Description: Climate change proceeding at unprecedented pace is currently redistributing life on Earth. Warming of the upper ocean and the atmosphere have altered sea ice extent and seasonal dynamics in the Arctic, and similar changes are observed in the Atlantic sector of the Southern Ocean and West Antarctic. This affects stocks of major pelagic species such as krill. A so far neglected key player, the predatory amphipod genus Themisto, covers a distribution from temperate to polar regions where it regionally represents the dominant food source for higher trophic levels. Poleward range expansions of several Themisto species have been already been documented or predicted as warming continues. In the Atlantic sector of the Southern Ocean, changes in seasonal sea ice extension are causing strong annual variations of krill stocks, whilst salps are on the rise and shifting their distribution poleward. A third key player, Themisto gaudichaudii, is now also anticipated to extend its range southward. In the Arctic, the ice-dependent Arctic T. libellula and the sub-Arctic boreal T. abyssorum co-exist. Considering the ongoing Atlantification of the Arctic, a range expansion of T. abyssorum concomitant with a retraction of T. libellula’s range are very likely to occur. Due to major knowledge gaps in the ecology, biology and genetic connectivity of Themisto species, the likelihood of range shifts and their consequences for food web structures and biogeochemical cycles remain largely unexplored. We conducted a comparative study of Themisto populations to investigate their distribution, connectivity and trophic ecology. Using population genetic methods, the species status and phylogeography of T. gaudichaudii, were characterized. A genetic homogeneity and high degree of phenotypic plasticity related to feeding morphology were revealed, indicating the potential of T. gaudichaudii lineages to thrive in regions further south. In order to predict the consequences of an increased predation pressure of T. gaudichaudii which may influence the standing stock of Antarctic krill and salps, feeding experiments and molecular diet analyses were carried out. Similar analyses were applied on Themisto species from the northern hemisphere, in order to characterize their genetic connectivity within and across different water masses (Atlantic vs. polar) in Fram Strait and East Greenland and compare with the Antarctic populations.

Description: The Asian shore crab H. sanguineus first appeared at the French coast in the late 1990’s. It rapidly extended its range further north to the German Wadden Sea and recently to western Sweden. In the intertidal area, it co-occurs with the European green crab C. maenas. As the ecophysiology of H. sanguineus is virtually unknown, the physiological capacities of both species and their potential for intra-guild competition were investigated. The aim of this study was to specifically compare the energy deposition and dietary preferences of ovigerous females of both species. Females of H. sanguineus and C. maenas carrying immature or mature eggs were collected in April, June, August and October 2015 in an intertidal area of the Island of Helgoland, North Sea. Total lipid levels and fatty acid compositions were determined of both midgut glands and eggs. In H. sanguineus, total lipid levels of the midgut glands were clearly higher than those of C. maenas (40% vs. 10% dry mass, DM). Immature eggs were quite lipid-rich in both species with 30% and 25%DM, respectively, whereas in mature eggs, lipid levels decreased to ~15%DM each. A Principal Component Analysis of the fatty acid compositions of midgut glands and eggs revealed separate clusters for both species with C. maenas lipids more characterized by membrane fatty acids. In C. maenas fatty acids of midgut glands and eggs clustered together largely dominated by carnivory biomarkers. Fatty acids of midgut glands and all eggs of H. sanguineus formed separate clusters and trophic markers indicate a more herbivorous diet. Higher lipid levels and thus more pronounced energy deposition in H. sanguineus midgut glands indicate higher starvation tolerance for females, a potential competitive advantage over C. maenas. Direct food competition, however, seems negligible, as H. sanguineus prefers a more herbivorous diet than C. maenas. Deviating fatty acid compositions in H. sanguineus midgut glands and eggs suggest that this species may represent an income breeder, utilizing energy from both the midgut gland but also from dietary input. Most brachyuran crabs are capital breeders, which rely exclusively on internal reserves.

