The Presence of Bioactive Compounds in European Eel (Anguilla anguilla) Skin: A Comparative Study with Edible Tissue.

The presence of bioactive compounds in European eel (Anguilla anguilla) skin was studied. Proximate and lipid class compositions and analysis of the fatty acid (FA) profile (individual FAs; FA groups, i.e., saturated, monounsaturated, and polyunsaturated; FA ratios, i.e., polyunsaturated/saturated, ω3/ω6) were determined and compared to the composition of the eel muscle. As a result, higher (p < 0.05) levels of proteins (271.6 g·kg-1), lipids (38.0 g·kg-1), ash (27.7 g·kg-1), and ω6 FAs were observed in the skin tissue. Contrary, the muscle tissue showed higher (p < 0.05) moisture, ω3 FA, and ω3/ω6 ratio values. Regarding lipid classes, a higher (p < 0.05) proportion of phospholipids (111.1 g·kg-1 lipids), free sterols (104.7 g·kg-1 lipids), α-tocopherol (274.0 mg·kg-1 lipids), and free FAs (43.6 g·kg-1 lipids) was observed in the skin tissue. No differences (p > 0.05) between both tissues could be detected for triacylglycerol and FA group (saturated, monounsaturated, and polyunsaturated) values and for the polyunsaturated/saturated FA ratio. It is concluded that European eel skin, a by-product resulting from commercial processing, can be considered a valuable source for the food and pharmaceutical industries by providing value-added constituents such as proteins, lipids, ω3 FAs, phospholipids, and α-tocopherol.

Reaching the steady state: 30 years of Anguillicola crassus infection of European eel, Anguilla anguilla L., in Northern Germany.

A 30 years long data series on the infection dynamics of European eel (Anguilla anguilla L.) with the non-native invasive nematode Anguillicola crassus Kuwahara, Niimi & Hagaki, 1974 is presented. Parasite burden was evaluated for 30 years in inland and coastal waters in Mecklenburg-Western Pomerania from 1991 to 2020. The total prevalence, mean intensity and damage status of the swim bladders were very high during the first decade (1991­2000), and significantly decreased in both marine and freshwater eel populations in the following decades (2001­2010, 2011­2020). The parasite intensity of eels in coastal waters was significantly lower compared with the freshwater systems (61.3% vs 79.5% in the first decade), indicating the vulnerability of the parasites to brackish water conditions and the fact that the life cycle of A. crassus cannot be completed under high saline conditions. Eel caught in the western part of the Baltic Sea (west of Darss sill) had the lowest mean infection (51.8% in first decade) compared to the eastern part with 63.8%. Thus, besides different infection patterns caused by the environmental conditions, a temporal trend towards a reduced parasite intensity and a more balanced parasite­host relationship developed in the 30 years of interaction after the first invasion. Possible reasons and mechanisms for the observed trends in parasite­host interactions are discussed.

Rearing European Eel (Anguilla anguilla) Elvers in a Biofloc System.

European eel (Anguilla anguilla) elvers (initial body weight (BW) = 3 g) were raised in triplicate for 60 days in a biofloc system (BFT) at 21 °C. Data from the current first study evaluating this farming technology indicated that European eel elvers adapted well to BFT systems as data on growth performance (specific growth rate = 1.48% ± 0.13 BW/day and FCR = 1.05 ± 0.09) indicated, with production costs using BFT being lower than conventional RAS units. The most critical issues associated with this aquaculture system were the maintenance of the biofloc in tanks by the regular addition of refined sugar (46% C) to keep a relationship for C:N of 20:1, and the prevention of emergence of opportunistic pathogens like the monogenean Pseudodactylogyrus sp. The overall results of this study in terms of elvers' performance and quality and the composition of the biofloc material and its microbial composition indicated that BFT, which is considered to be one of the most cost-effective, sustainable, and environmentally friendly farming systems due to its zero water exchange and improvement of feed conversion ratio by the dietary contribution of bioflocs, may be satisfactorily used for farming European eels elvers at a density of 2 kg/m3. However, further studies are needed to test this technology with older eel stages.

