Sea Bass Dicentrarchus Labrax Research Articles

Intraspecific variation in freshwater tolerance has consequences for telomere dynamics in the euryhaline teleost Dicentrarchus labrax

Stressful events can alter organism physiology at several levels triggering allostatic responses. Telomeres are well-conserved repetitive DNA sequences mainly localised at chromosome's ends, playing a crucial role in DNA stability. Analyses of telomere dynamics are new tools to assess consequences of environmental stress in non-model organisms like fish. In this study, the relationship between freshwater tolerance and telomere dynamics was investigated in the gills of the European sea bass Dicentrarchus labrax. Fluorescent in situ hybridisation of telomeric sequences revealed distal telomeres as well as intrachromosomal telomeres known as interstitial telomere sequences. In order to better understand telomere dynamics in the gills of D. labrax, we used quantitative PCR to measure telomere length and mRNA expression of the catalytic subunit of telomerase reverse transcriptase tert. For the calculation of the relative telomere length, two reference genes were tested: the single copy gene mc2r, encoding melanocortin 2 receptor and the multicopy gene 18S, encoding the 18S ribosomal RNA. We proposed a novel normalisation method to calculate the relative telomere length using both, single and multiple copy genes as references. Cell dynamics was also investigated by measuring mRNA expression of genes involved in apoptosis (caspase 8 and 9), cell proliferation (proliferation cell nuclear antigen), aerobic mitochondrial metabolism (ATP citrate-synthase), anaerobic metabolism (lactate dehydrogenase a) and antioxidant enzymatic defences (superoxide dismutase 1 and 2, catalase). Following a 15-days fresh water exposure, telomere dynamics was not significantly modified in the gills of freshwater tolerant fish. But freshwater intolerant fish exhibited telomere attrition relative to saltwater controls, and lower expression of tert in gills relative to freshwater tolerant fish. This modification of telomere dynamics in intolerant individuals was found to be correlated with lower antioxidant enzymatic defences, a higher aerobic metabolic marker and a lower cellular turnover. These data bring new perspectives for the use of telomere dynamics as an integrative marker to study environmental stress in fish, while considering individual phenotypic plasticity in response to freshwater exposure.

Antioxidant Activities and Selenogene Transcription in the European Sea Bass (Dicentrarchus labrax) Liver Depend, in a Non-linear Manner, on the Se/Hg Molar Ratio of the Feeds

Feeding 3.9 and 6.7 mg Hg/kg (Se/Hg molar ratios of 0.8 and 0.4, respectively) for 14 days negatively affected Dicentrarchus labrax growth and total DNTB- and thioredoxin-reductase (TrxR) activities and the transcription of four redox genes (txn1, gpx1, txnrd3, and txnrd2) in the liver, but a diet with 0.5 mg Hg/kg (Se/Hg molar ratio 6.6) slightly increased both reductase activities and the transcription of txn1, gpx1, and txnrd2. Feeding 6.7 mg Hg/kg for 53 days downregulated the genes of the thioredoxin system (txn1, txnrd3, and txnrd2) but upregulated gpx1, confirming the previously proposed complementarity among the antioxidant systems. Substitution of 20% of the feed by thawed white fish (hake) slightly counteracted the negative effects of Hg. The effects were not statistically significant and were dependent, in a non-linear manner, on the Se/Hg molar ratio of the feed but not on its Hg concentration. These results stress the need to consider the Se/Hg molar ratio of the feed/food when evaluating the toxicity of Hg.

