Flatfish Species Research Articles

A Review of Flatfish (Order: Pleuronectiformes) Diversity in Indian Waters: A Literature Overview

The present study provides a comprehensive overview of the taxonomic diversity of flatfish in Indian waters. Flatfish play a significant role in artisanal and commercial fisheries in India and are frequently caught as by-catch in trawling operations. However, their benthic nature makes them particularly vulnerable to the pressures of overfishing and bottom trawling practices. Approximately 13% of all documented flatfish species worldwide have been recorded from India, contributing significantly to the global diversity, which includes 815 species across 124 genera and 14 families. Based on available literature, this study documented 108 flatfish species from India, represented by 35 genera in 10 families. The most dominant families identified were Cynoglossidae, Bothidae, Soleidae, and Paralichthyidae. Among the four major oceanic boundaries of India-the Arabian Sea, the Bay of Bengal, the Andaman and Nicobar regions, and the Lakshadweep waters, the Bay of Bengal was identified as having the highest recorded diversity of flatfish species. According to the IUCN classification, approximately 71 species reported from India are categorized as “Least Concern,” 34 as “Data Deficient,” two species as “Vulnerable,” and one species as “Near Threatened.” One species, Paraplagusia blochii, is still not assessed.

The effects of dissolved organic carbon and model compounds (DOC analogues) on diffusive water flux, oxygen consumption, nitrogenous waste excretion rates and gill transepithelial potential in Pacific sanddab (Citharichthys sordidus) at two salinities.

Many flatfish species are partially euryhaline, such as the Pacific sanddab which spawn and feed in highly dynamic estuaries ranging from seawater to near freshwater. With the rapid increase in saltwater invasion of freshwater habitats, it is very likely that in these estuaries, flatfish will be exposed to increasing levels of dissolved organic carbon (DOC) of freshwater origin at a range of salinities. As salinity fluctuations often coincide with changes in DOC concentration, two natural freshwater DOCs [Luther Marsh (LM, allochthonous) and Lake Ontario (LO, autochthonous) were investigated at salinities of 30 and 7.5 ppt. Optical characterization of the two natural DOC sources indicate salinity-dependent differences in their physicochemistry. LO and LM DOCs, as well as three model compounds [tannic acid (TA), sodium dodecyl sulfate (SDS) and bovine serum albumin (BSA)] representing key chemical moieties of DOC, were used to evaluate physiological effects on sanddabs. In the absence of added DOC, an acute decrease in salinity resulted in an increase in diffusive water flux (a proxy for transcellular water permeability), ammonia excretion and a change in TEP from positive (inside) to negative (inside). The effects of DOC (10mg C L-1) were salinity and source-dependent, with generally more pronounced effects at 30 than 7.5 ppt, and greater potency of LM relative to LO. Both LM DOC and SDS increased diffusive water flux at 30 ppt but only SDS had an effect at 7.5 ppt. TA decreased ammonia excretion at 7.5 ppt. LO DOC decreased urea-N excretion at both salinities whereas the stimulatory effect of BSA occurred only at 30 ppt. Likewise, the effects of LM DOC and BSA to reduce TEP were present at 30 ppt but not 7.5 ppt. None of the treatments affected oxygen consumption rates. Our results demonstrate that DOCs and salinity interact to alter key physiological processes in marine flatfish, reflecting changes in both gill function and the physicochemistry of DOCs between 30 and 7.5 ppt.

Genetic Variability and Population Genetic Structure of Marbled Flounder Pseudopleuronectes yokohamae in Korea and Japan Inferred from mtDNA Control Region Sequences

The marbled flounder (Pseudopleuronectes yokohamae) is a demersal flatfish species, widely distributed in the northwestern Pacific region. In the present study, the mitochondrial DNA (mtDNA) control region was used to determine the genetic diversity and population genetic structure of this species. We obtained a 380 bp segment of the mtDNA control region after the alignment of 78 individual sequences from P. yokohamae collected from two locations in Korea (Biungdo and Mukho) and one location in Japan (Tohoku) and 103 individual sequences from a previous study (Yokjido and Namhae). The overall haplotype diversity and nucleotide diversity were 0.983 ± 0.003 and 0.016 ± 0.008, respectively. The genealogical relationships of the mtDNA control region did not exhibit any specific genealogical association according to sampling location. The pairwise FST value indicated that the Biungdo (west coast of Korea) and Tohoku (Japan) populations showed genetically differentiated structures (but the Tohoku and Mukho populations did not). However, there was no discernible difference between the Mukho population from Korea’s eastern coast and the Yokjido and Namhae populations from the southern coast. The biological characteristics of P. yokohamae and oceanographic barriers may have contributed to producing genetically distinct populations.

