Mass occurrences of scyphozoan jellyfish, such as Aurelia coerulea, have been reported in various waters; however, little is known about the hydrozoans and ctenophores in Tokyo Bay. In this study, the abundance of these jellyfishes was monitored for four years(April 2015–November 2016, April 2018–July 2019)in Tokyo Bay. Thirteen hydrozoan species and three ctenophores were observed. The dominant hydrozoan species identified were Cytaeis uchidae, Podocoryne minima, Rathkea octopunctata, Diphyes chamissonis, and Liriope tetraphylla, while Bolinopsis mikado was the most dominant among the ctenophores. Cytaeis uchidae, P. minima, and R. octopunctata appeared for a short period with extremely high abundance. Cytaeis uchidae occurred when the salinity was low, but P. minima occurred under high water temperature and high salinity conditions during summer. The abundance of R. octopunctata increased with decreasing water temperature during winter. On the other hand, D. chamissonis and L. tetraphylla presented prolonged abundance during most of the studied period. Diphyes chamissonis occurred in summer when the water temperature was high, and L. tetraphylla was observed at low temperatures and high salinities in autumn. It was suggested that these two species, without polyp stages, had entered from the adjacent waters in Sagami Bay. Diversity for the species composition of hydrozoans and ctenophores is very low, with some species dominant in Tokyo Bay; the occurrence of these dominant species is greatly affected by water temperature and salinity in the ambient seawater. These species play an important role in the gelatinous plankton ecosystem in Tokyo Bay.

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Phyllosoma, a zoeal larva with extremely flat body and long appendages of spiny and slipper lobsters, is morphologically different from other decapod zoeas. In natural environments, phyllosomas often associate with planktonic gelatinous organisms such as jellyfish although its ecological implication is still unclear. Here we review the interaction between phyllosomas and gelatinous organisms. Our recent examinations showed that phyllosomas feeding exclusively on jellyfish successfully develop into juvenile, and that phyllosomas could associate with and prey on various species of venomous jellyfish. These results suggest that phyllosomas utilize gelatinous organisms as food sources as well as floating vehicles.

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Six specimens of Thamneus rostratus Bovallius, 1887 (Amphipoda: Hyperiidea) including a shining with iridescent glitter mature female attaching under surface of the umbrella of a warm water medusa, Pelagia noctiluca (Forsskål, 1775) (Cnidaria: Scyphozoa) were found from a shore coast in Jogashima, southern end of the Miura peninsula, Pacific coast of central Japan. The morphological features of the present specimens were in close agreement with the descriptions of Thamneus rostratus (or as Euthamneus rostratus) in previous literatures. This hyperiid have widely distributed, known from records in tropical and temperate regions of the world’s oceans, but it is relatively uncommon. This finding represents the first record of T. rostratus from Japanese water.

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The occurrences of cnidarian medusae and ctenophores have been monitored monthly for 15 years (Jan. 1981-Jun. 1996) in Tokyo Bay and are compared with previous records. Four species of cnidarians, Aurelia aurita (Linnaeus), Chrysaora melanaster Brandt, Pelagia noctiluca (Forskal) and Spirocodon saltator (Tilesius), and three of ctenophores, Beroe cucumis Fabricius, Bolinopsis mikado (Moser) and Pleurobrachia pileus (O. F. Muller), were identified. Among them, only A. aurita seems to reproduce in the inner bay and often forms dense aggregations. Bolinopsis mikado appeared frequently except at the innermost part of the bay. The other species occurred rarely before 1988 but the occurrence increased after 1988. In the early reports in the 1910s, when eutrophic water occupied only in the innermost part of the bay, A. aurita occurred sporadically at much lower concentrations comparing to our observations. Bolinopsis mikado and Beroe cucumis often occurred from autumn to winter and C. melanaster in early summer. Due to the acceleration of eutrophication in the 1960s, the zooplankton community composition, including medusae and ctenophores, changed drastically. Large swarms of A. aurita began to appear contemporary wity the extreme increase of a small copepod, Oithona davisae. The hypereutrophication condition of Tokyo Bay has not been ameliorated so far, the occurrences of outer-bay species, however, tend to increase recently. The fresh water discharge into Tokyo Bay has increased, and that suppose to strengthen the density current. Another possibility of increasing water exchange is the long-term hydrographic variation caused by the change of current path of the Kuroshio in relation with the large meander. The increase of non-resident medusae after 1988 may be caused by the enhanced water exchange between the adjoining sea and the bay.

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