Replacement of fish meal by Antarctic krill meal in diets for European sea bass Dicentrarchus labrax: Growth performance, feed utilization and liver lipid metabolism

A sustainable growth of the aquaculture sector implies the use of sustainable novel raw materials as replacers of the traditional fishmeal (FM) and fish oil (FO) ingredients. This fact has led to the development of sustainable and functional diets as part of a management strategy to reduce the effects on fish growth performance and health derived from low FM/FO dietary contents. In this sense, Antarctic krill (Euphausia superba) is considered a potential candidate in dietary inclusions to potentiate fish growth and health status. In this study, European sea bass (Dicentrarchus labrax) were fed a practical diet with either a 15% fishmeal content (KM0; control diet) or the same diet substituted by 30% (KM5; 50 g KM/kg diet) or 50% (KM7.5; 75 g KM/kg diet) Antarctic krill meal (KM) for 12 weeks in triplicates. At the end of the feeding trial, growth performance, liver morphology, liver proximate composition, lipid classes and fatty acid profiles, as well as the expression of hepatic genes related with lipid metabolism were evaluated. Fish fed KM-based diets presented higher (p < 0.05) final weight, protein and lipid efficiency ratios, specific growth rate (SGR) and improved feed conversion ratio (FCR), irrespective of the KM dietary level. Whole body and muscle proximate composition and fatty acid profiles were similar among dietary groups. Livers of European sea bass fed the experimental diets presented similar (p > 0.05) biochemical composition and fatty acid profile. However, smaller hepatocellular area and lower grade of cytoplasm vacuolization as well as a better alignment around sinusoidal spaces were found. The analyses of liver lipid classes revealed a positive correlation between the level of dietary KM and the pigmented material such as astaxanthin and free fatty acid content, as well as a negative correlation with the cholesterol levels. The expression of hepatic genes studied demonstrated a downregulation of 3-hydroxy-3-methylglutaryl-coenzyme A reductase (hmgr) and delta-6-desaturase (fads2) expression levels in fish fed KM-based diets. Besides, gene expression levels of fatty acid binding protein 7 (fabp7) and lipoprotein lipase (lpl) were significantly correlated with KM dietary levels. Altogether, these results profile KM as a potential promoter of growth and liver health in European sea bass fed low fish meal and oil diets.

Phenotypic Convergence in Sea Bass (Dicentrarchus labrax) Escaping From Fish Farms: The Onset of Feralization?

The impact of fish escaping from fish farms may depend on the extent to which escapees adapt to the natural environment, resemble wild conspecifics, and become feral. Yet, little is known about the process of feralization in marine fish. We examined phenotypic changes in body shape, body condition, and scale growth profiles of sea bass escaping from fish farms in the Canary Islands and quantified the extent to which escapees had diverged from farmed conspecifics. Most feral sea bass had sizes that overlapped with those of farmed fish, indicating that they had escaped throughout the production cycle. However, 29% of escapees were larger than the maximum size at harvesting, indicating growth in the wild. Analysis of scale growth profiles showed that some escapees had grown in the wild as fast as cultured fish, albeit at more variable growth rates. Feral sea bass tended to converge towards a similar body shape, having more streamlined bodies, lower body condition, and lower hepatosomatic indices (HSI) than fish in cages. Although our study cannot discriminate between phenotypic plasticity and differential mortality of escapees, we interpret phenotypic convergence as the likely result of a period of initial starvation, phenotypic plasticity, and selection against maladapted phenotypes. Our results warn against the risks of rearing sea bass in open-net cages and suggest that sea bass escapees could pose a threat to shallow coastal assemblages, particularly in areas where the species is not naturally found.

Influence of oceanic and climate conditions on the early life history of European seabass Dicentrarchus labrax

Understanding how marine fish early-life history is affected in the long-term by environmental and oceanographic factors is fundamental given its importance to population dynamics and connectivity. This work aimed at determining the influence of these processes on the interannual variability in hatch day and early-life growth patterns of European seabass, over a seven-year period (2011–2017) in the Atlantic Iberian coast. To accomplish this, otolith microstructure analysis was used to determine seabass hatch day and to develop early-growth correlations. In most years, hatching occurred from February to April, with two exceptions: in 2012, hatching started in early-January, and in 2016 an exceptionally long hatching period was registered. Using generalized additive models (GAM), we observed that sea surface temperature (SST), the North Atlantic Oscillation index (NAOi) and Chlorophyll-a (Chla) were the main drivers behind the inter-annual variability in seabass hatch day. Analysis of correlations between growth increments allowed assessing important periods of seabass growth and how future growth is affected. Since seawater temperature is among the main drivers for seabass recruitment and growth, its life cycle may be hampered due to ocean warming and an increasingly unstable climate, with consequences for the natural marine stocks and their harvest.

