On the distribution of ‘gonionemus vertens’ a. Agassiz (Hydrozoa,Limnomedusae), a new species in the eelgrass beds of Lake Grevelingen (S.W. Netherlands)

Summary Data about morphology, life history and habits ofGonionemus vertens A. Agassiz (Hydrozoa, Limnomedusae) are given. The distribution of the species is treated in more detail, especially its mechanism in relation to large-scale (global) and small-scale (local) range extensions.The species is endemic in the coastal North Pacific Ocean, possessing a strictly littoral medusa stage (bound to eelgrass beds and algal belts) and a very small sessile solitary polyp phase (attached to stones and shells).The hypotheses trying to explain the spread to Western Europe and the Atlantic coast of North America strongly suggest human influences,i.e. distribution of the polyp via oyster transports (EDWARDS, 1976) and transport of the polyp as a member of the fouling community on ships hulls (TAMBS-LYCHE, 1964). Arguments pro and contra these hypotheses are evaluated. The occurrence ofGonionemus polyps in offshore waters, as mentioned by EDWARDS (1976) but not further discussed by this author, deserves more attention.It is the assumed occurrence of the polyps in shallow as well as in deeper waters with strong tidal movements that explains satisfactorily the previous and recent findings of the medusa in the S.W.-Netherlands. The medusa has been found in theZostera marina beds of the saline Lake Grevelingen. From 1976 onwards the abundance of the medusa increased synchronously with the extension of the eelgrass beds. Medusae were collected during the summer of 1980, fed with isopod crustaceans or mussel meat and kept alive for several months in seawater aquaria in the lab. Polyps have not yet been observed.Communication no. 211 of the Delta Institute for Hydrobiological Research.