Comparative evaluation of the effects of different activating media and temperatures on European eel (Anguilla anguilla) sperm motility assessed by computer assisted sperm analysis.

The European eel is a critically endangered teleost fish with very poor success rate for captive breeding and artificial reproduction. Therefore, to support its conservation, new strategies are needed to ensure fertilization. Objective analysis of sperm motility may be critical as it potentially represents one of the most important reproductive quality parameters. Spermatozoa acquire motility once in contact with hyperosmotic solutions as saltwater, yet the exact mechanisms and the role of temperature are still to be clarified. The main aim of the study was to assess the effects of 3 activating media (artificial sea water, tank water and commercial Actifish®) at 4 and 20 °C on sperm motility, by means of computer assisted sperm analysis. Secondary aim was to test 2 different concentrations of Actifish® mimicking sea water pH/osmolality, at 4 °C. The results suggested how both temperature and activating media have effects on spermatozoa motility and kinematics, with temperature mainly acting upon interaction with the media type. The samples activated with tank water at 20 °C showed the poorest motility outcomes (mean 38.1%), while the ones activated with Actifish® diluted 1:4 and artificial sea water, at 4 °C, the highest (means 51.8 and 51.5% respectively). Additionally, diluting Actifish® to reach same pH and osmolality of seawater led to worse motility outcomes, suggesting that composition may be the critical factor for activation rather than osmolality itself.

Timing is everything; operational changes at a pumping station with a gravity sluice to provide safe downstream passage for silver European eels and deliver considerable financial savings.

Catadromous European eel (Anguilla anguilla) are a critically endangered fish species due in part to in-river anthropogenic barriers (e.g., pumping stations, weirs, hydropower facilities). European legislation stipulates that safe downstream passage must be provided at hazardous intakes. Where present, gravity sluices have the potential to act as safe and low-cost downstream passage for seaward migrating silver eels at pumping station, but operational changes are required. This study used catchment-wide and fine-scale acoustic telemetry to investigate if operational changes (OC) at a pumping station (PS) with a co-located gravity sluice (GS) facilitated safe downstream passage for silver European eels. Specifically, night-time pump operations were ceased, river levels prior to sluicing were elevated and the GS was opened during key eel migration windows, i.e., at night during the new moon phase in autumn. No tagged eels passed through any pumps and the majority (2018 = 87.5%, 2020 = 88.9%) that approached the PS during OC passed downstream through the GS. Most eels approached during the first period of night sluicing after release (2018 = 73.9% and 2020 = 76.5%) and passed downstream during the first sluice event they experienced at the PS (2018 = 66.7% and 2020 = 75.0%). During the final approach prior to passage, very few retreats back upstream occurred at a median (IQR) distance of 34 (7.25) m from the GS and were predominantly a short distance (1-8 m). Overall, OC at a PS with a GS are considered a win-win-win, despite opening the sluice for <3% of the study period, given safe downstream passage was maximised, the financial benefits of sluicing water (∼£14,670 in direct operational costs over two years) and the relative ease of implementation.

A psychiatric drug found in waste-water plant effluents alters the migratory behavior of critically endangered Anguilla anguilla juveniles.

Migratory fishes cross or settle in several environments potentially polluted. Psychiatric drugs, which represent one growing pollution and are found in discharges from waste-water treatment plants, may alter individual behaviors. Here, we assessed behavioral alterations in the upstream migratory behavior of Anguilla anguilla caused by diazepam, an anxiolytic. We monitored the swimming activity, swimming behavior, and boldness to assess whether diazepam impacts them or not. Our 7-day behavioral follow-up allowed us to test the kinetics of the potential effects of diazepam. We found diazepam reduced swimming activity and altered individual swimming behavior, with fewer individuals swimming against the current, so swimming upstream. Those effects varied over time and were stronger at the end of our monitoring, suggesting chemical pollutants encountered in estuaries may act as a chemical burden for individuals, despite metabolisation. We also found diazepam favored bolder behavior in glass eels. Our results provide new knowledge on chemical pollution and psychiatric drugs inducing behavioral alterations. Those alterations may have ecological and evolutionary consequences for glass eels, by diminishing predator avoidance and impacting spatial colonization, and thus, local density.