An investigation of links between metabolic rate and feed efficiency in European sea bass Dicentrarchus labrax.

Feed efficiency (FE) is the amount of body weight gain for a given feed intake. Improving FE through selective breeding is key for sustainable finfish aquaculture but its evaluation at individual level is technically challenging. We therefore investigated whether individual routine metabolic rate (RMR) was a predictor of individual FE in the European sea bass Dicentrarchus labrax, a major species in European mariculture. The European sea bass has three genetically distinct populations across its geographical range, namely Atlantic (AT), West Mediterranean (WM), and East Mediterranean (EM). We compared FE and RMR of fish from these three populations at 18 or 24 °C. We held 200 fish (62 AT, 66 WM, and 72 EM) in individual aquaria and fed them from ad libitum down to fasting. FI was assessed for an ad libitum feeding rate and for a fixed restricted ration (1% of metabolic body weight·day-1, with metabolic body weight = body weight0.8). After being refed 12 wk in a common tank, individual RMR was measured over 36 h by intermittent flow respirometry. There was a significant effect of temperature whereby fish at 18 °C had greater mean FE (P < 0.05) and lower RMR (P < 0.001). There was also a significant effect of population, where AT fish had lower FE (P < 0.05) and greater RMR (P < 0.001) than WM and EM, at both temperatures. Despite these differences in temperature and population means, individual FE and RMR were not significantly correlated (P > 0.05). Therefore, although the results provide evidence of an association between metabolic rate and FE, RMR was not a predictor of individual FE, for reasons that require further investigation.

Molecular, Cellular and Functional Analysis of TRγ Chain along the European Sea Bass Dicentrarchus labrax Development.

In jawed vertebrates, adaptive immune responses are enabled by T cells. Two lineages were characterized based on their T cell receptor (TcR) heterodimers, namely αβ or γδ peptide chains, which display an Ig domain-type sequence that is somatically rearranged. γδ T cells have been less extensively characterized than αβ and teleost fish, in particular, suffer from a severe scarcity of data. In this paper, we worked on the well-known model, the European sea bass Dicentrarchus labrax, to broaden the understanding of teleost γδ-T cells. The T cell receptor chain (TR) γ transcript was expressed at a later developmental stage than TRβ, suggesting a layered appearance of fish immune cells, and the thymus displayed statistically-significant higher mRNA levels than any other organ or lymphoid tissue investigated. The polyclonal antibody developed against the TRγ allowed the localization of TRγ-expressing cells in lymphoid organs along the ontogeny. Cell positivity was investigated through flow cytometry and the highest percentage was found in peripheral blood leukocytes, followed by thymus, gut, gills, spleen and head kidney. Numerous TRγ-expressing cells were localized in the gut mucosa, and the immunogold labelling revealed ultrastructural features that are typical of T cells. At last, microalgae-based diet formulations significantly modulated the abundance of TRγ+ cells in the posterior intestine, hinting at a putative involvement in nutritional immunity. From a comparative immunological perspective, our results contribute to the comprehension of the diversity and functionalities of γδ T cells during the development of a commercially relevant marine teleost model.

