Hidden neozoans in macrozoobenthos

Klotz, Ramona Ursula (2022): Hidden neozoans in macrozoobenthos: the polyp stage of the freshwater jellyfish Craspedacusta sowerbii. Dissertation, LMU München: Fakultät für Biologie

Abstract

Craspedacusta sowerbii is a freshwater jellyfish species that has been invading freshwater almost all over the world. In marine environments, which are usually associated with jellyfish, increasing jellyfish observations are related to eutrophication, temperature increases and habitat degradation. Reports of jellyfish observations in freshwater have also been increasing over recent decades. The question arises as to whether population dynamics of freshwater jellyfish are affected by similar factors than observed in marine systems. Difficulties in studying freshwater jellyfish are related to the fact that most scientific interest is focused on the easily visible medusa stage. However, in the complex life cycle of C. sowerbii medusae play only a minor role. The inconspicuous polyp stage is much more important, as it is present throughout the year and is almost exclusively responsible for reproduction. The polyp stage is therefore crucial for a successful invasion of new habitats, and for current and future distribution patterns of this species. In my thesis I investigated the following aspects: (1.) Distribution patterns of medusae and polyps of C. sowerbii, (2.) Factors affecting the growth of polyps of C. sowerbii (chemical environment, such as nitrate and biocides) (3.) Technical aspects of polyp handling during monitoring and experi-mental analyses. With a “Citizen Science” project and a literature research, recent distribution patterns of C. sowerbii medusae in Germany were revealed and evaluated. Analyses of the distribution patterns show that rivers probably act as important pathways for distribution. To determine how well the observation of easily detectable medusae reflect the “real” distribution of the species, including the polyp stage, the distributions of both life stages were analysed in lakes in Upper Bavaria. The analysis revealed that the polyp stage is approximately twice as abundant as the medusa stage; many more lakes than previously thought are therefore inhabited by this species. Additional comparison of lakes inhabited by only the polyp stage with lakes inhabited by both polyps and medusae show a clear difference in mean alti-tude—lakes inhabited by polyps and medusae are located at considerably lower altitudes. This could reflect a predicted influence of temperature on the development of medusae. The distribution of polyps of C. sowerbii is affected by environmental parameters. Recently, the im-portance of parameters related to the chemical constitution of freshwater environments has gained increased attention. Nitrate is seen as an abundant and common threat to freshwater organisms; la-boratory experiments on the effect of nitrate on the growth of the polyp population of C. sowerbii were therefore performed. The results of these experiments show that nitrate decreases the growth of polyp populations. This decrease was observable under both acute and chronic exposure to nitrate. A comparison with field data supports the potential importance of nitrate as shown in the laboratory. In two rivers in Upper Bavaria with differing nitrate concentrations, polyp distributions were quantified. In the river with higher nitrate concentrations the number of polyps was much lower than in the river with lower nitrate concentrations. Additionally, experiments with certain agricultural pesticides known to enter freshwater systems by run-off were performed. Results show that the polyp stage of C. sowerbii is sensitive to at least some pesticides, also potentially affecting species distribution. As mentioned above, the polyp stage of C. sowerbii is studied less frequently than its medusa stage. Detection in the field of this highly inconspicuous stage is extremely difficult, and needs some experi-ence. Experiments on staining the polyp stage show that neutral red is able to permeate and stain the species, showing a clear visible red colour in the polyp stage. This staining does not seriously harm the polyps and no higher mortality of polyps with staining was observed. After collecting polyps in the field, it is possible to successfully maintain and grow them in the laboratory and use individuals for further experiments. The necessary medium changes during cultivation have no influence on the reproduction of polyps, allowing experiments that need controlled environmental parameters that can only be achieved by regular growth medium changes. During such experiments polyps have to be transferred between culture and experimental environments and injuries of the fragile polyps can occur. However, my results show that almost 100% of the polyps used in experiments regenerated within 24 to 96 hours, even after heavy injuries. This demonstrates the high regeneration capabilities many cnidarian species are known for. My results provide important knowledge for planning and conducting further experi-ments investigating the ecology of the polyp stage of C. sowerbii. In summary, my results help to understand recent and future distribution patterns of this highly mobile, invasive species. It became clear that the inconspicuous and under-investigated polyp stage of the jelly-fish is key to understanding the dynamics of C. sowerbii. My findings show that C. sowerbii is much more broadly distributed than originally thought from medusa observations. Additionally, temperature plays an important role for medusae production, hence increased abundances, and increasing food web effects, of C. sowerbii medusa with ongoing Global Change can be predicted.

• Dokumententyp: Dissertationen (Dissertation, LMU München)
• Themengebiete: 500 Naturwissenschaften und Mathematik ; 500 Naturwissenschaften und Mathematik > 570 Biowissenschaften, Biologie
• Fakultäten: Fakultät für Biologie
• Sprache der Hochschulschrift: Englisch
• Datum der mündlichen Prüfung: 18. Juli 2022
• 1. Bericht­erstatter:in: Stibor, Herwig