Can operational tactics compensate for weaker tensile properties of biodegradable gillnets?

This study aimed at discriminating between the effects of physical strain due to the interactive effect of gear operation and degradation, and degradation only, on the differences in breaking strength, strain, and stiffness at break of polybutylene succinate-co-adipate-co-terephthalate (PBSAT) (biodegradable) and PA6 (nylon) gillnet materials, and to compare the effect of fish species and resulting capture mode on catch efficiency for a roundfish species, Atlantic cod ( Gadus morhua), and a flatfish species, European plaice ( Pleuronectes platessa) in the Danish coastal gillnet fishery. The PBSAT meshes were much weaker than PA6 already at the start of the experiment. There were 58% (95% Confidence Interval (CI): 38%–79%) less cod and 32% (CI: 17%–49%) less plaice after 4 months in the PBSAT gillnets. We did not observe significant differences between the interactive effect of gear operation and degradation, and degradation only, and in capture modes. We conclude that reduction in catch efficiency results from a combination of weaker mechanical properties worsened by degradation during, e.g., transport and storage, and a species given shape and swimming ability.

Patterns of persistence: Genetic and behavioral population complexity of winter flounder amid population declines.

Winter flounder Pseudopleuronectes americanus (Walbaum 1792) are a coastal flatfish species of economic and cultural importance that have dwindled to <15, % of their historic abundance in the southern New England/Mid-Atlantic region of the United States, with evidence indicating near-extirpation of certain local populations. This species exhibits intricate behaviors in spawning and migration that contribute to population complexity and resilience. These behaviors encompass full or partial philopatry to natal estuaries, the generation of multiple pulses of larval delivery, and partial migration. The patterns of genetic diversity within and among estuaries and cohorts presented here carry important implications in understanding the susceptibility to demographic shocks, even if the full extent of genetic diversity within and among winter flounder stocks on the US East Coast remains unresolved. Our findings reveal connectivity between estuaries in Long Island, New York, suggesting the potential for genetic rescue of depleted subpopulations. Family reconstruction and relatedness analysis indicate that split cohorts and migration contingents are not the result of genetically distinct lineages. We found no evidence for genetic structure separating these groups, and in some instances, we were able to detect closely related individuals that belonged to different migratory contingents or cohorts. Characterizing the spatial and behavioral organization of this species at the population level is crucial for comprehending its potential for recovery, not only in terms of biomass but also in reinstating the complex population structure that supports resilience. The search for generality in winter flounder spawning and migration behavior remains elusive, but perhaps the lack of generalities within this species is what has allowed it to persist in the face of decades of environmental and anthropogenic stressors.

Physiological sensitivities to hypoxia differ between co-occurring juvenile flatfishes

Coastal hypoxia threatens estuarine nursery habitat for juvenile demersal fishes worldwide, and is expected to intensify with climate change. This study examines the physiological tolerances of hypoxia in two ecologically and economically important flatfish species that co-occur in a highly eutrophic estuary and biodiversity hotspot on California's central coast, the Elkhorn Slough. Juveniles of English sole (Parophrys vetulus) occur less frequently in areas of the slough where dissolved oxygen (DO) ≤ 4.0 mg/L O2 compared to speckled sanddabs (Citharichthys stigmaeus), suggesting that English sole may be more sensitive to hypoxia. We exposed both species to an ecologically relevant, acute six-hour exposure to six DO levels ranging from ambient to severely hypoxic: 8.0, 6.0, 4.0, 3.0, 2.0, and 1.5 mg/L O2 and measured known physiological and biochemical indicators of hypoxia stress. As DO declined, metabolic rate and aerobic scope decreased for English sole, while anaerobic activity (measured by L-lactate) and ventilation rate increased for both species. Biochemical responses to hypoxia were observed only at very low DO levels (≤ 4.0 mg/L O2), indicating that both species appear to have a higher tolerance for relatively short-term hypoxia than other teleost fishes. Speckled sanddabs increased ventilation rate and anaerobic activity at higher DO levels than English sole, suggesting that earlier onset of compensatory mechanisms may contribute to the greater relative hypoxia tolerance in sanddabs. Examining the link between physiological and ecological thresholds for hypoxia tolerance will help determine suitable nursery habitat areas and management targets for estuarine restoration, and aid in predictions of fishery success under eutrophication and climate change.