The influence of dietary Coenzyme Q10 on growth performance, antioxidant capacity and resistance against Aeromonas hydrophila of juvenile European eel (Anguilla anguilla).

The present study was conducted to investigate the effects of dietary Coenzyme Q10 (CoQ10) on the growth performance, body composition, digestive enzyme activity, antioxidant capacity, intestinal histology, immune-antioxidant gene expression and disease resistance of juvenile European eel (Anguilla anguilla). Fish were fed a diet supplemented with CoQ10 at concentrations of 0, 40, 80 and 120 mg/kg for 56 days. The results indicated that dietary CoQ10 supplementation did not significantly affect final body weight (FBW), survival rate (SR), weight gain (WG), feed rate (FR), viscerosomatic index (VSI) or hepatosomatic index (HSI) among all experimental groups. However, the highest FBW, WG and SR were found in the 120 mg/kg CoQ10 group. Dietary 120 mg/kg CoQ10 markedly improved feed efficiency (FE) and the protein efficiency ratio (PER). The crude lipid in the body and triglycerides (TG) and total cholesterol (TC) in serum were obviously lower in the 120 mg/kg CoQ10 group than in the control group. For digestive enzymes, protease activity in the intestine was markedly boosted in the 120 mg/kg CoQ10 group. The serum activities of SOD, CAT and GST in the 120 mg/kg CoQ10 group were significantly higher than those in the control group. Dietary 120 mg/kg CoQ10 efficiently enhanced superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx) and glutathione S-transferase (GST) activities in the liver, while the malondialdehyde (MDA) content was significantly decreased. No significant histological changes in the liver were identified in any group. Dietary supplementation with 120 mg/kg CoQ10 improved antioxidant capacity and immunity by upregulating the expression of cyp1a, sod, gst, lysC, igma1, igmb1 and irf3 in the liver. Furthermore, the cumulative survival rate of juvenile European eel against challenge with Aeromonas hydrophila was significantly elevated in the 80 and 120 mg/kg CoQ10 supplemented groups. Conclusively, our study suggested that supplementing the diet of juvenile European eel with CoQ10 at a concentration of 120 mg/kg could promote their feed utilization, fat reduction, antioxidant capacity, digestibility, immune-antioxidant gene expression and resistance to Aeromonas hydrophila without negative effects on fish health status.

The rete mirabile: a possible control site for swimbladder function.

In a recent study, a large number of transport proteins was detected in the transcriptome and proteome of saline perfused rete mirabile tissue of the European eel. In this study, the data set was reanalyzed for the presence of receptor proteins and proteins involved in intracellular signaling pathways. A large number of expressed receptor proteins and proteins involved in intracellular signal transduction was detected. Several G-protein-coupled receptor signal pathways were significantly enriched in their expression level, in particular receptors and signaling pathways involved in the control of blood flow. The enriched signaling pathways also include pathways involved in trafficking of crucial transport proteins like, monocarboxylate transporters, V-ATPase, and aquaporin. The data, therefore, suggest that the rete mirabile has the capacity to control swimbladder function by regulating blood flow and by modifying countercurrent multiplication.

Factors influencing the downstream passage of European silver eels (Anguilla anguilla) through a tidal sluice.

