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Environmental DNA (eDNA) studies have burgeoned over the last two decades and the application of eDNA has increased exponentially since 2010, albeit at a slower pace in the marine system. We provide a literature overview on marine metazoan eDNA studies and assess recent achievements in answering questions related to species distributions, biodiversity and biomass. We investigate which are the better studied taxonomic groups, geographic regions and the genetic markers used. We evaluate the use of eDNA for addressing ecological and environmental issues through food web, ecotoxicological, surveillance and management studies. Based on this state of the art, we highlight exciting prospects of eDNA for marine time series, population genetic studies, the use of natural sampler DNA, and eDNA data for building trophic networks and ecosystem models. We discuss the current limitations, in terms of marker choice and incompleteness of reference databases. We also present recent advances using experiments and modeling to better understand persistence, decay and dispersal of eDNA in coastal and oceanic systems. Finally, we explore promising avenues for marine eDNA research, including autonomous or passive eDNA sampling, as well as the combined applications of eDNA with different surveillance methods and further molecular advances.

Gelatinous zooplankton (GZP) comprising ctenophores, cnidarians and tunicates, gained more interest in recent years. During favorable conditions GZP is known to rapidly increase in biomass and several dominant species are able to exploit the zooplankton standing stock. It is assumed that GZP will increase in biomass with the ongoing warming and Atlantification of the Arctic Ocean. Hence, it is crucial to gain insights into the role of gelatinous zooplankton in the current and future Arctic marine food web . This study targets both the benthic and pelagic components of the Arctic marine food web . First, we aim to determine the role of so-called “jellyfalls”, or jelly carcasses on the seafloor, as a food source for the benthic community. To do so, we investigate the stomach content of benthic scavenging amphipods in Kongsfjorden sampled during Polar night with DNA metabarcoding. By doing so,. Second, we look at the role of jellyfish as predators in the pelagic system. We focus on a widely distributed boreal-Arctic hydrozoan species, Aglantha digitale, which is also assumed to be a climate-change winner in an “Atlantified” Arctic. Its feeding ecology is scarcely studied, in particular in the Arctic region? This jellyfish is believed to feed on a wide range of zooplankton including copepods. Due to its high abundances in the Arctic regions, both in summer and in winter, it is crucial to investigate spatio-temporal patterns in its diet. Hence, we use DNA metabarcoding of Aglantha’s stomach content to reveal its prey composition in different seasons and regions in order to better predict its future role in an ice-free and Atlantified Arctic.

Pelagic hyperiid amphipods represent an important trophic link between the herbivore-omnivore zooplankton community and various higher trophic levels. The two pelagic hyperiid amphipods, Themisto gaudichaudii and Vibilia armata, are abundant in the southern Benguela upwelling system. T. gaudichaudii is recognized as a free-swimming species, while V. armata has shown an association with gelatinous zooplankton, particularly during their juvenile stages. So far, morphological identification of prey remains in the stomach contents has been commonly performed to reveal the food items in the diet of hyperiid species. But the stomach content examination technique often overlooked the highly degraded, fragile, and fragmented prey organisms (e.g., gelatinous zooplankton). Thus, the dietary spectra of both species remain poorly understood, especially in the Benguela upwelling system. DNA metabarcoding is an emerging and powerful molecular technique that can be used to investigate diet compositions using DNA fragments available in stomachs or feces. This molecular approach allows prey identification at a high taxonomic resolution to species or genus level, depending on the completeness of the reference database. In this study, we apply DNA metabarcoding to identify the prey composition in the stomachs of T. gaudichaudii and V. armata, targeting the mitochondrial cytochrome c-oxidase subunit 1 (COI) and the nuclear 18S rDNA regions. Further, we assess variations of prey compositions among the different size categories of each species and different localities in the southern Benguela upwelling system. The outcome of this study provides an insight into the trophic status of the two hyperiids and ultimately contributes to a comprehensive understanding of their ecological role in the southern Benguela food web.

