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 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.