Tidal sluices are a frequent element in the tidal regions of Europe's rivers and may hinder downstream migrating European eels Anguilla anguilla. Sea level rise will reduce the possibility for tidal sluices to freely discharge water, further compressing windows of opportunity for the passage of eels. Understanding how eels utilize the discharge events of tidal sluices and which conditions facilitate successful passage is pivotal for the design of effective fish migration measures. To investigate eel migration at a tidal sluice, acoustic receivers were placed at the tidal sluice Nieuwe Statenzijl and in its tributary of the Westerwoldse Aa, the Netherlands. Of the 30 tagged eels, 26 eels reached the tidal sluice and passage success was 100%. The mean migration speed of eels in the unobstructed part of the tributary was slow (0.14 m s-1 ). The eels were delayed in their migration by the sluice and delay was right-skewed distributed with most eels showing moderate delays (<2 days), while about 10% of the tagged individuals experienced extensive delays of more than 3 weeks. The number of missed sluicing events prior to successful passage was influenced by biological characteristics such as migration speed in the tributary, weight and condition. In addition, sluicing events with rapidly increasing and high maximum discharge levels increased the success rate of an individual eel to pass the sluice. Compromising sluicing duration in favour of higher and faster increasing discharge could facilitate eel migration at tidal barriers and contribute to the recovery of this endangered species.

Mortality in farmed European eel (Anguilla anguilla) in Italy due to Streptococcus iniae.

BACKGROUND: Streptococcal infections are one of the main causes of fish disease. During the last decade, Streptococcus iniae has become one of the most important aquatic pathogens worldwide, causing high losses in marine and freshwater finfish. Clinical signs in farmed fish include loss of appetite, lethargy and grouping at the bottom of the tank. Gross changes comprise darkening of the skin and haemorrhage at the basis of fins and opercula. To date, S. iniae has been isolated from several wild and farmed fish species but never in the European eel (Anguilla anguilla). In Europe, eel production from aquaculture is around 4500 tonnes and Italy is the third largest producer. This communication represents the first report of an outbreak of S. iniae infection in European eels. CASE PRESENTATION: The outbreak occurred at an eel farm in northern Italy between May 2021 and September 2021. The outbreak caused about 2% mortality per month, resulting in the loss of about 10% of the farmed fish. The diseased eels showed apathy, lethargy, inactivity and inappetence. In July 2021, three eels were necropsied. Necropsy revealed skin and branchial hyperaemia, a few skin ulcers, and diffuse peritoneal congestion with a few haemorrhagic-like spot lesions. Swab samples for bacteriology were taken from the kidneys, liver, spleen, and brain. Additionally, four eels were opened and swap samples as above were taken. All the investigated eels were found dead. Bacteriological examination revealed growth of Streptococcus spp. from all samples. Identification of S. iniae was done by biochemical characterization, the API20STREP microsystem, 16S rDNA sequencing, and MALDI-TOF. Antimicrobial therapy (oxytetracycline and erythromycin) was ineffective. CONCLUSIONS: This is the first report of S. iniae infection in the European eel. Although this may be an isolated outbreak, it is of concern due to the losses associated with this pathogen in fish worldwide and because the European eel is an endangered species. Due to the difficulties of controlling the disease with antimicrobials, it is advisable to plan other effective control measures, such as improving water quality and the environmental conditions, reducing fish density, improving biosecurity, and by using immunostimulants and, when possible, vaccines.

Monitoring for Anguillicoloides crassus, Anguillid herpesvirus 1, aquabirnavirus EVE and rhabdovirus EVEX in the European eel population of southern Spain.

European eel is critically endangered in Europe. Among other stressors, pathogens are well-known to harm eels' fitness. One hundred and eighty-two eels were captured in three Eel Management Units in Andalucía (SE Spain) and analysed for Anguillicoloides crassus, Anguillid herpesvirus 1 (AngHV1), the rhabdovirus Eel Virus European X (EVEX) and the aquabirnavirus Eel Virus European (EVE). A. crassus adults and preadults were isolated and morphometrically identified, and the eel swimbladders were artificially digested to count A. crassus larvae. Also, eel tissues were examined by PCRs for the presence of viruses. EVEX and EVE were not detected in any of the eels. The estimated prevalence (95% confidence limits) was 71 (64-78)% for A. crassus and 35 (28-42)% for AngHV-1, varying these prevalences significantly between and within EMUs. Moreover, A. crassus prevalence was highest in smaller eels, in sites closest to the sea and eels sampled in the autumn. By contrast, AngHV-1 prevalence was highest in biggest eels, in sites far from the sea and sampled in the summer or winter. However, in mixed effects logistic models including site as a random variable, the risk of infection was associated with distance to the sea in both A. crassus and AngHV-1 infections and also to winter sampling in the case of AngHV-1 and not to other variables. These results are evidence that both pathogens are highly endemic in eels from Andalusian habitats. Further studies are needed to better understand the risk factors associated with these pathogens on eel populations.