Gelatinous zooplankton (GZ) or jellies, consisting of cnidarians, ctenophores and tunicates, can reach high biomasses, but so far, have been overlooked as a food source for higher trophic levels. Traditionally, GZ have been seen as trophic “dead-ends” in the food web, as they are rarely observed during visual inspections of predators’ stomach contents. This is particularly the case in fishery surveys, which conventionally rely on morphological stomach content analysis. However, high water content and fragility can result in quick digestion of gelatinous prey, meaning only those that were ingested shortly before sampling can be visually recorded. Modern molecular methods, which can detect the DNA of gelatinous species longer after ingestion, are increasingly showing that GZ are part of the diet of numerous animals, including various invertebrate groups, seabirds, turtles and fish. The evaluation of GZ as a potential food source for marine animals is particularly important in context of global warming, as the biomass of GZ is expected to further increase. This study uses DNA metabarcoding (18S and COI) to identify prey items in the stomachs of common Southern Greenlandic fish species, several of which are commercially exploited: Gadus morhua, Sebastes sp., Anarhichas sp., Argentina silus and Hippoglossoides platessoides. The analysis and comparison of the prey taxa detected with each of the two genes, should yield a comprehensive picture of the prey spectrum of the different fish species, including readily digested and fragile organisms like GZ. The selection of investigated fish species will allow a prey-spectrum assessment of different trophic groups across different habitats: East vs. West Greenland; pelagic vs. demersal fish; juvenile vs. adult.

Rapid warming in the Arctic is drastically impacting marine ecosystems, affecting pelagic communities and food web structure. Themisto amphipods are dominant in the Arctic zooplankton community and represent a key link between secondary producers and higher trophic levels. In the Arctic seas, two coexisting species are found in high biomass: Themisto libellula, considered a true Arctic species and Themisto abyssorum, a sub-Arctic, boreal species. Many aspects of the ecology and genetic structure of these two species are not well studied, despite their importance in the food web. First, we tested both species for levels of genetic diversity and then assessed their diet spectrum with molecular methods, with a regional focus on the Greenland shelf, Fram Strait and Svalbard. Spatial genetic structure was evaluated using the mitochondrial cytochrome c oxidase subunit 1 gene (COI). Our results revealed strikingly different levels of genetic diversity: low levels in T. libellula contrasted with higher diversity in T. abyssorum. No spatial genetic structure was found, and both species exhibited high levels of connectivity and evidence of historic demographic expansion. The observed low genetic diversity, in combination with cold adaptations, could cause T. libellula to be more susceptible to the warming Arctic. In contrast, high diversity likely increases adaptive potential in T. abyssorum. In order to comprehensively characterize the prey spectrum of both Themisto species, we also applied DNA metabarcoding, also using COI, on gut contents. Both species showed a regional variation in prey spectrum. T. abyssorum’s diet showed a clear dominance of reads identified as chaetognath species, whereas T. libellula had a broader prey spectrum, including ice-associated taxa such as polar cod. Calanoid copepods did not appear as important as prey as assumed from previous (morphological) studies. Several previously overlooked jellyfish taxa were found in the stomachs of both species. T

Gelatinous zooplankton or “jellies” (ctenophores, cnidarians, tunicates) are known to be major drivers of ecosystem changes. Increases in jelly biomass, referred to as “jellification”, have been observed in several marine ecosystems, causing, amongst others, the collapse of major fisheries. For the Arctic region, abundance data on jellies are virtually non-existent, impeding our ability to detect changes of a similar magnitude. To better understand the role of jellies in the Arctic seas, the Helmholtz Young Investigator Group ARJEL (2019-2026), aims to combine the most recent technologies in optics, acoustics, and environmental DNA analyses. Based on data collected during recent international campaigns, we attempt to link distributional patterns of jellies to sea-ice and oceanographic features. Furthermore, we apply species distribution models to a broad set of archived data to understand observed species and community patterns and to predict changes under future climate-change scenarios. The role of jellies in the Arctic food web, their importance for planktonic predators and fish and their link to the sea-ice trophic pathway is assessed with molecular diet studies. Physiological and transcriptomic studies serve to predict range expansions, and the consequences of expansion will be predicted based on food web models. An overview of the project’s goals, methods and first results will be given. One of our first research highlights include the comparison of species composition and abundances of ctenophores and cnidarians in Arctic vs. Atlantic-influenced Svalbard fjords. We also demonstrate a seasonality in species composition of the gelatinous component of the zooplankton observed during the year-long expedition MOSAiC in the central-Arctic.

