Genetic population structure and environmental impact on Craspedacusta at the medusa and polyp stages

Wang, Yuanyuan (2022): Genetic population structure and environmental impact on Craspedacusta at the medusa and polyp stages. Dissertation, LMU München: Fakultät für Biologie

Abstract

Craspedacusta, commonly known as the freshwater jellyfish, has spread worldwide rapidly and has been invading Europe for more than a century. It is thought to originate from the Yangtze River area in China. It features a metagenetic lifestyle, including the free swimming, planula larvae and dioecious medusae stages, as well as the benthic stages, comprising sessile polyps, cylindrical frustules and spherical podocysts. Benthic stages are easily dispersed and are so-called “good hitchhikers”. Hence, Craspedacusta sp. shows important traits that are advantageous for effective dispersal and successful invasion into new habitats. Increasing anthropogenic activities profoundly accelerates the expansion. For example, intense international trade is a major source for intercontinental dispersal of non-native species. The presence of freshwater jellyfish can lead to a significant decrease and mortality of zooplankton, thus potentially influencing freshwater ecosystem dynamics. Therefore, there are growing concerns about the potential future impact of Craspedacusta on aquatic food webs. The debate about the taxonomic classification within Craspedacusta, has lasted for over a century since the first discovery of the medusae stage. Craspedacusta has been given numerous names over time, according to ambiguous morphologic traits which are unreliable and confusing. In the present study, I investigated the spatio-temporal genetic diversity and structure among Craspedacusta population samples from central Europe, including medusa and polyp stages. This study is a first insight into the genetic diversity and the differentiation of Craspedacusta populations on large scales that allows a broader and deeper integrative understanding of the dispersal and invasion of this species. My results support recent findings that at least two mitochondrial lineages (Type 1, Type 2) with four main haplotypes (Type 1.1, 1.2, 2.1, 2.2), invaded Europe. Except for one lake, all lakes in this study were inhabited by only a single haplotype and gender of medusae, even when comparing different years. However, in contrast to medusae, polyps with distinct haplotypes co-existed in the same lake, even on very small scales, such as on a small stone. The haplotype identity of medusae and polyps that were found in a lake was sometimes inconsistent, for example, only one medusa haplotype appeared, while more haplotypes co-existed among polyps. At the medusa stage, gender and mitochondrial haplotype corresponded in large samples from numerous populations analysed. A known environmental parameter that is thought to trigger life cycle transitions in Craspedacusta is temperature. Hence, in order to explore the temperature effect on the growth dynamics of regional Craspedacusta at the polyp stage, I studied their asexual reproduction and population growth pattern in controlled laboratory experiments. Craspedacusta polyp populations were showing distinct growth rates at 18°C and 26°C, and developed significantly faster at a higher temperature. The number of frustules released by a single polyp, and the increase in polyp population size, were temperature dependent. Colonies with two connected polyps were the most common type at both temperatures. I conclude that increasing temperatures will further promote the rapid expansion of polyp populations and may thereby induce cascading effects in freshwater ecosystems. Additionally, I also investigated the effects of freshwater acidification on Craspedacusta at the polyp stage. I cultured polyps of Craspedacuta sp. from different clonal assemblages and clones at different pH levels, manipulated by weak acidification. My results indicated that Craspedacusta sp. polyps were quite insensitive to weak acidification with slight pH variations, at both low and high water temperatures. My findings further support the opinion that the polyps of Craspedacusta have a very broad tolerance to a large variety of environmental parameters, making them successful invaders into diverse freshwater ecosystems.

• 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: 11. Juli 2022
• 1. Bericht­erstatter:in: Stibor, Herwig