Explosives leaking from dumped munition contaminate fish from German coastal waters: a reason for chronic effects?

BackgroundConventional munition dumped into the North Sea and the Baltic Sea close to the German coastline is corroding. A major concern is that biota, including fish, are negatively affected by toxic explosives leaking into marine environments. With the present study, we investigated fish living in close proximity to munition dumping sites for contamination and for signs of health impairments. The flat fish species common dab (Limanda limanda) was used as a model, since it lives in the vicinity of dumping sites and exhibits minor migratory activity. Since explosives are excreted via the bile, the bile fluids from dab were analysed. Further on we inspected the health status of the fish.ResultsDab caught in German coastal waters of the Baltic Sea and the North Sea were contaminated with explosives. Probably due to rapid metabolization, concentrations of the explosive 2,4,6-trinitrotoluene (TNT) were always below limit of detection, but its metabolites 2-amino-4,6-dinitrotoluene and 4-amino-2,6-dinitrotoluene were detected in bile fluid up to 26.36 ng/ml and 95.91 ng/ml, respectively. Only few fish from the Baltic Sea were positive for the explosive HMX, octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine with a maximum concentration of 0.89 ng/ml. Highest concentrations of TNT metabolites in bile fluid were detected in dab collected near the dumping site “Kolberger Heide” in the bay of Kiel (Baltic Sea). However, also dab from the North Sea were significantly contaminated with TNT metabolites.ConclusionsThe present study showed for the first time that fish living close to near shore munition dumping sites in the North Sea are contaminated with explosives. Various health indicators (body condition factors, externally visible fish diseases, parasites or liver anomalies) showed differences in health status between fish living in the North Sea and in the Baltic Sea, respectively. However, the health status of fish caught at the most contaminated site in the Baltic Sea was not worse compared to fish living in less contaminated areas. We conclude that fish living in the vicinity of dumping sites in the North Sea and the Baltic Sea can be significantly contaminated with explosives. However, obvious health impairments of the fish were not observed.

Evolution of rhodopsin in flatfishes (Pleuronectiformes) is associated with depth and migratory behavior.

Visual signals are involved in many fitness-related tasks and are therefore essential for survival in many species. Aquatic organisms are ideal systems to study visual evolution, as the high diversity of spectral properties in aquatic environments generates great potential for adaptation to different light conditions. Flatfishes are an economically important group, with over 800 described species distributed globally, including halibut, flounder, sole, and turbot. The diversity of flatfish species and wide array of environments they occupy provides an excellent opportunity to understand how this variation translates to molecular adaptation of vision genes. Using models of molecular evolution, we investigated how the light environments inhabited by different flatfish lineages have shaped evolution in the rhodopsin gene, which is responsible for mediating dim-light visual transduction. We found strong evidence for positive selection in rhodopsin, and this was correlated with both migratory behavior and several fundamental aspects of habitat, including depth and freshwater/marine evolutionary transitions. We also identified several mutations that likely affect the wavelength of peak absorbance of rhodopsin, and outline how these shifts in absorbance correlate with the response to the light spectrum present in different habitats. This is the first study of rhodopsin evolution in flatfishes that considers their extensive diversity, and our results highlight how ecologically-driven molecular adaptation has occurred across this group in response to transitions to novel light environments.