The pelagic amphipods Themisto libellula and T. abyssorum represent a key zooplankton group in the polar regions. These two hyperiid amphipods are an important food source for higher trophic levels, and being carnivores, they are able to control the zooplankton standing stock in some Arctic regions. T. abyssorum is a boreal species reaching the Arctic with incoming Atlantic water and is thus likely to benefit from the ongoing Atlantification of the Arctic Ocean, whereas it may heavily impact the genuine Arctic T. libellula. 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 food web in a changing Arctic. In recent years, several studies were conducted concerning the diet of T. libellula and T. abyssroum. In those studies, mainly using biomarkers or stereomicroscopy, a diet consisting of the most abundant zooplankton species including calanoid copepods, euphausiids and chaetognaths, was suggested. T. abyssorum and T. libellula were also found to occupy distinct niches in the Arctic ecosystem. In this study, DNA metabarcoding was used to compare the species-level prey spectrum of the two Themisto amphipods and compare its regional variation. We found that T. libellula feeds on larvae of fish like Boreogadus saida, while this was not the case for T. abyssorum. The chaetognath Eukrohnia hamata was detected in most samples of T. abyssorum, as well as DNA of hydrozoans like Aglantha digitale or Nanomia cara, suggesting that gelatinous zooplankton represent a food source for those amphipods.

Gelatinous zooplankton or “jellies” (ctenophores, cnidarians, tunicates) are known to be major drivers of ecosystem changes. Increases in jelly biomass, referred to as “jellification”, have been observed in several marine ecosystems, causing, amongst others, the collapse of major fisheries. For the Arctic region, abundance data on jellies are virtually non-existent, impeding our ability to detect changes of a similar magnitude. To better understand the role of jellies in the Arctic seas, the Helmholtz Young Investigator Group ARJEL (2019-2026), aims to combine the most recent technologies in optics, acoustics, and environmental DNA analyses. Based on data collected during recent international campaigns, we attempt to link distributional patterns of jellies to sea-ice and oceanographic features. Furthermore, we apply species distribution models to a broad set of archived data to understand observed species and community patterns and to predict changes under future climate-change scenarios. The role of jellies in the Arctic food web, their importance for planktonic predators and fish and their link to the sea-ice trophic pathway is assessed with molecular diet and biomarker studies. Physiological and transcriptomic studies serve to predict range expansions, and the consequences of expansion will be predicted based on food web models. An overview of the project’s goals, methods and first results will be given, with scope for collaborations.

Jellyfish (ctenophores and cnidarians) are known to be major drivers of ecosystem changes. Increases in biomass, referred to as “jellification”, have been observed in several marine ecosystems, causing, amongst others, the collapse of major fisheries. For the Arctic, comprehensive datasets on jellyfish are currently missing, impeding our ability to detect changes of a similar magnitude. The Helmholtz Young Investigator Group “ARJEL” aims to combine the most recent technologies in optics and environmental DNA analyses, to better understand the role of jellies in the Arctic seas. We apply species and community distribution models to a broad set of archived and newly obtained data to understand distribution patterns and to predict range and community shifts under future climate-change scenarios. The role of jellies in the Arctic food web, their seasonal and regional variation in feeding habits and their importance as prey for planktonic predators and fish is assessed with DNA metabarcoding and biomarker studies. We investigate the role of “jelly-falls” in sustaining the benthic food web. Experimental studies will determine jellyfish’ thermal windows and resilience. The outcomes of the models, trophic data, and insights into the connectivity and adaptability of jellyfish species, will allow us to improve food web and ecosystem models, currently neglecting jellyfish. An understanding of jellyfish-fish interactions, and how these will be impacted by climate-change driven range shifts, will shed light on the fate of commercially exploited Arctic fish stocks. We will present the project aims and first results, as well as our planned research activities during our stay at AWIPEV, Kongsfjorden, in January 2022. Our ongoing foci include: i) the comparison of jellyfish’ communities in Arctic vs. Atlantic-influenced Svalbard fjords to forecast the impact of the ongoing Atlantification; ii) the comparison between various methods for assessing jellyfish diversity; iii) the ecology