Is the Illegal Trade of Glass Eels (Anguilla anguilla) Increasing the Spread of Disease? A Case of EVEX.

The European eel (Anguilla anguilla) is a catadromous species that inhabits the rivers of the Adriatic watershed in Croatia. It is a critically endangered fish species, according to the IUCN, due to its declining abundance in European rivers caused by overfishing and trafficking and by diseases caused by nematodes, pathogenic bacteria and viruses. An illegal parcel of glass eels was confiscated at the Zagreb Airport and was intended to re-populate Croatian rivers. Barcoding was employed to determine species affiliation, and a thorough health check was carried out. This study reports the evaluation of gross lesions, histological findings, and EVEX virus isolation and identification. Since the confiscated glass eels were of unknown origin and given the serological and genetic similarities of EVA and EVEX, we designed primers and probes for almost whole genome sequencing to elucidate the origin of glass eels based on viral phylogeny. Bayesian phylogeny showed that the isolated strain had the most common ancestor with a Danish isolate and likely evolved from the French isolate of EVEX. These findings are discussed in light of the divergence of recently isolated strains and their possible contribution to the decrease of the abundance of the European eel in European waters.

Diversification of habenular organization and asymmetries in teleosts: Insights from the Atlantic salmon and European eel.

Habenulae asymmetries are widespread across vertebrates and analyses in zebrafish, the reference model organism for this process, have provided insight into their molecular nature, their mechanisms of formation and their important roles in the integration of environmental and internal cues with a variety of organismal adaptive responses. However, the generality of the characteristics identified in this species remains an open question, even on a relatively short evolutionary scale, in teleosts. To address this question, we have characterized the broad organization of habenulae in the Atlantic salmon and quantified the asymmetries in each of the identified subdomains. Our results show that a highly conserved partitioning into a dorsal and a ventral component is retained in the Atlantic salmon and that asymmetries are mainly observed in the former as in zebrafish. A remarkable difference is that a prominent left-restricted pax6 positive nucleus is observed in the Atlantic salmon, but undetectable in zebrafish. This nucleus is not observed outside teleosts, and harbors a complex presence/absence pattern in this group, retaining its location and cytoarchitectonic organization in an elopomorph, the European eel. These findings suggest an ancient origin and high evolvability of this trait in the taxon. Taken together, our data raise novel questions about the variability of asymmetries across teleosts and their biological significance depending on ecological contexts.

Glass eels and viruses - a lesson learnt from stocking the eastern German Baltic Sea coast.

Concerns about Anguillid herpesvirus 1 (AngHV-1) in European eels, especially due to stocking measures, is increasingly coming into focus and raises questions regarding disease monitoring and prevention. In the past, stocking of AngHV-1-positive eels into waters assumed AngHV-1-free has led to a rapid increase of infected eels in the wild. For this study, a glass eel stocking experiment was conducted in the eastern German Baltic coast from 2014 - 2016. Retrospective analysis of stocked glass eels shows, that virus prevalence varied from 5.3 - 37.4 % during stocking years and appeared to be influenced by a prolonged holding period in the catching region. On average, 16 % glass eels were AngHV-1 positive. Given that stocked eels were alizarin red s marked, it was possible to monitor the local eel stock between 2018 and 2019 for AngHV-1 infection and additionally two commonly coinfections, namely European Virus Eel (EVE) and Europe eel virus X (EVEX), of marked and not marked eels. In recaptured eels, mortality was significantly higher in the glass eel cohort which was found to be heavily AngHV-1 infected (stocked in 2014) compared to the less infected cohorts. Furthermore, virus-positive stocked eels turned out to be disadvantaged compared to non-infected conspecifics in terms of total length and fat content. However, co-infections with EVE and EVEX could not be detected in this study. In conclusion, eel conservation and maintaining an economically successful eel fishery is probably best achieved by stocking eels that have been shown to be free of AngHV-1. This article is protected by copyright. All rights reserved.

