The Ponto-Caspian mysid Limnomysis benedeni was first recorded in Lake Constance in summer 2006, and a stable population developed at the site of discovery. Although this mysid is common in the Rhine and Danube rivers, little is known about its ecology and impact in systems of invasion. We investigated the autecology of L. benedeni in habitatchoice and food experiments. In the habitat-choice experiments, highly structured habitats, i.e., stones covered with zebra mussels (Dreissena polymorpha), macrophytes, and especially stonewort, were strongly preferred. In food experiments, L. benedeni fed mostly on food sources with a small particle size, e.g., biofilm on leaf litter, biodeposited material of zebra mussels, epilithon, and phytoplankton. We also compared the L. benedeni population data from Lake Constance with that from rivers. In Lake Constance, female L. benedeni were nearly 30% larger and carried more than three times more eggs in spring (9.470.6 mm and 28.475.7 eggs) than in summer (6.770.8 mm and 8.772.9 eggs). The mysids present in spring might be the generation that over-wintered; in summer, this generation was probably replaced by a new generation of smaller individuals. The large brood size and the detritivorous feeding strategy might allow L. benedeni to colonize Lake Constance rapidly.

The numbers of alien species in freshwater systems and their detrimental impacts on the stability of ecosystems and global species diversity are increasing. To predict and assess such impacts, a thorough knowledge of the autecology and life cycle of the alien species is required. Limnomysis benedeni is common and one of the most invasive mysids in Europe. Here we show a clear dependency of the development time of the brood of L. benedeni on water temperature. In laboratory experiments (one in spring 2008 and two in 2009, in spring and summer) 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. Since the development time is one of the key characteristics of the life cycle and is therefore necessary to calculate, for example, birth and mortality rates, we determined an equation for the development time as a function of the water temperature. This information will be useful to understand the distribution potential of this invasive species.

The mysid Limnomysis benedeni, one of the most important ponto-caspian invaders, was found in Lake Constance (southern Germany) in 2006. As part of larger studies to evaluate the effects of L. benedeni on the ecosystem, we studied its life-cycle strategies over an entire seasonal cycle in intervals of 3-5 weeks, addressing factors (predation, temperature) which we expected to be most important triggers of the observed changes. The size class distribution and the reproductive pattern indicated that the life cycle 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, which fed size selectively on larger mysids. In conclusion, our results suggest predation was the reason for the dominance of juveniles and the observed size class distribution in summer. In contrast, the smaller adults in summer were most likely a physiological adaptation, perhaps evolved to avoid predation or as a reaction on metabolic losses at higher temperatures.

No abstract

Among the mechanisms that allow competing species to coexist are resource partitioning and dietary segregation. The current study uses multiple stable isotopes, carbon (␦ 13 C), nitrogen (␦ 15 N) and sulphur (␦ 34 S), to test the hypothesis that dietary segregation in cohabiting invasive mysids (Limnomysis benedeni and Katamysis warpachowskyi) and gammarids (Dikerogammarus villosus and Gammarus roeselii) will be reflected by differences in isotope values. Furthermore, IsoError mixing models were used to estimate the relative contributions of periphyton and seston to the invaders' diets. Whole tissue ␦ 13 C, ␦ 15 N and ␦ 34 S analysis in L. benedeni and K. warpachowskyi imply that these sympatric, non-native mysids maintain differentiated feeding niches or resource partitioning by feeding on distinct components of the available food resources (predominantly seston by L. benedeni and periphyton by K. warpachowskyi). By contrast, the gammarids D. villosus ('killer shrimp') and G. roeselii exhibited no significant difference in ␦ 13 C and ␦ 15 N, indicating a considerable overlap between the dietary sources of these sympatric invaders. Feeding niche differentiation, irrespective of season or the nature of habitat invaded (lake or river), might facilitate the coexistence of invasive mysids in their 'new' environment by minimizing direct resource competition. The mutual interaction by the invasive gammarids, coupled with voracious behavior, could assist their success at co-invasion with serious implications for local biodiversity including the potential extinction of native species.