of the East China Sea could have originated from the coast of eastern China. Most of the animals found on the floating Sargassum were pelagic taxa that complete their life cycles on the rafts in situ. Typical coastal animals, including gammaridean and caprellidean amphipods, tanaidaceans, nematodes, gastropods, polychaetes, halacarid mites, bryozoans, and hydrozoans, were also obtained from the floating Sargassum, but only occasionally and their densities were quite low. These facts suggest that dispersal events among benthic habitats via Sargassum rafts are relatively rare.