Investigating the prey spectrum of two co-occurring Themisto amphipods in the Fram Strait (Atlantic-Arctic gateway) using DNA metabarcoding

Pelagic amphipods are a key zooplankton group in polar regions. In the Arctic Ocean, the two hyperiid amphipod species Themisto libellula and Themisto abyssorum are dominating the pelagic community. They are not only an important food source for higher trophic levels like birds, fish and marine mammals, but they are also important predators able to control the zooplankton standing stock in some Arctic regions. Due to its association with incoming warm Atlantic waters, T. abyssorum will be benefitting from the ongoing Atlantification of the Arctic Ocean whereas the Arctic water associated T. libellula may suffer from it. Since the Arctic is undergoing drastic changes in terms of warming and sea ice loss, it is crucial to gain knowledge about the prey spectrum of these two important amphipods, to predict the future of pelagic communities and the food web in a changing Arctic. In recent years, several studies were conducted concerning the diet of T. libellula and T. abyssorum. These studies suggested a diet mainly containing the most abundant zooplankton species including copepods, euphausiids and chaetognaths. As these studies were often conducted using stereomicroscopy they are limited, as it remains impossible to identify soft-bodied organisms and strongly digested prey. Other studies using a biomarker approach like stable isotopes or fatty acids, suggested that the two Themisto species are covering different niches in the Arctic ecosystem, with T. abyssorum covering a slightly higher trophic position than T. libellula. Additionally, they suggested that T. libellula has a diet associated to the sea-ice algal pathway, leading to the assumption that it will be more heavily affected by the ongoing sea-ice retreat. But these approaches are also limited to a certain point since there are only specific markers for primary production and primary consumers and so the whole prey spectrum of these organisms cannot be estimated with these methods. We used DNA barcoding to compare the species-level prey spectrum of the two Themisto amphipods and investigate its regional variation. Therefore, samples from different locations in the Fram Strait were taken, and stomach contents from 30-75 individuals per station were extracted. When comparing prey spectra between locations, we aim to identify drivers impacting the diet composition of both species and to estimate how environmental changes will affect the food web structure. Finally, we would like to detect whether and to which extent Themisto feeds on gelatinous zooplankton, known to be very abundant in the region