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This study and the sampling at sea was funded by a grant from the UK Natural Environment Research Council (NE/E010350/1) to ASB and VAS. EGM was funded by a NERC grant (NE/P002412/1) and Independent Research Fellowship (NE/S014756/1). Robust time-series of direct observations of jellyfish abundance are not available for many ecosystems, leaving it difficult to determine changes in jellyfish abundance, the possible causes (e.g. climate change) or the consequences (e.g. trophic cascades). We sought an indirect ecological route to reconstruct jellyfish abundance in the Irish Sea: since zooplankton are jellyfish prey, historic variability in zooplankton communities may provide proxies for jellyfish abundance. We determined the Bayesian ecological network of jellyfish–zooplankton dependencies using jellyfish- and zooplankton-abundance data obtained using nets during a 2-week cruise to the Irish Sea in 2008. This network revealed that Aurelia aurita abundance was dependent on zooplankton groups Warm Temperate and Temperate Oceanic as defined by previous zooplankton ecology work. We then determined historic zooplankton networks across the Irish Sea from abundance data from Continuous Plankton Recorder surveys conducted between 1970 and 2000. Transposing the 2008 spatial dependencies onto the historic networks revealed that Aurelia abundance was more strongly dependent over time on sea surface temperature than on the zooplankton community. The generalist predatory abilities of Aurelia may have insulated this jellyfish over the 1985 regime shift when zooplankton composition in the Irish Sea changed abruptly, and also help explain its globally widespread distribution. Publisher PDF

The widely distributed snail species Theodoxus fluviatilis (Linneaus, 1758) is rather diverse in shell size and patterning. It has formed regional subgroups in northern Germany that appear separate in either fresh- or brackish water, yet, according to literature, are indistinguishable by morphology, anatomy or mtRNA markers. Tolerance towards challenging salinities, however, is distinct. The question as to whether the observed difference in salinity tolerance is a result of local adaptation or of phenotypic plasticity remains. In this study, physiological limits to salinity stress and tolerance shift abilities were compared between individuals from both subgroups in a salinity transfer experiment and acclimation regime followed by a survival analysis. Brackish water animals struggled in freshwater but performed much better than freshwater individuals when directly transferred into higher salinities. With acclimation to their challenging salinities, individuals of both subgroups achieved shifts in the slopes of their reaction norms and were able to survive salinity levels otherwise lethal to them. Even with this shift freshwater animals were not able reach the tolerance levels of brackish water individuals. The results indicate that phenotypic plasticity as well as genetic adaptation may determine osmotolerance in regional subgroups of T. fluviatilis.

Hyperosmotic stress may result in osmotic volume loss from the body to the environment in animals which cannot control the water permeability of their integument. Euryhaline (having a wide tolerance range of environmental salinities) animals have generally evolved the ability to counteract cell volume shrinkage by accumulating inorganic and organic osmolytes within their cells to balance internal and external osmolalities. Molluscs use very different combinations of amino acids and amino acid derivatives to achieve this goal. Theodoxus fluviatilis (Linneaus, 1758) is a neritid gastropod that is distributed not only in limnic habitats in Europe but also in brackish waters (e.g. along the shore line of the Baltic Sea). Animals from brackish sites survive better in high salinities than animals from freshwater locations. The results of this study indicate that these differences in salinity tolerance cannot be explained by differences in the general ability to accumulate amino acids as organic osmolytes. Although there may be differences in the metabolic pathways involved in osmolyte accumulation in foot muscle tissue, both groups of animals accumulate amino acid mixtures equally well when stepwise acclimated to their respective maximum tolerable salinity for extended periods. Among these amino acids, alanine, proline as well as the osmolyte urea hold a special importance for cell volume preservation in Theodoxus under hyperosmotic stress. It is possible that the accumulation of various amino acids during hyperosmotic stress occurs via hydrolysis of storage proteins, while alanine and proline are most likely newly synthesised under conditions of hyperosmotic stress in the animals.

Hyperosmotic stress may result in osmotic volume loss from the body to the environment in animals that cannot control the water permeability of their integument. Euryhaline animals (which have a wide tolerance range of environmental salinities) have generally evolved the ability to counteract cell volume shrinkage by accumulating inorganic and organic osmolytes within their cells to balance internal and external osmolalities. Molluscs use very different combinations of amino acids and amino acid derivatives to achieve this goal.

Hyperplasia and hypertrophy are elements of phenotypic plasticity adjusting organ size and function. Because they are costly, we assume that they are beneficial. In this review, the authors discuss examples of tissue and organ systems that respond with plastic changes to osmotic stress to raise awareness that we do not always have sufficient experimental evidence to conclude that such processes provide fitness advantages. Changes in hydranth architecture in the hydroid Cordylophora caspia or variations in size in the anal papillae of insect larvae upon changes in medium salinity may be adaptive or not. The restructuring of salt glands in ducklings upon salt-loading is an example of phenotypic plasticity which indeed seems beneficial. As the genomes of model species are recently sequenced and the animals are easy to rear, these species are suitable study objects to investigate the biological significance of phenotypic plasticity and to study potential epigenetic and other mechanisms underlying phenotypic changes.