Abstract
Hydromedusae are diverse zooplankton that sometimes have great impacts on aquatic ecosystems. We investigated the seasonal occurrences and assemblage structure of hydromedusae in the offshore area of Sagami Bay, central Japan. Throughout the study, 33 holoplanktonic and 9 meroplanktonic hydromedusae species were collected. Unlike the nearshore hydromedusan assemblage, low species richness of meroplanktonic hydromedusae was a characteristic feature offshore. Total abundances drastically changed seasonally, with a spring peak. Muggiaea atlantica , Obelia sp., Solmundella bitentaculata , Rathkea octopunctata , Sugiura chengshanense , Liriope tetraphylla , and Aglaura hemistoma were abundant, with these seven species together comprising 58.7%–97.9% of the hydromedusan assemblage. Seasonal occurrence patterns differed among species. For example, M. atlantica was the most abundant and occurred throughout the year, with a spring peak that was probably affected by water temperature; Obelia sp. showed multiple abundance peaks, which might be driven by advections. A redundancy analysis (RDA) showed that seasonal occurrence of hydromedusae was closely related to both physical and biological factors. Our findings indicate that succession of the offshore hydromedusan assemblage was seasonal, but it was also influenced by short‐term hydrographic processes such as the strong intrusion of oceanic water into the bay.