Contamination levels and habitat use influence Hg accumulation and stable isotope ratios in the European seabass Dicentrarchus labrax

Hg accumulation in marine organisms depends strongly on in situ water or sediment biogeochemistry and levels of Hg pollution. To predict the rates of Hg exposure in human communities, it is important to understand Hg assimilation and processing within commercially harvested marine fish, like the European seabass Dicentrarchus labrax. Previously, values of Δ199Hg and δ202Hg in muscle tissue successfully discriminated between seven populations of European seabass. In the present study, a multi-tissue approach was developed to assess the underlying processes behind such discrimination.We determined total Hg content (THg), the proportion of monomethyl-Hg (%MeHg), and Hg isotopic composition (e.g. Δ199Hg and δ202Hg) in seabass liver. We compared this to the previously published data on muscle tissue and local anthropogenic Hg inputs.The first important finding of this study showed an increase of both %MeHg and δ202Hg values in muscle compared to liver in all populations, suggesting the occurrence of internal MeHg demethylation in seabass. This is the first evidence of such a process occurring in this species. Values for mass-dependent (MDF, δ202Hg) and mass-independent (MIF, Δ199Hg) isotopic fractionation in liver and muscle accorded with data observed in estuarine fish (MDF, 0–1‰ and MIF, 0–0.7‰). Black Sea seabass stood out from other regions, presenting higher MIF values (≈1.5‰) in muscle and very low MDF (≈-1‰) in liver. This second finding suggests that under low Hg bioaccumulation, Hg isotopic composition may allow the detection of a shift in the habitat use of juvenile fish, such as for first-year Black Sea seabass.Our study supports the multi-tissue approach as a valid tool for refining the analysis of Hg sourcing and metabolism in a marine fish. The study’s major outcome indicates that Hg levels of pollution and fish foraging location are the main factors influencing Hg species accumulation and isotopic fractionation in the organisms.

Family-effects in the epigenomic response of red blood cells to a challenge test in the European sea bass (Dicentrarchus labrax, L.)

BackgroundIn fish, minimally invasive blood sampling is widely used to monitor physiological stress with blood plasma biomarkers. As fish blood cells are nucleated, they might be a source a potential new markers derived from ‘omics technologies. We modified the epiGBS (epiGenotyping By Sequencing) technique to explore changes in genome-wide cytosine methylation in the red blood cells (RBCs) of challenged European sea bass (Dicentrarchus labrax), a species widely studied in both natural and farmed environments.ResultsWe retrieved 501,108,033 sequencing reads after trimming, with a mean mapping efficiency of 73.0% (unique best hits). Minor changes in RBC methylome appeared to manifest after the challenge test and a family-effect was detected. Only fifty-seven differentially methylated cytosines (DMCs) close to 51 distinct genes distributed on 17 of 24 linkage groups (LGs) were detected between RBCs of pre- and post-challenge individuals. Thirty-seven of these genes were previously reported as differentially expressed in the brain of zebrafish, most of them involved in stress coping differences. While further investigation remains necessary, few DMC-related genes associated to the Brain Derived Neurotrophic Factor, a protein that favors stress adaptation and fear memory, appear relevant to integrate a centrally produced stress response in RBCs.ConclusionOur modified epiGBS protocol was powerful to analyze patterns of cytosine methylation in RBCs of D. labrax and to evaluate the impact of a challenge using minimally invasive blood samples. This study is the first approximation to identify epigenetic biomarkers of exposure to stress in fish.

Phylogeography of betanodavirus genotypes circulating in Tunisian aquaculture sites, 2012-2019.

The purpose of this study was to determine the phylogenetic relationships among the primary betanodavirus strains circulating in Tunisian coastal waters. A survey was conducted to investigate nodavirus infections at 15 European sea bass Dicentrarchus labrax and gilthead sea bream Sparus aurata farming sites located along the northern and eastern coasts of Tunisia. The primary objective of the study was to create epidemiological awareness of these infections by determining phylogenetic relationships between the main betanodavirus strains circulating during the period 2012-2019, using RNA1 and/or RNA2 genome segments. Approximately 40% (118 of 294) tissue pools tested were positive for betanodavirus. Positive pools were distributed across all of the sampling sites. While fish mortalities were always correlated with the presence of virus in sea bass, a severe outbreak was also identified in sea bream larvae in 2019. Phylogenetic analysis revealed that almost all Tunisian strains from both sea bass and sea bream irrespective of outbreaks clustered within the RGNNV genotype. It is noteworthy that samples collected during the 2019 outbreak from sea bream contained both RNA1 and RNA2 fragments belonging to the RGNNV and SJNNV genotype, respectively, an indication of viral genome reassortment. To our knowledge, this is the first report of reassortant betanodavirus in Tunisia.