Finding the right plaice at the right time: Multi‐molecular analysis of flatfish reveals historical catch habitats

AbstractFlatfish are ecologically diverse species that commonly occur in marine environments, but also in estuarine and riverine habitats. This complicates the examination of the potential role of flatfish in the ‘marine fish event horizon’, an economic shift in human exploitation from freshwater to marine fish species during the 10–11th centuries CE around the southern North Sea. This study represents the first multi‐disciplinary investigation of flatfish remains to make species‐specific interpretations of flatfish exploitation. Peptide mass fingerprinting and multi‐isotope analysis of carbon (δ13C), nitrogen (δ15N) and sulphur (δ34S) was performed on collagen from 356 archaeological flatfish and 120 comparative archaeological marine or freshwater species to explore the catch habitat of individual flatfish species between 600 and 1600 CE from the North Sea area. European flounder show signals reflecting both freshwater and marine environments, while other flatfish show only those of marine habitats. A subtle shift towards more marine exploitation towards the end of the period is identified, corresponding to the observed transition in targeted species from flounder to plaice throughout the medieval period. Sites show slight differences in δ13C and δ34S within the same species, related to the local environments. Remarkable is the high abundance of marine plaice and flounder during the early medieval period, which shows clear marine or coastal exploitation of flatfish early on, well before the previously accepted onset of the marine fish event horizon. This indicates a gradual shift from coastal to open marine fish exploitation over the medieval period.

Foraging by larval fish: a full stomach is indicative of high performance but random encounters with prey are also important

Abstract This study contrasts diet composition patterns of larval fish categorized as strong and weak foragers, identified from quadratic relationships between larval length and the number of prey eaten, for 11 fish species. Two sets of alternative hypotheses test whether strong foragers (1) exhibit precocious behaviour by eating later developmental stages of copepods, and (2) take advantage of random encounters with zooplankton, based on the contrast between the two categories in each 1 mm length-class. Results indicate that strong foragers shift their feeding toward earlier copepod developmental stages, which was most apparent in four flatfish species, and demonstrate stronger overall prey selectivity than weak foragers. Inverse modeling revealed the latter is achieved through increases in apparent prey perception and/or responsiveness to dominant prey types (i.e. nauplii and copepodites) and declines for less frequent prey (e.g. veliger and Cladocera). Foraging strength increased modestly with larger eye diameter and mouth gape. Two possible explanations for prey selection patterns are that strong foragers have inherently different capacity to perceive and attack prey, or that after initially eating sufficient large prey to meet metabolic requirements fuller stomachs depend on the ability of larval fish to take advantage of random encounters.

Post-settlement dynamics of Japanese flounder Paralichthys olivaceus in Tango Bay: Seasonal patterns in growth, mortality, and recruitment potential.

Japanese flounder Paralichthys olivaceus is one of the most valuable coastal flatfish species in East Asia. To investigate post-settlement growth and mortality, juveniles were sampled in Tango Bay (Japan) weekly throughout the settlement period in 2007 and 2008. Otolith (lapillus) microstructure analysis enabled the categorization of juveniles into six biweekly cohorts each year. Later cohorts exhibited higher growth rates possibly because of higher water temperatures. A key observation was the direct relationship between high mortality and high density in mid-season cohorts in both years, pointing to density-dependent mortality. This increased mortality may be attributed to predation, including cannibalism by earlier cohorts. Furthermore, growth-selective mortality was evident soon after settlement, underscoring the vulnerability of slow growers to predation during the early juvenile stage. Although earlier and later cohorts were less abundant but showed promising recruitment potential, the prospective contribution of mid-season cohorts to the adult population remained uncertain. The results clearly highlight the importance of density-dependent mortality in population regulation in post-settlement Japanese flounder.

Ecological and phylogenetic components of flatfish ectoparasites (Pleuronectiformes: Paralichthyidae) from the Southern Gulf of Mexico.