Impact of environmental micropollutants and diet composition on the gut microbiota of wild european eels (Anguilla anguilla).

In fish, the gut microbiome plays a crucial role in homeostasis and health and is affected by several organic and inorganic environmental contaminants. Amphidromous fish are sentinel species, particularly exposed to these stressors. We used whole metagenome sequencing to characterize the gut microbiome of wild European eels (Anguilla anguilla) at a juvenile stage captured from three sites with contrasted pollution levels in term of heavy metals and persistent organic pollutants. The objectives were to identify what parameters could alter the gut microbiome of this catadromous fish and to explore the potential use of microbiota as bioindicators of environment quality. We identified a total of 1079 microbial genera. Overall, gut microbiome was dominated by Proteobacteria, Firmicutes and Actinobacteria. Alpha and beta diversity were different amongst sites and could be explained by a reduced number of environmental and biological factors, specifically the relative abundance of fish preys in eels' diet, PCB101, γHCH (lindane), transnonachlor and arsenic. Furthermore, we identified a series of indicator taxa with differential abundance between the three sites. Changes in the microbial communities in the gut caused by environmental pollutants were previously undocumented in European eels. Our results indicate that microbiota might represent another route by which pollutants affect the health of these aquatic sentinel organisms.

Changes in Dendritic Spine Morphology and Density of Granule Cells in the Olfactory Bulb of Anguilla anguilla (L., 1758): A Possible Way to Understand Orientation and Migratory Behavior.

Olfaction could represent a pivotal process involved in fish orientation and migration. The olfactory bulb can manage olfactive signals at the granular cell (GC) and dendritic spine levels for their synaptic plasticity properties and changing their morphology and structural stability after environmental odour cues. The GCs' dendritic spine density and morphology were analysed across the life stages of the catadromous Anguilla anguilla. According to the head and neck morphology, spines were classified as mushroom (M), long thin (LT), stubby (S), and filopodia (F). Total spines' density decreased from juvenile migrants to no-migrant stages, to increase again in the adult migrant stage. Mean spines' density was comparable between glass and silver eels as an adaptation to migration. At non-migrating phases, spines' density decreased for M and LT, while M, LT, and S density increased in silver eels. A great dendritic spine development was found in the two migratory phases, regressing in trophic phases, but that could be recreated in adults, tracing the migratory memory of the routes travelled in juvenile phases. For its phylogenetic Elopomorph attribution and its complex life cycle, A. anguilla could be recommended as a model species to study the development of dendritic spines in GCs of the olfactory bulb as an index of synaptic plasticity involved in the modulation of olfactory stimuli. If olfaction is involved in the orientation and migration of A. anguilla and if eels possess a memory, these processes could be influenced by the modification of environmental stimuli (ocean alterations and rapid climate change) contributing to threatening this critically endangered species.

Corrigendum: A Transcriptomic Study Reveals That Fish Vibriosis Due to the Zoonotic Pathogen Vibrio vulnificus Is an Acute Inflammatory Disease in Which Erythrocytes May Play an Important Role.

[This corrects the article DOI: 10.3389/fmicb.2022.852677.].

Identification of virus infections of European eels intended for stocking measures.

