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The numbers of alien species in freshwater systems and their detrimental impacts on the stability of ecosystems and global species diversity are increasing. The introduction of alien species leads to changes in species composition inter alia. Functional traits of the biota thereby also change, which in turn will likely alter ecosystem processes, e.g. by modifying the availability, capture, and use of nutrients or by affecting the feeding relationships (trophic structure) within a community. To predict and assess such impacts, a thorough knowledge of the autecology and life cycle of the alien species is required. The mysid Limnomysis benedeni, one of the most important ponto-caspian invaders, was found in Lake Constance (southern Germany) in 2006. Since 1958, 14 alien species of the makrozoobenthos have invaded the littoral of Lake Constance. About 1/3 of the species belong to the Peracarida (including Amphipoda, Isopoda and Mysida), 1/3 to the Mollusca (Gastropoda and Bivalvia) and the last 1/3 to other groups of the makrozoobenthos. The main part of the alien species originated in the ponto-caspian region and it is remarkable that formerly, their first records were mainly made in the Upper Lake Constance, but since 10 years has shifted to bay of Bregenz.The aim of the study was to gain knowledge on the possible impacts of L. benedeni on the littoral of Lake Constance. L. benedeni was not a well examined species, so basic studies on the autecology were necessary first. Studies on the life-cycle strategies over an entire seasonal cycle, addressing factors (predation, temperature) which we expected to be most important triggers of the observed changes, indicated that the size class distribution and the reproductive pattern of L. benedeni changes seasonally. During winter (November to March), the mysid invested energy in growth and delayed reproduction until April, when the population was dominated by adults. In summer (June to September), the adults reproduced at a smaller body size and the population was disproportionately dominated by juveniles. In a mesocosm experiment that excluded fish predators, the mysids followed the same seasonal patterns of growth and energy investment as in the field population, but the size class distribution differed. Even in summer, the population in the mesocosm was dominated by adults. Stomach analyses of fish showed that L. benedeni is preyed upon by juvenile Perca fluviatilis in summer, which fed size selectively on larger mysids. Because of the smaller size at maturity, we state that the adults in summer really showed a physiological adaption on the season, perhaps evolved to avoid predation or as a reaction on metabolic losses at higher temperatures, but the shifted length distribution was probably caused by the high fish predation.L. benedeni shows a distinct dependency of the development time of the brood on water temperature. In laboratory experiments under controlled conditions, we determined embryonic development times and the probability of survival of the females and juveniles at water temperatures ranging from 4° to 25° C. At 6.5° and 25° C, the probability of survival of both the females and the larvae was lower than at 10°, 15°, or 20° C. With the determined equation for the development time as a function of water temperature, it was now possible to estimate population rates, e.g. birth and mortality rates. Direct field measurements of the main population characteristics (abundance, biomass, clutch size, sex ratio) were combined with general ecological equations developed for species with other ecological behaviours. L. benedeni reproduces continuously instead of producing cohorts, with high, fluctuating abundances. We estimated instantaneous mortality rates of L. benedeni in the littoral by calculating instantaneous birth population growth rates. The formulae used served well for the estimations. During the reproduction period in summer, the instantaneous mortality rate was high, but the instantaneous birth rate was high enough to balance the population growth rate. The instantaneous mortality rates reinforced the known life-cycle patterns of L. benedeni and confirmed the assumption that the life-cycle shift in summer is an adaption to high predation on the population by fish.The feeding mode and food preference of adult L. benedeni collected from the field and from laboratory feeding experiments were examined by analysing their stomach contents. In a second setup with two sets of laboratory growth experiments, we determined the growth of juveniles by feeding newly hatched juveniles with different natural food sources at different water temperatures. The stomach analyses showed clearly that L. benedeni fed non-selective on organic particles smaller than ~ 200 µm both by filtering or grazing. They fed well on biofilm in the field as well as in laboratory growth experiments and we suppose a high disturbance potential of the mysid on the biofilm community. As a reason of their feeding mode, we conclude that L. benedeni has no predatory impact on zooplankton in the field and probably affects nutrient cycles in the littoral. L. benedeni seams to be a good food source for perch, but will not cause a massive decline of the fish community as it is describes from other mysids. However, the effect on the perch growing, if there is any, will be small in both ways, negative or positive. Perhaps the perch will profit from the new food source, but it could be that the effect will be negated because L. benedeni only replaces other prey species.With this study, we contribute a huge step in understanding freshwater mysid ecology, life-cycle physiology and population dynamics. In sum, L. benedeni has the potential to play an important role in the trophic relations in the littoral, via “bottom-up” mechanisms as well as via “top-down” control, and will persist an important invasive species in Europe. |