For many years, parasite ecologists have debated the relative importance of phylogeny and ecology as drivers of parasite community structure. Here, we address this issue using data on the metazoan ectoparasite communities of different flatfish species. Twenty species of flatfish were collected along the continental shelf of the Southern Gulf of Mexico and examined for ectoparasites. Eight flatfish species were parasitized by at least one ectoparasite species. In total, 326 ectoparasites, representing 11 species (4 monogeneans, 4 copepods, 1 isopod, 1 branchiurid, and 1 leech) were removed from 1622 hosts examined. The highest prevalence (37.5%) occurred in Bomolochus sp1 from Trinectes maculatus, while the lower (0.1%) for Argulus sp., Gnathia sp. and Trachellobdella lubrica from Cyclopsetta chittendeni. Changes in the ectoparasite community structure per host species and region were evaluated using a Permutational Multivariate Analysis of Variance and represented by a multidimensional scaling analysis. Significant differences in the parasite species composition among regions and hosts were detected, but no significant interaction between regions and hosts occurred. A multivariate pairwise t-test detected significant differences in the parasite infracommunities between the Yucatan Shelf and the other two regions; in addition, significant differences were detected between C. chittendeni and the Syacium species as well as between Ancylopsetta dilecta and Syacium papillosum. In this case, there is no relationship between the patterns of ectoparasitic community structure and the inherent phylogenetic affinity of the hosts; instead, the variations in ectoparasitic communities are determined by a regional ecological component.

Endocrine patterns associated with ovarian development in female Pacific halibut (Hippoglossus stenolepis)

The Pacific halibut (Hippoglossus stenolepis) is a large migratory demersal flatfish species that occupies a top trophic role in the North Pacific Ocean and Bering Sea ecosystems, where it also supports various fisheries. As a first attempt to characterize the endocrine mechanisms driving sexual maturation in this important species, we collected pituitary, ovarian and blood samples from Pacific halibut females captured in the wild that were classified histologically into various female developmental stages. We conducted gene expression analyses of gonadotropin beta subunits in the pituitary and observed that mRNA expression levels of fshb gradually increased throughout vitellogenesis, remained elevated until before ovulation and declined after spawning. In contrast, the mRNA expression levels of lhb markedly increased during oocyte maturation and remained elevated until after spawning. Ovarian mRNA expression levels of the gonadotropin receptor genes fshr and lhr peaked during oocyte maturation and before spawning, respectively, immediately following the developmental stage at which pituitary fshb and lhb mRNA expression first reached maximum levels. The ovarian gene expression patterns of steroidogenic enzyme genes cyp19a1 and hsd20b2 paralleled those of fshr and lhr, respectively. Testosterone and 17β-estradiol (E2) plasma levels increased concomitantly with fshr and cyp19a1 mRNA expression levels, and vitellogenin plasma levels increased throughout vitellogenesis and reached maximum levels prior to spawning. These results are consistent with the notion that in female Pacific halibut, as in other teleosts, vitellogenesis and oocyte maturation and ovulation are likely under the control of pituitary gonadotropic hormones Fsh and Lh, respectively.

Relationship between stable cesium concentration and body size of Japanese flounder Paralichthys olivaceus and the effect of a size-dependent shift in diet.

To understand the relationship between the radioactive cesium (Cs) concentration in muscle of Japanese flounder Paralichthys olivaceus and the species' biological characteristics (size, sex, and age) under conditions of ecological equilibrium (i.e., distributed among ecosystem components over sufficient time, and with nearly constant ratios of Cs concentration in organisms to the concentration in water) as existed before the accident at the Fukushima Dai-ichi Nuclear Power Station (FDNPS), Japan, in 2011, we examined stable Cs, as it is thought to exist in equilibrium in the environment and behave similarly to radioactive Cs in aquatic animals. The concentration of stable Cs in 241 P. olivaceus (range 216-782 mm total length [TL]) collected in Sendai Bay, approximately 90 km north of the FDNPS, in June-July 2015 was expressed as an exponential function with size as an independent variable; the results show the concentration of stable Cs doubled with an increase in TL of 442 mm. Next, to evaluate the cause of the size-dependent change in stable Cs concentration, we examined 909 individuals (200-770 mm TL) collected in September 2013-July 2015 to determine their feeding habit based on size. Analysis of the frequency of occurrence of prey organisms in stomach contents showed that sand lance Ammodytes japonicus (55-180 mm standard length [SL]) was the most consistently consumed across size classes. Analysis on a wet-mass basis showed that A. japonicus and anchovy Engraulis japonicus (65-130 mm SL) were the main food of P. olivaceus sized 200-599 mm TL, whereas chub mackerel Scomber japonicus (120-230 mm SL) and two species of flatfishes (180-205 mm SL) were abundant in the diet of P. olivaceus sized ≥600 mm TL. All these prey items were presumed to have similar concentrations of stable Cs. Based on the above, the effect of diet on the relationship between stable Cs in muscle and fish size was considered negligible. That the diet of P. olivaceus largely did not change with size was also confirmed by C and N stable isotope ratios in P. olivaceus and their prey species. Therefore, the Cs-size relationship is probably determined by changes in the balance between the rate of Cs intake from food and seawater and the excretion rate during growth, both of which change as functions of body mass. Values of stable Cs concentrations among environmental components and animals appear to be a valid indicator for understanding the radioactive Cs distribution in the marine environment and aquatic animals under the equilibrium state, as existed before the 2011 nuclear accident.

