Description: The Asian shore crab Hemigrapsus sanguineus and its congener, the Asian brush-clawed crab H. takanoi, were first found in Europe in 1999 and 1994, respectively. Hemigrapsus sanguineus was initially detected in Le Havre (France) and in the Oosterschelde (The Netherlands). Now it occurs from northern France to the German Wadden Sea. Hemigrapsus takanoi was originally found in La Rochelle (France) and is now established from the Bay of Biscay to the German Wadden Sea. In 2014, a Baltic Sea population was detected in Kiel Fjord (Germany), most likely introduced via the Kiel Canal. Here we summarize the most recent findings on the northern expansion of H. sanguineus and H. takanoi and their competition with the native counterpart, the green shore crab Carcinus maenas. Two population studies in the intertidal areas around the rocky island of Helgoland and the Wadden Sea island of Sylt (Germany) revealed that the three species favor different levels of wave exposure. Our studies presented the highest densities of Hemigrapsus spp. in Europe so far with 144 H. sanguineus m-2 on Helgoland, and more than 200 ind. m-2 of either species on Sylt. The Baltic population of H. takanoi is increasing in Kiel Fjord and adjacent bays. Individuals of H. takanoi were recently found in Wismar Bay, further east along the Baltic coast. In the framework of a citizen-science project, specimen of both species were found at the Swedish west coast, representing the northernmost records in Europe so far. The success of established populations and the continuous extension of the distribution areas suggest that the presence and impact of both Hemigrapsus species on European coasts will significantly increase in the future.

Description: The western Pacific crabs Hemigrapsus takanoi and H. sanguineus were first introduced to Europe in the 1990s and have afterwards spread along the coast of the European Wadden Sea (south-eastern North Sea) between 2005 and 2010. The crab fauna in the intertidal zone of the Wadden Sea was formerly dominated by the European Shore Crab Carcinus maenas, itself being a successful invader in many coastal regions around the world and well known as a strong competitor. Therefore, the rapid establishment of Hemigrapsus spp. was very surprising. We hypothesized that timing of crab recruitment and subsequent growth of the cohorts play an important role to explain the success of both Hemigrapsus species. Since December 2014, a bi-weekly survey on the recruitment of all three crab species is conducted in the northern Wadden Sea. To our knowledge, this is the first temporally highly resolved, long-running data series on the recruitment of Hemigrapsus spp. for Europe. First results of the survey provide evidence that recruitment phases of C. maenas and Hemigrapsus spp. are temporally shifted against each other, leading to consequences on interactions between the species. We will present the survey data, along with experimental results on competition and predation among juvenile crabs.

Description: Life-history traits and interactions with native species play an important role for the successful establishment of non-native species in new habitats. We investigated the recent successful invasion of the Pacific crabs Hemigrapsus takanoi and H. sanguineus to the southeastern North Sea coast with respect to their recruitment patterns, as well as interactions of juvenile with sub-adult individuals among the Pacific crabs and with native shore crabs Carcinus maenas. A field survey of juvenile native and introduced crab abundances (carapace width 1.4–10 mm) was conducted in the northern Wadden Sea, spanning 24 months from 2014 to 2016. The survey revealed different seasonal recruitment patterns of native C. maenas and both introduced Hemigrapsus species. Native shore crabs showed a single recruitment peak from June to July, while Hemigrapsus spp. mainly recruited from August to early September, but recruits occurred in low densities throughout the winter until the end of the following spring season. Field experiments on the effects of larger crabs on the recruitment intensity showed that recruitment of H. takanoi was enhanced by the presence of larger congeners, but remained unaffected by larger C. maenas. Recruitment of juvenile C. maenas, by contrast, was reduced by the presence of larger Hemigrapsus spp. Additional laboratory experiments revealed high rates of cannibalism on newly recruited C. maenas by subadult conspecifics as well as strong predation by larger Hemigrapsus spp. In contrast, newly recruited Hemigrapsus spp. had a much lower risk of being preyed on by subadult conspecifics and native shore crabs. Our results suggest that the timing of recruitment in combination with low intraspecific competition and reduced predation pressure by native shore crabs are crucial for the rapid and ongoing establishment of Hemigrapsus spp. in the Wadden Sea.

Description: Drei Jahre nach Beendigung der Sandentnahme ist das Entnahmegebiet auf dem Rungholtsand wieder nahezu vollständig verfüllt. Die Sedimentzusammensetzung aus homogenem Feinsand entspricht weitgehend dem Zustand vor der Entnahme. Die Benthos-Lebensgemeinschaft im ehemaligen Entnahmegebiet ist arten- und individuenarm, Besiedlungsdichte und Biomasse unterscheiden sich jedoch nicht mehr signifikant von den angrenzenden Wattflächen. Noch verbleibende Unterschiede im Artenspektrum und der Größenverteilung der Muscheln dürften sich binnen Jahresfrist ausgeglichen haben. Der rasche Renaturierungsverlauf – 3 Jahre bis zur Verfüllung des Entnahmekraters und dem Angleich des Artenspektrums und etwa 4-5 Jahren bis zum Angleich des Größenspektrums der langlebigen Arten – ist eine Besonderheit der Lage des Entnahmegebietes am Rand eines Tiefs mit hoher Sedimentdynamik. In weniger strömungsexponierten Lagen ist mit viel längeren Renaturierungsdauern zu rechnen. In dieser Hinsicht sind im Falle erneuter Sandentnahmen aus dem Wattenmeer die hochdynamischen Randgebiete der tieferen Gezeitenrinnen als Entnahmegebiete zu bevorzugen.

Description: From the depths of the oceans to the shallow estuaries and wetlands of our coasts, organisms of the marine environment are teeming with unique adaptations to cope with a multitude of varying environmental conditions. With millions of years and a vast volume of water to call their home, they have become quite adept at developing specialized and unique techniques for survival and – given increasing human mediated transport – biological invasions. A growing world human population and a global economy drives the transportation of goods across the oceans and with them invasive species via ballast water and attached to ship hulls. In any given 24-hour period, there are about 10,000 species being transported across different biogeographic regions. If any of them manage to take hold and establish a range in an exotic habitat, the implications for local ecosystems can be costly. This review on marine invasions highlights trends among successful non-indigenous species (NIS), from vectors of transport to ecological and physiological plasticity. Apart from summarizing patterns of successful invasions, it discusses the implications of how successfully established NIS impact the local environment, economy and human health. Finally, it looks to the future and discusses what questions need to be addressed and what models can tell us about what the outlook on future marine invasions is.