Isabelle Ewers

• Ewers, Isabelle & Dunthorn, Micah (2026). A review of protistan parasites in and around Norwegian marine fish farms. Ocean Microbiology. 2. doi: 10.1186/s44375-026-00008-2. Full text in Research Archive Show summary

  Protistan parasites are an integral part of marine ecosystems and can serve as indicators for environmental health and ecosystem status. This study provides a review of the protistan parasite taxa reported in and around Norwegian marine fish farms, the functional groups they represent, and which potential implications their occurrences have for environmental stress. By focusing on the parasite life cycles, pathogenicity, and host specificity, we aimed to determine whether they are internal or external, monoxenous or heteroxenous, and generalist or specialist parasites for inference of the functional groups. Our findings reveal that the majority of reported parasites are external or internal, monoxenous, and generalist taxa, such as Paramoeba perurans, Spironucleus salmonicida, Ichthyobodo, and Ichthyophonus hoferi, which can indicate that the environment along the Norwegian marine shoreline is experiencing moderate to high levels of environmental stress and some degree of degradation. By contrast, the presence of only a few parasitic taxa such as Trichodina cooperi and Trypanosoma pleuronectidium is indicative of low levels of stress and mildly degraded conditions. Notably, parasites typically associated with healthy environments, such as external, heteroxenous specialists, were largely absent. These patterns suggest that Norwegian coastal ecosystems in and around fish farms are subject to continuous environmental stress, potentially due to the farming conditions and other anthropogenic pressures. While not all detected protistan parasites are currently associated with disease outbreaks in Norway, many are known pathogens elsewhere or may act as opportunists under changing environmental conditions. Their presence highlights the importance of continued research efforts on protistan parasites of fish, especially in the context of climate change and evolving fish farming practices.

• Mahlangu, Sibusiso; Ewers, Isabelle; Carvalho, Cintia Oliveira; Schrøder-Nielsen, Audun; Dunthorn, Micah & Karlsen, Hans Erik [Show all 12 contributors for this article] (2025). Investigating the dynamics of the aquatic community in Oslofjord through time series analysis of eDNA. Journal of Fish Biology. ISSN 0022-1112. doi: 10.1111/jfb.70105. Full text in Research Archive Show summary

  Understanding the temporal dynamics of marine communities is critical for assessing ecosystem health and guiding conservation efforts. Here, we conducted a survey using environmental DNA (eDNA) metabarcoding with two primer sets, MiFish and Elas02, to investigate seasonal and interannual changes in the fish community of Oslofjord (Norway) over two consecutive years. Using the MitoFish reference database through the Global Biodiversity Information Facility querying tool, we identified 63 fish species and found significant changes in the dominant taxa between seasons and years. Clupea harengus (Atlantic herring) consistently peaked in early spring, while Scomber scombrus (Atlantic mackerel) dominated winter months in the second year. The MiFish primer set showed increased species richness in the second year, whereas the Elas02 primer set showed stable richness despite compositional turnover. Non-metric multidimensional scaling revealed distinct community separation between years in presence/absence of data, driven by species turnover rather than abundance read changes. Our findings support the use of eDNA metabarcoding to capture fine-scale (bi-monthly) temporal dynamics and emphasise the importance of multi-year datasets for distinguishing ecological trends from stochastic changes. This strategy improves monitoring practices for marine ecosystems under anthropogenic stressors.

• Ewers, Isabelle; Rajter, Ľubomír; Czech, Lucas; Mahé, Frédéric; Stamatakis, Alexandros & Dunthorn, Micah (2023). Interpreting phylogenetic placements for taxonomic assignment of environmental DNA. Journal of Eukaryotic Microbiology. ISSN 1066-5234. 70, p. e12990–e12990. doi: 10.1111/jeu.12990. Full text in Research Archive

View all works in NVA

• Ewers, Isabelle & Dunthorn, Micah (2025). Functional groups of protistan parasites occurring in and around Norwegian marine fish farms. Full text in Research Archive Show summary

  Marine fish farming is continually increasing along the Norwegian coast. While this industry has a socioeconomic benefit, it is also significantly impacting environmental health. One aspect of this environmental health is the diversity and distribution of parasites, as well as the functional groups they belong to. I have found that most protistan parasites in and around Norwegian fish farms are generalists with monoxenous life cycles (requiring one host), indicating there is a high level of environmental stressors. By contrast, healthy ecosystems are often thought to be rich in parasites that are specialists with heteroxenous life cycles (requiring two hosts).

• Dunthorn, Micah & Ewers, Isabelle (2025). Nothing new under the sun: global ciliate diversity is already well known. Full text in Research Archive Show summary

  We are still in an era of morphological and molecular methods finding new protistan taxa. What we wanted to know is if there are still ciliate taxa that remain to be discovered and described. To do this, we used 18,812 OTUs from free-living ciliates derived from EukBank’s global metabarcoding dataset of the V4 region of the SSU-rRNA locus. OTUs were taxonomically assigned to the main ciliate subtaxa using pairwise alignment and phylogenetic placement methods. Both methods agreed in their assignment for 99.17% of the OTUs to already known ciliate subtaxa. The conflicting OTU assignments were between incertae sedis taxa with pairwise alignments and deep branches with phylogenetic placements. Support for these placements, though, were low. These data show that we likely already know all the main ciliate subtaxa, although closely related species to already-known taxa remain to be described.

• Mahlangu, Sibusiso; Ewers, Isabelle; Carvalho, Cintia Oliveira; Schrøder-Nielsen, Audun; Dunthorn, Micah & Karlsen, Hans Erik [Show all 12 contributors for this article] (2025). Investigating the dynamics of the aquatic community in Oslofjord through time series analysis of eDNA. BioRxiv. ISSN 0362-4331. doi: 10.1101/2025.02.05.636659. Full text in Research Archive
• Ewers, Isabelle & Dunthorn, Micah (2024). A review of the protistan parasite diversity in and around Norwegian marine fish farms. Full text in Research Archive
• Ewers, Isabelle (2023). Observing protistan communities using Leray-XT COI primers. Full text in Research Archive

View all works in NVA