Associated Immune Status of Cultured Sea Bass Dicentrarchus Labrax Vaccinated with Killed Vaccine Against Vibriosis

Associated Immune Status of Cultured Sea Bass Dicentrarchus Labrax Vaccinated with Killed Vaccine Against Vibriosis

Evaluation of absorption and depletion of florfenicol in European seabass Dicentrarchus labrax.

The pharmacokinetic properties and residue elimination of florfenicol (FLO) and its amine were investigated in European seabass Dicentrarchus labrax at 24°C. The trial mainly included analysis of FLO in plasma after a single dose dietary administration of 10 mg/kg and in muscle plus skin following a multiple dosing (10 mg kg−1 day−1 for 7 days) to estimate pharmacokinetics and residue depletion, respectively. The maximum plasma concentration of FLO was measured to be 1.64 μg/ml, 4 hr post administration. The elimination half‐life (t 1/2b) and the area under the concentration‐time curve extrapolated to infinity (AUC0‐∞) were calculated to be 13.0 hr and 34.7 μg h−1 ml−1, respectively. Withdrawal times of FLO and its amine were calculated to be 46.3 degree‐days, indicating a fast removal from the edible tissues of treated European seabass. Overall, FLO can be considered as a potentially efficient antibacterial agent for farmed European seabass provided that additional efforts will be devoted towards its in vitro and clinical efficacy.

Fshr: a fish sex-determining locus shows variable incomplete penetrance across flathead grey mullet populations.

SummaryWhole-genome sequencing data were produced from a single flathead grey mullet female and assembled into a draft genome sequence, whereas publicly available sequence data were used to obtain a male draft sequence. Two pools, each consisting of 60 unrelated individuals, respectively, of male and female fish were analyzed using Pool-Sequencing. Mapping and analysis of Pool-Seq data against the draft genome(s) revealed >30 loci potentially associated with sex, the most promising locus of which, encoding the follicle-stimulating hormone receptor (fshr) and harboring two missense variants, was genotyped on 245 fish from four Mediterranean populations. Genotype data showed that fshr represents a previously unknown sex-determining locus, although the incomplete association pattern between fshr genotype and sex-phenotype, the variability of such pattern across different populations, and the presence of other candidate loci reveal that a greater complexity underlies sex determination in the flathead grey mullet.

Draft Genome Sequence of Mycobacterium hippocampi DL, Isolated from European Sea Bass (Dicentrarchus labrax).

Mycobacterium hippocampi is an acid-fast opportunistic pathogen associated with infections in aquatic animals. Here, we report the draft genome sequence of M. hippocampi strain DL, an isolate from cultured European sea bass (Dicentrarchus labrax) associated with systemic symptomatology.

Nerocila orbignyi, Cymothoid isopoda infestation in European sea bass, Dicentrarchus labrax. Trials for treatment with evaluation of immune and antioxidative responses.

Cymothoid isopods cause serious infestations in fish that may adversely affect the aquaculture. Current study targeted investigating the enormous infestation of European sea bass Dicentrarchus labrax with cymothoid isopod, Nerocila species in a marine fish farm at Kafr El Sheikh Governorate, Egypt; accompanied with trials for treatment with malathion 57% at 0.15 and 0.30 mg/L concentrations and evaluation of the immune and antioxidants responses. For this purpose, 450 D. labrax were randomly collected alive during April 2020 and thoroughly examined for detection of external parasites. Malathion 57% treatment trial was carried out. Blood and tissue samples were collected from infested fish at zero-day (before malathion treatment) and at 24hrs, 48hrs, 3rd day, 4th day and 5th day (after malathion treatment). Isopods collected from infested fish were identified as N. orbigny and fish's external body surface and gills were the predilection sites. Both malathion concentrations were effective in eradicating N. orbigny and decreasing the isopod prevalence from 100 to 0% at the 5th day sampling point. Infested fish revealed improved hematological parameters (Erythrocyte counts, hemoglobin, and hematocrit readings), however leucocyte and differential cells counts showed significant decrease in infested fish. Immunological parameters (Lysozyme and IgM activities) decreased, but nitric oxide increased by time after malathion application. Nerocila is considered serious parasite in aquaculture that negatively affects fish's immune and antioxidative responses; and malathion denoted as a promising treatment to control N. orbigny in infested D. labrax.