Gelatinous zooplankton or “jellies” (ctenophores, cnidarians, tunicates) are known to be major drivers of ecosystem changes. Increases in jelly biomass, referred to as “jellification”, have been observed in several marine ecosystems, causing, amongst others, the collapse of major fisheries. For the Arctic region, abundance data on jellies are virtually non-existent, impeding our ability to detect changes of a similar magnitude. To better understand the role of jellies in the Arctic seas, the Helmholtz Young Investigator Group ARJEL aims to combine the most recent technologies in optics, acoustics, and environmental DNA (eDNA) analyses. Based on data collected during recent international campaigns, we attempt to link distributional patterns of jellies to oceanographic features and sea ice. Furthermore, we apply species distribution models to a broad set of archived data to understand observed species and community patterns and to predict changes under future climate-change scenarios. The role of jellies in the Arctic food web and their importance as prey for planktonic predators and fish is assessed with molecular diet studies, which will improve food web models currently neglecting jellies as major predators and prey. We also explore the genetic connectivity of dominant jelly species across the Arctic Ocean and its adjacent seas. An overview of the project’s goals, methods and first results will be given. Our ongoing research foci include: 1. the comparison of species composition and abundances of ctenophores and cnidarians in Arctic vs. Atlantic-influenced Svalbard fjords to better understand the impact of the ongoing Atlantification of the Arctic; and 2. the comparison between optical methods, net catches and eDNA for assessing jelly diversity and abundances.

Climate change proceeding at unprecedented pace is currently redistributing life on Earth. In the Arctic region, climate change is acting more rapidly than elsewhere on this planet, and has dramatically altered sea ice thickness and extent. However, for many Arctic taxa, the distribution ranges and population connectivity have remained undocumented. This is particularly so for Arctic gelatinous zooplankton, of which the diversity, abundances and role in the food web are understudied. The hydromedusa Aglantha digitale is highly abundant in the Arctic Ocean, and characterized by a widespread distribution, ranging from temperate waters to the central Arctic. Its distribution in the water column has been linked to the presence of Atlantic water masses, which renders it a likely candidate to benefit from the ongoing “Atlantification” of the Arctic. Despite its ubiquity and abundance, its genetic diversity remains unknown, and it is unclear whether this species is composed of different geographic lineages throughout its distribution range. To compare the genetic diversity and assess the phylogeography of A. digitale, we collected samples from several recent international cruises. Geographic populations from temperate waters, sub-Arctic Greenland, Svalbard and the central Arctic are compared based on sequences of the mitochondrial cytochrome c oxidase subunit I (COI). A better understanding of the distribution and connectivity will help to predict potential range shifts of A. digitale in an “Atlantified” Arctic.

Gelatinous zooplankton (GZP), comprising ctenophores, cnidarians, and tunicates, gained more interest in recent years. During favourable conditions, GZP is known to rapidly increase in biomass and several dominant species are able to exploit the zooplankton standing stock. Since GZP biomass may increase with ongoing warming in the Arctic Ocean, similar to elsewhere in the World Ocean, it is likely that GZP range shifts will take place concomitant with the Atlantification of the Arctic. Thus, it is crucial to gain knowledge about their role in the Arctic marine food web. Because of GZP having low nutritional values, they were considered to be a “trophic dead end”, until modern methods like video logging, biomarkers, and molecular diet studies introduced a paradigm shift regarding the role of jellyfish as prey for different fish and seabird species worldwide. In this project, we will use DNA metabarcoding to reveal the role of GZP in the diet of Arctic and sub-Arctic fish species. Additionally, the role of so-called “jellyfalls” as a carbon source for the Arctic deep-sea benthic communities will be investigated by applying DNA metabarcoding on the stomach contents of scavenging benthic amphipods. The role of GZP as predators in the Arctic Ocean during different seasons will also be investigated. To do so, DNA metabarcoding of the gastric pouch of dominant GZP species in the Arctic Ocean will be performed, to detect, amongst others, predation on ichthyoplankton. Additionally, biomarkers will be used to determine the trophic position of GZP in the Arctic marine food web and to reveal the role of the ice-algal pathway in the diet of GZP. The obtained data will reveal which fish species may be competitors of predators on jellyfish and, hence, will contribute to improve food web models of the Arctic marine ecosystem, currently neglecting the role of GZP.