Ecological Characteristics and Genetic Population Structure of the Invasive Freshwater Jellyfish Craspedacusta sowerbii Species Complex

The genus Craspedacusta, a freshwater jellyfish, has successfully spread worldwide in the last 140 years. Numerous global reports on its occurrence are in contrast to lacking knowledge about its ecological importance for biodiversity in invaded aquatic food webs. Recent molecular data indicate that invasive Craspedacusta include distinct molecularly defined lines with uncertain species status. Recently, it has even been suggested that Craspedacusta “sowerbii” should be considered a species complex. The fact that invaded lakes are almost all populated by unisexual medusae suggests that sexual reproduction may play a minor role and the fate of natural populations depends on the success of clonal lineages. The aim of this Special Issue is to present new insights into ecosystem functioning, aspects of evolutionary ecology and the genetic population structure of different life stages.

The plastic metagenetic life cycle of members of this genus, which includes asexually reproducing benthic polyps and bisexually reproducing pelagic medusae, allows species to benefit from benthic and pelagic food sources as convenient. Craspedacusta is also known for forming large jellyfish blooms. However, little is known about their ecological impact on native food webs at the medusa or polyp stage. As with many invasive species, such as herbivorous invasive plant species, or other invasive freshwater species (e.g., Daphnia), its invasion success benefits from the clonal reproduction of initial colonizers without a need to find a sexual partner to become locally established. The multiple invasion of different clonal strains would favor local genetic diversity among polyps, and the competitiveness of certain lineages with native species would determine their long-term establishment. An example for rapid invasion success by one single parthenogenetically reproducing clone has recently been reported for the freshwater marbled crayfish, which has now, 25 years after escaping its aquarium origin in Germany, been found in more than ten countries in Africa, Asia and Europe. Habitats in an early succession and/or disturbed habitats are particularly vulnerable to invasion, and high fertility, a generalist lifestyle or phenotypic plasticity have been shown to be beneficial for clonal invaders as well as the production of resting stages to overcome adverse environmental conditions. All of this taken together affects the differential invasion success of clonal variants. Humans are not only the main vectors for the global transfer of alien species, but their activities contribute to the disturbance of all kinds of habitats by promoting climate change and CO₂ rise. In the case of Craspedacusta, ecological impacts on aquatic food webs are expected to increase during climate change, as jellyfish bloom formation, density, and duration are all correlated with rising temperatures.

Dear Colleagues,

We are planning a Special Issue in Biology regarding the invasive freshwater jellyfish Craspedacusta sowerbii. The focus of the Special Issue will be on the ecological characteristics and population genetics of Craspedacusta sp., which are increasingly observed in freshwater systems globally. The Special Issue will bring together different research fields such as the experimental and observational limnology, population genetics and ecology of invasive species. This should help us to gain a more complete overview of the recent state of the art and integrative insights into the highly dynamic processes underlying the worldwide spread of this species. 

To date, publications about Craspedacusta have been rare and often published in very different and specialized journals. This Issue will supplement existing literature by providing an integrated picture of recent developments including modern experimental and molecular approaches.

Dr. Sabine Gießler
Prof. Dr. Herwig Stibor
Guest Editors

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• ecosystem functioning
• evolutionary ecology
• genetic population structure
• species complex
• clonality
• polyp
• medusa life stage