ALBERT

Description: To predict the risk associated with future introductions, ecologists seek to identify traits that determine the invasiveness of species. Among numerous designated characteristics, tolerance towards environmental stress is one of the most favored. However, there is little empirical support for the assumption that non-native species generally cope better with temporarily unfavorable conditions than native species. To test this concept, we ran five pairwise comparisons between native and non-native marine invertebrates at temperate, subtropical, and tropical sites. We included (natives named first) six bivalves: Brachidontes exustus and Perna viridis, P. perna and Isognomon bicolor, Saccostrea glomerata and Crassostrea gigas, two ascidians: Diplosoma listerianum and Didemnum vexillum as well as two crustaceans: Gammarus zaddachi and G. tigrinus. We simulated acute fluctuations in salinity, oxygen concentration, and temperature, while we measured respiration and survival rates. Under stressful conditions, non-native species consistently showed less pronounced deviations from their normal respiratory performance than their native counterparts. We suggest that this indicates that they have a wider tolerance range. Furthermore, they also revealed higher survival rates under stress. Thus, stress tolerance seems to be a property of successful invaders and could therefore be a useful criterion for screening profiles and risk assessment protocols. Highlights ► Non-native species showed higher survival rates in the face of stress than native. ► Respiratory performance under stress was closer to normal in non-native species. ► Strong evidence for stress tolerance as a general trait of non-native species. ► Robust results due to a global-scale, modular experimental approach.

Description: Marine bioinvasions are becoming more and more frequent due to shipping traffic and the increasing importance of aquaculture. The interaction of introduced species with resident communities can be neutral, negative or positive. In parallel, changing climatic conditions are altering environmental regimes, amplifying the frequency and intensity of extreme events. Such enhanced abiotic stresses are acting on marine organisms, especially in intertidal systems where fluctuations in various environmental factors are high. Both processes, climate change and the increasing number of bioinvasions, are amplified by human activities and mostly affect coastal areas. The question arises, if enhanced stress might be tolerated better by invasive species, and thus, be of advantage when competing with native species. As an approach to these questions, salinity stress tolerance of the native intertidal bivalves Saccostrea glomerata and Perna canaliculus was compared with the tolerance of the invasive oyster Crassostrea gigas. Experimental studies were conducted with populations of the south-western Pacific at the north-eastern coast of New Zealand. Oxygen consumption rates were measured as an immediate, and mortality and growth as a longterm response towards hypo- and hyper-salinity stress. Under severe low salinity regimes, oxygen consumption decreased significantly compared to respiration under ambient salinity conditions, indicating a strong response towards stress. The invasive oyster, however, showed a smaller decrease in respiration under stress than the native oyster and lower mortality rates compared to the native mussel, indicating a higher tolerance under these stress regimes. Different models are discussed as possible explanations for the better performance of the invasive species under stress. Besides bioinvasions and climate change, human activities also cause pollution and eutrophication, that have direct impacts on coastal regions, especially in harbour areas. This puts additional strain on the animals in these systems. Thus, a second aspect investigated in this thesis was, whether populations of the same species, but from sites with different anthropogenic impacts, differ in their tolerance towards additional stress. This question was assessed in similar experimental designs, comparing populations of P. canaliculus from a pristine site, Pakiri, and a disturbed yacht harbour, Weiti. This comparison, however, could not answer the study question. Since stressful events are becoming more important, in the long-term, a higher salinity stress tolerance of the invasive oyster, might result in modifications of competitive hierarchies in favour of the invasive species. Competitive symmetries between local and introduced species can have consequences for the structure of whole benthic communities in costal ecosystems.

Description: The successful invasion of a non-native species depends on several factors, including initial colonization and establishment of a self-sustaining population. Populations of the non-native paddle crab Charybdis japonica were first recognized in the Waitemata Harbour, Auckland, New Zealand in 2000, most likely arriving in ballast waters of an Asian merchant vessel. A survey completed in 2003 found C. japonica throughout the Waitemata Harbour, and further sampling in 2009 has revealed several well established populations in estuaries up to 70 km from the putative invasion point. As the potential for further establishment of C. japonica beyond this area may depend on the temperature and salinity tolerances of their free swimming larvae, we quantified the survival of newly-hatched Stage 1 C. japonica zoeae subjected to temperatures ranging from 11 to 43°C or salinities from 5 to 45‰ in the laboratory. Upon hatching, replicate C. japonica zoeae were directly transferred from 21°C and 34.6‰ seawater to either an experimental temperature or salinity level. Behaviour and death rates of the larvae were monitored over a 24 h period in the absence of food. Comparisons of zoeal survival rates to historical sea surface temperatures and salinities show that C. japonica Stage 1 zoeae tolerate a broad range of temperatures and salinities and could survive natural conditions throughout New Zealand. This gives C. japonica the potential to invade many other New Zealand estuaries and harbours.