Summary 1. The Ponto‐Caspian invader, Hemimysis anomala, was recently found in large numbers in a gravel‐pit lake connected to the Lower Rhine. Mysids were sampled with Perspex traps between September 2002 and April 2003 to study the population dynamics, vertical migration and feeding. 2. The abundance (as catch per unit effort, CPUE) of H. anomala declined from 270 individuals (ind.) trap−1 (4 h)−1 in December to below 4 ind. trap−1 (4 h)−1 in April. Average lengths ranged from 4.9 mm in autumn to 9.9 mm in March. The length–weight relationship of virgin females changed throughout the sampling period, from a size‐corrected wet weight of 5.1 mg in September to 16.6 mg in April for a female of 7.8 mm. 3. Successive monthly samples taken over 24 h revealed that H. anomala preferred the surface at twilight and night. During dawn the mysids migrated to the middle and bottom layers and were hardly found during day. Diel vertical migration clearly depends on the proximate factor light. 4. Stomach analyses revealed that larger H. anomala preferred zooplankton, whereas small individuals fed more on phytoplankton. The seasonal comparison showed an increasing percentage of zooplankton with increasing length. The proportion of zooplankton in the stomachs of large H. anomala individuals was highest during night and lowest during day. 5. The results suggest that H. anomala may become an important link between primary/secondary production and higher trophic levels in the food web of its new environments at the Lower Rhine.

a b s t r a c tDrift is described as a dispersal mechanism inherent in many freshwater fish species. The main aim of this study was a comprehensive description of drift patterns of three invasive gobies living in sympatry, and to compare these results with similar sampling efforts before the occurrence of invasive gobies at the River Rhine. More than 26,500 larvae and juvenile fishes were caught with drift nets in 2000 and 2012-2014. Though some species were missing in 2012-2014 (e.g. bullhead Cottus gobio and gudgeon Gobio gobio), only low differences were found in the drift of autochthones before and after the goby invasion with respect to abundances or sizes. Roach (Rutilus rutilus) and bighead goby (Ponticola kessleri) increased in size over the season, indicating a somewhat different drift strategy than for barbel (Barbus barbus), monkey goby (Neogobius fluviatilis) and round goby (N. melanostomus) that drifted over several months, but remained at a similar size. The drift data give a first indication that the potential impact of gobies in the River Rhine should act on other stages than on the very first larvae within the life-cycle of important members of the local fish communities. The data also clearly reveal that bighead goby is actually at the bust phase within typical boom-and-bust cycles of invasive species, while round and monkey goby have not yet left the boom phase so far at the Lower Rhine.

1. The Ponto-Caspian polychaete Hypania invalida (Archiv fü r Naturgeschichte, 1860, 26, 109) is undergoing rapid range expansion in the Rhine and other central European waterways. We examined its reproductive traits in an attempt to account for its remarkable invasive success. 2. For the first time in this species, we found males, dioecy (gonochorism) and an exclusively sexual mode of reproduction; no indication for hermaphroditism or (unisexual) parthenogenesis, that could explain the rapid range expansion of H. invalida, was found.3. Our experimental evidence shows that H. invalida reproduces by males discharging their sperm into the water column while eggs are retained and fertilised in the female dwelling tubes. This mode of 'spermcast' reproduction is common in sessile benthic animals on wave-washed shores. 4. Fecundity of H. invalida ranged between 60 and 260 eggs per clutch and is increased by iteroparity (i.e. multiple cycles of oogenesis and egg-laying per specimen). Offspring are brooded for about 2 weeks before they leave the maternal dwelling tubes and disperse in the water column. 5. After connection to a supply of Rhine water, sediments in flumes were rapidly colonised up to a density of 12 000 ind. m )2 via the settlement of small larvae (<300 lm) from the water column; these reached sexual maturity approximately 12 weeks after settlement. 6. The reproductive traits of H. invalida give the species a high rate of dispersal. The capacity of its larvae to survive passage through pumps before settlement enables them to travel in the ballast water of ships. This might explain the upstream expansion of H. invalida in rivers. We conclude that further range expansion, and especially intercontinental transport of this polychaete, can only be prevented by improved ballast water treatment.