Seasonal dynamics of parasite Lernanthropus kroyeri (van Beneden, 1851) on cultured sea bass Dicentrarchus labrax (Linnaeus, 1758) from the Adriatic Sea

Due to the great economic importance of sea bass Dicentrarchus labrax (Linnaeus, 1758) in Mediterranean mariculture, the aim of this study was to explore the seasonal dynamics of the copepod parasite Lernanthropus kroyeri (van Beneden, 1851) on the gills of cultured sea bass. This is the first study of L. kroyeri on cultured sea bass from the Adriatic Sea. Sea bass (N = 457) samples were taken monthly from October 2012 to October 2014, from the same floating net cage. A significant positive correlation (r = 0.24; P < 0.0001), was found between the sea bass length and the total number of parasites, and between the fish length and egg string length of L. kroyeri (r = 0.647; P < 0.0001). Prevalence of L. kroyeri ranged from 94.4% to 100%. Female parasites dominated the population throughout the research period (72.6%), followed by males (25%) and immature parasites (2.4%). Gravid females were most dominant during the summer season, with their numbers significantly dropping in the winter. Observed seasonal dynamics of L. kroyeri reproductive patterns suggest that the winter season may be the best period for control and treatment of the infection. Furthermore, this new data may provide a basis for the development of epidemiological models with a focus on preventive measures in order to improve health management of mariculture in the Adriatic Sea, as well as in the whole Mediterranean area.

A spatially explicit individual-based model to support management of commercial and recreational fisheries for European sea bass Dicentrarchus labrax

The European sea bass (Dicentrarchus labrax) is a slow growing and late maturing high value fish that is exploited by both commercial and recreational fisheries. In recent years, scientific assessments have shown a rapid decline in spawning stock biomass around the UK attributed to poor recruitment (driven by environmental factors) and high fishing mortality. This resulted in significant reductions in the harvest of sea bass following technical measures implemented by the European Commission to conserve stocks. Individual-based models (IBMs) are simulations of individual ‘agents’ of organisms that interact with each other and their environment locally and have been shown to be effective management tools in many systems. Here, an IBM that simulates the population dynamics and spatial distribution of sea bass was developed to assess how technical management measures applied to subsets of the population impact the overall stock. Conventional stock assessment techniques were used to model the processes affecting population dynamics, while the spatial distribution was simulated using a combination of temperature preferences and information from tagging studies. The IBM was parameterised using existing knowledge from the literature and can mimic key assessment outputs used to inform management and advice on fishing opportunities. Utility of the IBM is demonstrated by simulating the population consequences of several key management scenarios based on those implemented by the European Commission, including short-term bans on pelagic trawling in spawning areas, commercial and recreational catch limits and increasing the minimum conservation reference size. The IBM has potential to complement the annual stock assessment in managing European sea bass because it models individual movement, environmental drivers and emergent spatial distribution, thereby providing enhanced predictions of management strategy outcomes that could inform spatial advice on fishing opportunities and policy.

