Seasonal shifts in the life cycle of the ponto-caspian invader Limnomysis benedeni (Crustacea: Mysida): a physiological adaptation?

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.