Differences on the level of hepatic transcriptome between two flatfish species in response to liver cancer and environmental pollution levels

Environmental factors can cause cancer in both wild animals and humans. In ecological settings, genetic variation and natural selection can sometimes produce resilience to the negative impacts of environmental change. An increase in oncogenic substances in natural habitats has therefore, unintentionally, created opportunities for using polluted habitats to study cancer defence mechanisms. The Baltic and North Sea are among the most contaminated marine areas, with a long history of pollution. Two flatfish species (flounder, Platichthys flesus and dab, Limanda limanda) are used as ecotoxicological indicator species due to pollution-induced liver cancer. Cancer is more prevalent in dab, suggesting species-specific differences in vulnerability and/or defence mechanisms. We conducted gene expression analyses for 30 flatfishes. We characterize between- and within-species patterns in potential cancer-related mechanisms. By comparing cancerous and healthy fishes, and non-cancerous fishes from clean and polluted sites, we suggest also genes and related physiological mechanisms that could contribute to a higher resistance to pollution-induced cancer in flounders. We discovered changes in transcriptome related to elevated pollutant metabolism, alongside greater tumour suppression mechanisms in the liver tissue of flounders compared to dabs. This suggests either hormetic upregulation of tumour suppression or a stronger natural selection pressure for higher cancer resistance for flounders in polluted environment. Based on gene expression patterns seen in cancerous and healthy fish, for liver cancer to develop in flounders, genetic defence mechanisms need to be suppressed, while in dabs, analogous process is weak or absent. We conclude that wild species could offer novel insights and ideas for understanding the nature and evolution of natural cancer defence mechanisms.

Differences in biomarker responses and chemical contamination among three flatfish species in the Bay of Seine (NE Atlantic)

To assess the potential of the sole as sentinel species for ecotoxicological monitoring, the present study compares contaminant levels and biological responses with two closely related flatfish species: the common dab and European flounder. Trace metals, organic contaminants and biomarkers were measured in the three flatfish species collected during the same oceanographic cruise in the Bay of Seine (France). Overall, sole showed lower concentrations of Hg, met-Hg, Cd, Zn and PBDE (lw), higher concentrations of Ag, Cu, PFOS (ww), PCBs, p,p’-DDE (lw) and OH-pyrene, a higher ability to metabolize PBDEs and higher genotoxic (Comet, Micronuclei) and neurotoxic (AChE inhibition) alterations. Sole was the species most frequently occurring in the bay and appeared sensitive to chemical contamination. We therefore recommend promoting the use of the common sole for ecotoxicological monitoring.

Development of approaches to regulation of flatfish fishery in the West Bering Sea zone

It is demonstrated based on results of bottom trawl surveys, that basis of flatfish resource in the West Bering Sea zone is formed by flathead sole, Alaska plaice and northern rock sole, respectively 40.3, 35.1 and 19.6% in the total flatfish biomass averaged for the period 2000–2021. Almost three quarters of the estimated flatfish stock of the zone are within the Anadyr Gulf. In 2001–2021 the stock was relatively stable and demonstrated a slight trend to growth. Estimates of the total biomass increased from 153 thous. t in 2001 to 230–305 thous. t in 2020–2021. The analysis of the current flatfish fishery showed that the major contribution to annual catches is made by the specialized fishery by bottom trawls – 39.4% and by Danish seines – 25.4%, and about 12.8% is taken as by-catch in the walleye pollock fishery by pelagic trawls. About 77.1% of the annual catches of flatfish reported in the West Bering Sea zone are taken from 169°E in Olutor Based on the analysis of the annual flatfish catch by different types of fishery, species composition of the catches and seasonal catch dynamics, the average proportion of northern rock sole in the total annual catches was about 53.1%. It is shown, that stocks of northern rock sole in this area bear the main fishing load, and Alaska plaice and flathead sole are not developed in full. Optimal scheme of the flatfish TAC assessment for the West Bering Sea zone is developed. It includes calculation of commercial biomass of northern rock sole by cohort methods, development of its fishery regulation rule, the TAC assessment based on this rule and assessment of possible by-catch of “minor” flatfish species based on their long-term proportional contribution into the catches.

