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: 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.