Not a fairy tale: archives tell a story about clammed up natives and crabby invaders

Natural history museum collections are valuable archives of the past. They store selective moments in form of individual organisms that are linked to a particular date and place. Repeated collection events over several years of identical species from the same places results in time series. These time series are invaluable because they store information about the changes in biodiversity and adaptations of species. By applying modern analyses techniques like ancient DNA (aDNA) methods and shot- gun sequencing, the genetic data of worldwide 1.5 billion historic collection entries can be made available and investigated today. Because of this research potential, natural history museum collections represent an essential basis for scientific questions, particularly in relation to long-term environmental changes caused by climate change and globalisation. In the course of this thesis, three studies have been conducted on the basis of museum collections in the background of climate change and globalisation. First, the studies aimed to investigate the extinction event of the native European flat oyster Ostrea edulis, as well as the impact of the neozoan common limpet slipper Crepidula fornicata on the oyster, since it was soon after its introduction accused to displace O. edulis. Secondly, the historical phylogeography of O. edulis was reconstructed to enlighten the processes of the extinction event further. Thirdly, the genetic population structure of long-established neozoa like the Chinese mitten crab Eriocheir sinensis was investigated in order to find potential adaptation or differentiation processes to its new environment. Both native and neozoan species were selected for their ecological and economic importance, which is why many museum collections hold time series of these species. The results of these studies showed that the extinction event of O. edulis in the North Sea can be successfully traced based on collection material. The shallow oyster beds on the coasts went extinct first, whereas the last living individual of the deeper oyster beds in the central North Sea was found in 1939. Moreover, the reconstruction of the historical distribution revealed that the population of C. fornicata increased only after the oyster beds went extinct. Thus, C. fornicata is despite its bad reputation not to blame for the extinction event of the European flat oyster in the North Sea. Additionally, by using aDNA methods on dry shells from museum collections, the historical phylogeography of O. edulis was successfully depicted in its native range for the first time. These modern methods unveiled the historical population structure of the European flat oyster across Europe in the late 19th century – including the now extinct populations of the Wadden Sea. The innovation of this historical study in comparison with present-day studies was the discovery of the autochthon- ous haplogroup in the Wadden Sea. This haplogroup has not yet been detected in current oyster beds in other locations, assuming this haplogroup could be extinct today. This offers a possible explanation why the oyster has not resettled the Wad- den Sea until now. Lastly, the public databases (e.g. GenBank, BOLD) were used for population genetic analyses on the Chinese mitten crab E. sinensis. This invasive crab has been introduced to Europe at the beginning of the 20th century and spread rapidly over countries ever since. The results revealed unique haplotypes occurring in Northern Germany that has not been found in the native range. These haplotypes suggest genetic differentiation and adaptation processes in response to the new environment during the past century. Finally, the natural history collections proved to be a valuable tool for different research questions. Whereas the impact of climate change could not be verified on the basis of these preliminary studies, the influence of globalisation is evident. Therefore the results of the historical studies will help to better understand the processes in the future.