Molecular characterization and ontogenetic expression profiles of LPXRFa and its receptor in Japanese flounder (Paralichthys olivaceus)

Investigations concerning the LPXRFa system are rarely conducted in flatfish species. Here, we first identified and characterized lpxrfa and its cognate receptor lpxrfa-r genes in the Japanese flounder (Paralichthys olivaceus). The coding DNA sequence of lpxrfa was 579 bp in length, wich encoded a 192-aa preprohormone that can produce three mature LPXRFa peptides. The open reading frame (ORF) of lpxrfa-r was 1446 bp in size, and encoded a 481-aa LPXRFa-R protein that encompassed seven hydrophobic transmembrane domains. Subsequently, tissue distribution expression profiles of lpxrfa and lpxrfa-r transcripts were assayed by quantitative real-time PCR. The results indicated that expressions of lpxrfa transcripts were detected at the highest levels in the brain of both females and males, however, lpxrfa-r transcripts were remarkablely expressed in the brain tissue of female fish and in the testis tissue of male fish. Furthermore, transcript levels of lpxrfa and lpxrfa-r genes were investigated during early ontogenetic development, with the maximum expression levels at 30 days post-hatching. Overall, these data contribute to providing preliminary proof for the existence and structure of the LPXRFa system in Japanese flounder, and the study is just the foundation for researching physiological function of LPXRFa system in this species.

Fast and User-Friendly Detection of Flatfish Species (Pleuronectes platessa and Solea solea) via Loop-Mediated Isothermal Amplification (LAMP).

The detection of a Cytochrome b gene (cytb) for species differentiation in fish is intensively used. A fast alternative to expensive and time-consuming DNA barcoding is loop-mediated isothermal amplification (LAMP) in combination with efficient readout systems. For this reason, we developed LAMP assays for rapid species detection of Pleuronectes platessa and Solea solea, two economically important flatfish species in Europe that are prone to mislabeling. Species-specific primer sets targeting cytb were designed, and LAMP assays were optimized. With the optimized LAMP assays, we were able to detect up to 0.1 and 0.01 ng of target DNA of P. platessa and S. solea, respectively, and in each case up to 1% (w/w) of target species in mixtures with nontarget species. For future on-site detection, a lateral flow assay and a pocket-sized lab-on-phone assay were used as readout systems. The lab-on-phone assay with the S. solea specific primer set revealed cross-reactivity to Solea senegalensis. The assay targeting P. platessa proved to be highly specific. Both assays could be performed within 45 min and provided rapid and easy detection of fish species.

Proximate analysis, amino acid and fatty acid profiles of edible flatfish collected from Cuddalore coast, Southeast India

It is quintessential to know the nutritional value of foods we consume. Here, we have assessed the nutritional value, amino acid and fatty acid profiles of five commercialised flatfish species from the Cuddalore coastal waters in the southeast coast of India. Proximate analyses were performed to assess the moisture, ash, proteins, carbohydrates, and lipids in the muscle of edible marine flatfishes. Flatfish protein has a well-balanced amino acid composition, with major amino acids in the five fish species being valine (0.77%), methionine (0.55%), isoleucine (0.50%), lysine (0.49%), and arginine (0.49%). Five flatfish oil samples were analysed for seven fatty acids, including three saturated fatty acids and four unsaturated acids. Morotic acid (18:4) was the most abundant fatty acid, followed by palmitic acid (16:0), linolenic acid (18:3), margaric acid (17:0), oleic acid (18:1), and stearic acid (18:0). The findings of this study can be used to establish a baseline for analysing the nutritional profiles of diverse fishes in future.