Life history characteristics and distribution of Bythotrephes longimanus Leydig (Crustacea,Onychopoda) in the Biesbosch reservoirs

The diel and horizontal distributions of Bythotrephes longimanus Leydig, Leptodora kindti (Focke) and Anomopoda (cladocerans), as well as several life history parameters of each developmental stage of B. longimanus were studied in the non-stratified Biesbosch reservoirs. Anomopoda avoided the surface layer during the day only in the pelagic zone. In the surface layer near the shore, however, they were very abundant. Because densities in the pelagic zone during the day were very low, and high during the night, the population undoubtedly moved from the littoral zone or from the sediments to the pelagic. Accumulation of zooplankton at the littoral site is either the result of random dispersal or wind-induced movements. Bythotrephes' diel distribution pattern is possibly related to that of their prey, the anomopods. Leptodora mostly exhibited a normal diel vertical migration with a small amplitude. Wind most likely influenced the horizontal distribution of the zooplankton. Bythotrephes body length increased with higher temperatures, whereas spine length was constant throughout the year. Bythotrephes can already be fertile in the first developmental stage, indicating that a rapid adaptation to a favourable environment is possible. The number of parthenogenetically produced eggs per ovigerous female was higher at the start of the growing season and constant throughout the rest of the year. Relatively few resting eggs and males were found in autumn.