Ecological effects of the mass occurrence of the Ponto-Caspian invader, Hemimysis anomala GO Sars, 1907 (Crustacea : Mysidacea), in a freshwater storage reservoir in the Netherlands, with notes on its autecology and new records

A new Ponto-Caspian invader, the mysid Hemimysis anomala G.O. Sars, 1907 (C rustacea: Mysidacea) was recorded for the first time in the Netherlands in 1997. In the summer of 1998 extremely high densities (> 6 ind. l(-)1) of th is neozoon were recorded in one of the Biesbosch reservoirs (Honderd en Der tig). This high abundance can not be explained by a recent invasion. Either H. anomala reached the Netherlands via the River Rhine, probably aided by shipping, or through transport with ballast water from the Baltic or Black Sea. The invasion had dramatic effects on the zooplankton composition and a bundance: from the end of August onwards hardly any Anomopoda, Ostracoda, R otifera and invertebrate predators (Leptodora kindti and Bythotrephes longi manus) were present. Copepod densities, however, were not influenced. Chlor ophyll-a concentrations were significantly lower compared to previous years , possibly the result of mysids feeding on the algae. Laboratory experi men ts revealed that H. anomala is a voracious predator and also an omnivorous feeder. Life history characteristics and behaviour of H. anomala fit into g eneral mysid ecology. The hidden life style of H. anomala during the day ma kes it difficult to assess its present geographical distribution. More atte ntion should therefore be given to appropriate sampling of this mysid. Beca use of its broad prey-size range H. anomala may pose a threat to existing l ake restoration programmes (biomanipulation) in the Netherlands. In additio n to the records of the Biesbosch reservoirs, H. anomala was found in the R iver Meuse and in reservoir Andijk, in the northern part of the Netherlands .