Description: Biological invasions can entail major threats to marine biodiversity. Non-indigenous species may induce changes in community structure and ecosystem functioning, thereby affecting ecosystem services and human economic interest and health. Decapod brachyuran crabs are among the most invasive marine animal taxa. The originally European species Carcinus maenas and the Asian shore crab Hemigrapsus sanguineus invaded different coastal areas around the world. While their invasion history and ecology has been thoroughly investigated, physiological properties of H. sanguineus are virtually unknown. The aim of this study was to compare the metabolic energy demand of both crab species and - based on these data - to assess their potential dietary impact on the ecosystem. Respiration measurements were conducted with a flow-through system covering a temperature range naturally experienced by these crabs (5, 10, 15 and 20 °C). Both species were analyzed on the island of Helgoland in April, June and August 2015. A general linear mixed-effects model (LMM) was applied to test for the effects of species, temperature, biomass and sex on respiration rates. Overall rates increased with temperature but decreased with the mass of the crabs. Respiration rates did not differ significantly between sexes in both species. From the full model, two separated LMMs were created for either species. They allowed establishing species-specific equations for the prediction of respiration rates y (nmol d-1 g-1) for a crab of any given mass xMass (g) at any given ambient temperature xTemp (°C): ln y = 10.39 + (-0.34 * ln xMass) + (0.06 * xTemp) for C. maenas and ln y = 10.42 + (-0.39 * ln xMass) + (0.08 * xTemp) for H. sanguineus. The mass-specific respiration rates of C. maenas and H. sanguineus were quite similar. By applying the diet-dependent respiratory quotient, oxygen uptake may be used to calculate carbon uptake and metabolic energy demand either for single crabs or for entire populations of a given area. On the population level, the metabolic energy demand and thus ecosystem impact of both species depend primarily on their abundance in the field and, less so on their dietary preferences.

Description: The crabeater seal (Lobodon carcinophaga) is the most abundant Antarctic seal and inhabits the circumpolar pack ice zone of the Southern Ocean. Until now, information on important environmental factors affecting its distribution as well as on foraging behaviour is limited. In austral summer 1998, 12 crabeater seals of both sexes and different age classes were equipped with satellite-linked dive recorders at Drescher Inlet (72.85°S, 19.26°E), eastern Weddell Sea. To identify suitable habitat conditions within the Weddell Sea, a maximum entropy (Maxent) modelling approach was implemented. The model revealed that the eastern and southern Weddell Sea is especially suitable for crabeater seals. Distance to the continental shelf break and sea ice concentration were the two most important parameters in modelling species distribution throughout the study period. Model predictions demonstrated that crabeater seals showed a dynamic response to their seasonally changing environment emphasized by the favoured sea ice conditions. Crabeater seals utilized ice-free waters substantially, which is potentially explained by the comparatively low sea ice cover of the Weddell Sea during summer 1998. Diving behaviour was characterized by short (〉90 % = 0–4 min) and shallow (〉90 % = 0–51 m) dives. This pattern reflects the typical summer and autumn foraging behaviour of crabeater seals. Both the distribution and foraging behaviour corresponded well with the life history of the Antarctic krill (Euphausia superba), the preferred prey of crabeater seals. In general, predicted suitable habitat conditions were congruent with probable habitats of krill, which emphasizes the strong dependence on their primary prey.

Description: Adult Euphausia superba survive winter without or with little feeding. It is not exactly known whether the scarcity of food or an internal clock, set by the natural Antarctic light regime, are responsible for non-feeding. Our research questions were therefore the following: (1) How will physiological and biochemical conditions of krill change during long-term starvation at constant light regime? (2) If and how do enzyme activities change during such starvation? (3) What is the influence of food availability versus that of light regime? To answer these questions, adult krill were starved under laboratory conditions for 12 weeks with constant light regime (12:12; dark/light) and the impact on physiological functions was studied. Initial experimental condition of krill resembled the condition of late spring krill in the field with fully active metabolism and low lipid reserves. Metabolic activity and activities of enzymes catabolising lipids decreased after the onset of starvation and remained low throughout, whereas lipid reserves declined and lipid composition changed. Mass and size of krill decreased while the inter-moult period increased. Depletion of storage- and structural metabolites occurred in the order of depot lipids and glycogen reserves after onset of starvation until proteins were almost exclusively used after 6–7 weeks of starvation. Results confirmed various proposed overwintering mechanisms such as metabolic slowdown, slow growth or shrinkage and use of lipid reserves. However, these changes were set in motion by food shortage only, i.e.without the trigger of a changing light regime.

Description: Hyperiid amphipod crustaceans are abundant in Southern Ocean waters from the Polar Frontal Zone to shelf waters where they represent a major prey for fish and seabirds. Hyperiella dilatata has evolved a peculiar anti-predatory behaviour: it abducts chemically-protected, shell-less pteropods from the water column and carries them on their dorsal side. Pteropods such as Clione antarctica are known to produce de novo a predator-deterrent chemical. Hence, the tandem formation of amphipods, abducting pteropods from the water column, represents an efficient protection from visual predators. This phenomenon was known from shelf waters around the continent, however, in the open ocean it was so far undocumented. We report this association at four different open-water sites in the Southern Ocean. Molecular analyses based on COI “barcode” sequences revealed that one Hyperiella lineage abducted Spongiobranchaea pteropods in the Polar Frontal Zone whilst a distinct Hyperiella lineage associates with Clione further south in the Weddell Sea. Hypotheses on the origin and function of these species-specific associations are provided in the context of biogeographic data and hyperiid phylogenetic histories.