Quantifying microplastic translocation from feed to the fillet in European sea bass Dicentrarchus labrax

Uptake of microplastic (MP) particles has been detected in a large number of marine organisms, and humans may consequently be exposed to high MP levels in consumed seafood. Yet there remains a dearth of knowledge regarding uptake into commercially important finfish and tissues (fillets) most commonly consumed by humans. The current study detected translocation of MP into the fillet of European seabass (Dicentrarchus labrax) juveniles fed with a diet containing fluorescent MP particles (1–5 μm) for 16 weeks. Results indicate MP translocation to fillet, with mean fillet MP content calculated as 0.36 ± 0.29 MP/g−1 fillet on the basis of manual fluorescent microscopy counts and 0.15 ± 0.28 MP/g−1 fillet on the basis of automated fluorescent microscopy counts. The retention/translocation rate is estimated at approximately 1 MP reaching the fillet for every 1.87 × 107 ingested in the experimental diet. This study presents first data of MP translocation from feed to the tissue fillet in European seabass and aids future examinations of the MP load in seafood for human consumption.

Exploring the potential of seaweed Gracilaria gracilis and microalga Nannochloropsis oceanica, single or blended, as natural dietary ingredients for European seabass Dicentrarchus labrax

Gracilaria gracilis and Nannochloropsis oceanica, single or blended, were tested in European seabass (Dicentrarchus labrax) diets. A control (CTRL) diet was compared with experimental diets including either 8% G. gracilis (GRA8), 8% N. oceanica (NAN8), or a blend of 4% of each alga (NAN4GRA4). After 106 days of feeding, growth, nutrient utilization, antioxidant defense, immunological status, and end-product quality were evaluated. All fish exhibited similar feed intake (1.4–1.5%), body weight, growth, and feed conversion ratio (1.6). Dietary inclusion N. oceanica did not affect digestible N intake and gain. Fish fed GRA8 had the lowest digestible N and energy intake (P < 0.05), and simultaneously the highest nitrogen retention efficiency and energy retention efficiency, resulting in a N and energy gain similar to all other treatments. All fish had well-preserved intestinal morphology; feeding NAN8 resulted in a significant increase in neutral goblet cells compared with GRA8. Fish fed the algal diets had significantly lower (P < 0.05) hepatosomatic index (1.7–1.8 vs 2.1) and plasma triglyceride levels than CTRL, but whole body composition remained similar among treatments. The liver total antioxidant capacity of fish fed NAN8 was significantly higher than that of fish fed GRA8 but did not differ significantly from the CTRL group. NAN4GRA4 resulted in lower values of total glutathione, glutathione peroxidase, and alternative complement. N. oceanica decreased fillet springiness; however, with NAN4GRA4, the muscle fillet became less resilient. G. gracilis and N. oceanica biomass, either used single (8%) or blended (4% each), can be valuable natural ingredients for partial replacement of fish meal in European seabass diets.

Mate finding in the isopod parasite Ceratothoa oestroides (Risso, 1816) in an aquaculture environment: A case study in the sea bass Dicentrarchus labrax (Linnaeus, 1758)

The purpose of this in situ study was to determine the pattern of protandrous hermaphrodite Ceratothoa oestroides infestation in sea bass with respect to parasite sexual behaviour. Sea bass (N = 618) were divided into six net cages which presented three experimental groups. Group 1 represented infected fish, where female parasites were manually removed and male parasites were left in the buccal cavity of fish. Group 2 represented infected fish where both female and male parasites were removed, and group 3 represented uninfected fish (i.e., the control group). The duration of the experiment was 865.47 degree days (ºD), and average sea bass mortality rate was 11.97 %. No mature C. oestroides females were found at the end of the experiment. Fish in cages with removed female parasites (group 1) had a statistically higher number of parasites in pairs at the end of the experiment, compared to the group where all parasites - both males and females - were removed (group 2) and control group (group 3) (P < 0.005). Female parasites from group 1, being larger and older, had more success in mate finding. These results suggest that the female parasite uses some method of communication to attract the pulli II, i.e., the future male parasite, necessary for reproduction. Because there is a lack of parasite biology studies, especially in situ, our data on reproduction and mating behaviour of C. oestroides could help in the development of overall parasitic removal procedures and health management in aquaculture.