| Summary: | Gelatinous zooplankton, such as ctenophores, have attracted attention during the last decade,mainly as a result of the enigma around their potentially increased abundances around the world. Despite the increased attention, they remain either understudied or disregarded in most food web investigations and monitoring programs, and are defined as one of the most difficult groups of pelagic animals to study. Consequently, their diversity and ecological role are often grossly oversimplified and misunderstood, leading to biased views of ecosystem functioning. In addition, ctenophores share traits such as voracious predation behavior, the ability to starve and shrink during periods of low food availability and to tolerate increased temperatures, as well as high reproductive capacity, making them likely to take advantage of changing environmental conditions. In the Arctic, earlier ctenophore data consist of sparse abundance estimates and dietary studies lacking a systematic or integrative approach. In the Baltic Sea, despite the wellestablished routine plankton monitoring program conducted by the surrounding nations, the distribution and the role of the Arctic ctenophore Mertensia ovum in this ecosystem has been unknown since its first reported appearance in 2007. In this thesis, the biodiversity of cydippid ctenophores and their role in Svalbard waters and in the Baltic Sea were studied. Extensive in situ sampling, laboratory experiments, morphological and molecular identification analysis, traditional and molecular gut content analysis, as well as several direct measures of the ctenophores and the pelagic communities they inhabit were combined to address system-specific questions and to better understand how important a role the ctenophores might have in marine ecosystems. A combination of morphological species identification and molecular methods revealed Euplokamis sp. and an unidentified mertensiid species to co-occur with the dominating Mertensia ovum in Arctic waters. Similarly, the first recording of the ...
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