The spread of viral diseases in eels is suggested to severely affect the European eel (Anguilla anguilla) panmictic population. The European Commission has initiated the Eel Recovery Plan (Council Regulation No. 1100/2007) to try to return the European eel stock to more sustainable levels within that measures eel restocking. However, scientific evidence evaluating the efficacy of stocking remains scarce. In addition, knowledge about the impact and contribution of eel stocking on the distribution of infectious diseases is insufficient. In this study, we aimed to investigate virus infections in batches of eels intended for restocking. We analysed samples of glass eels from certified fisheries and farmed European eels from different aquaculture farms. All analysed eels were purchased within a North Rhine Westphalian conservation program. Via a combination of cell culture and qPCR-based techniques, we detected infections of glass eels with the rhabdovirus Eel Virus European X and anguillid herpesvirus 1 infections in farmed eels (10-15 cm).

A Transcriptomic Study Reveals That Fish Vibriosis Due to the Zoonotic Pathogen Vibrio vulnificus Is an Acute Inflammatory Disease in Which Erythrocytes May Play an Important Role.

Vibrio vulnificus is a marine zoonotic pathogen associated with fish farms that is considered a biomarker of climate change. Zoonotic strains trigger a rapid death of their susceptible hosts (fish or humans) by septicemia that has been linked to a cytokine storm in mice. Therefore, we hypothesize that V. vulnificus also causes fish death by triggering a cytokine storm in which red blood cells (RBCs), as nucleated cells in fish, could play an active role. To do it, we used the eel immersion infection model and then analyzed the transcriptome in RBCs, white BCs, and whole blood using an eel-specific microarray platform. Our results demonstrate that V. vulnificus triggers an acute but atypical inflammatory response that occurs in two main phases. The early phase (3 h post-infection [hpi]) is characterized by the upregulation of several genes for proinflammatory cytokines related to the mucosal immune response (il17a/f1 and il20) along with genes for antiviral cytokines (il12ß) and antiviral factors (ifna and ifnc). In contrast, the late phase (12 hpi) is based on the upregulation of genes for typical inflammatory cytokines (il1ß), endothelial destruction (mmp9 and hyal2), and, interestingly, genes related to an RNA-based immune response (sidt1). Functional assays revealed significant proteolytic and hemolytic activity in serum at 12 hpi that would explain the hemorrhages characteristic of this septicemia in fish. As expected, we found evidence that RBCs are transcriptionally active and contribute to this atypical immune response, especially in the short term. Based on a selected set of marker genes, we propose here an in vivo RT-qPCR assay that allows detection of early sepsis caused by V. vulnificus. Finally, we develop a model of sepsis that could serve as a basis for understanding sepsis caused by V. vulnificus not only in fish but also in humans.

Microplastics across biomes in diadromous species. Insights from the critically endangered Anguilla anguilla.

Microplastic pollution affects freshwater and marine biota worldwide, microplastics occurring even inside the organisms. With highly variable effects, from physical damage to toxicity of plastic compounds, microplastics are a potential threat to the biodiversity, community composition and organisms' health. This emerging pollutant could overstress diadromous species, which are exposed to both sea and river water in their life cycle. Here we have quantified microplastics in young European eel Anguilla anguilla, a critically endangered catadromous fish, entering three rivers in southwestern Bay of Biscay. River water, sediments and seawater were also analysed for microplastics. The microplastic type was identified using Fournier-Transform Infrared spectroscopy and then searched for their hazard potential at the European Chemical Agency site. Both riverine and sea microplastic pollution were predictors of eels' microplastic profile (types of microplastics by shape and colour): A. anguilla juveniles entering European rivers already carry some marine microplastics and acquire more from river water. Potentially hazardous plastic materials were found from eels, some of them dangerous for aquatic life following the European Chemical Agency. This confirms microplastics as a potential threat for the species. Between-rivers differences for microplastics profiles persistent over years highlight the convenience of analysing and preventing microplastics at a local spatial scale, to save diadromous species from this stressor. Since the origin of microplastics present in glass eels seems to be dual (continental + seawater), new policies should be promoted to limit the entry of microplastics in sea and river waters.