Flatfish Species Research Articles

Seasonality in the condition and variability in somatic relationships of megrim (Lepidorhombus whiffiagonis) in the northeast Atlantic

Somatic parameters' estimation of megrim (Lepidorhombus whiffiagonis), an economically important flatfish species and relevant one in the benthic community, is essential in fisheries biology. It helps us to deepen our understanding of the species life history traits and an adequate stock assessment and fishery management. Le Cren's condition factor (K), weight-length relationships and weight conversion factors were studied in megrim for more than two decades time-series (1998–2019) and wide area with a relevant fishing activity and the species is distributed into two stocks: northern Bay of Biscay-nBB and Celtic Sea-CS stock, and southern Bay of Biscay-sBB and Galician waters-Gw stock. Around 30,000 specimens, mostly collected from commercial landings by the research institutions IEO and AZTI, were sampled. Total fish length, total weight and commercial gutted weight were obtained, and five-year periods were defined throughout in the time-series. The influence of several factors, as seasonality, temporality, fish sex, maturity, abundance, and environmental conditions on the megrim condition were analyzed for each stock by GLM, being the season the most relevant factor. A clear seasonal pattern found for K, with the lowest values in spring, was found in both populations for the first time based on a multi-year period analysis. The pattern was more prominent in females and less marked in immature, related tothe reproduction process. The lowest condition value was observed in May in nBB-CS and a month earlier in sBB-Gw (both after the spawning period). A progressive increase (favorable feeding conditions) followed to highest values in November–February, showing the relevance of K as indicator of the nutritional/reproductive status of megrim. Temporality (five-year periods) also had a much smaller effect on condition than seasonality. Megrim condition was inversely related to its abundance, and to the NAO index in the nBB-CS stock. Total weight-length, gutted weight-length and total weight-gutted weight relationships were fitted for each megrim stock and their temporal variations were analyzed. The large sample size, size range and time-series available allowed obtaining robust somatic parameters for the total weight-length relationships (a = 0.0049, b = 3.1012; a = 0.0041, b = 3.1682, respectively in nBB-CS stock and sBB-Gw stock), gutted weight-length relationships (a = 0.0046, b = 3.1033; a = 0.0042, b = 3.1510, respectively in nBB-CS and sBB-Gw stock), and weight conversion factors (1.049; 1.056, respectively in nBB-CS and sBB-Gw stock). They were considered the most appropriate to be used in the stock assessment of both stocks. A historical analysis of the temporal variability of the weight-length relationships showed different trends between stocks.

Integration of Maps Enables a Cytogenomics Analysis of the Complete Karyotype in Solea senegalensis.

The Pleuronectiformes order, which includes several commercially-important species, has undergone extensive chromosome evolution. One of these species is Solea senegalensis, a flatfish with 2n = 42 chromosomes. In this study, a cytogenomics approach and integration with previous maps was applied to characterize the karyotype of the species. Synteny analysis of S. senegalensis was carried out using two flatfish as a reference: Cynoglossus semilaevis and Scophthalmus maximus. Most S. senegalensis chromosomes (or chromosome arms for metacentrics and submetacentrics) showed a one-to-one macrosyntenic pattern with the other two species. In addition, we studied how repetitive sequences could have played a role in the evolution of S. senegalensis bi-armed (3, and 5–9) and acrocentric (11, 12 and 16) chromosomes, which showed the highest rearrangements compared with the reference species. A higher abundance of TEs (Transposable Elements) and other repeated elements was observed adjacent to telomeric regions on chromosomes 3, 7, 9 and 16. However, on chromosome 11, a greater abundance of DNA transposons was detected in interstitial BACs. This chromosome is syntenic with several chromosomes of the other two flatfish species, suggesting rearrangements during its evolution. A similar situation was also found on chromosome 16 (for microsatellites and low complexity sequences), but not for TEs (retroelements and DNA transposons). These differences in the distribution and abundance of repetitive elements in chromosomes that have undergone remodeling processes during the course of evolution also suggest a possible role for simple repeat sequences in rearranged regions.

Contemporary analyses of comparative fishing data: a case study of Thorny skate on the Grand Banks (NAFO Divisions 3LNOP)

We re-analyze Thorny skate data from two comparative fishing experiments conducted by DFO in 1995 and 1996 using improved and more contemporary methods to estimate the relative efficiency of the Campelen 1800 demersal shrimp trawl survey protocol compared to the Engel 145 otter trawl. We correct possible bias in the method previously applied to these data. We investigate if there are size-based differences and if depth or spatial regions have important effects on results. We also investigate the influence and robustness of the estimation procedures, which was a concern in the original analyses of these data for other groundfish species. We did not find strong evidence that the relative efficiency of the Campelen trawl protocol compared to the Engel was different for smaller-sized Thorny skate compared to larger ones. However, we conclude that there is a potential that size-based differential catchability existed but there is insufficient information to reliably estimate these effects for Thorny skate. We also found evidence of significant differences in relative efficiency among NAFO Divisions and experiments, which is similar to other flatfish species. However, the mechanisms for these differences are unknown and it is not clear if spatial estimates should be used when converting Engel indices to Campelen equivalents. Hence, we do not recommend a different Engel-Campelen conversion factor than the one currently used in stock assessments for Thorny skates on the Grand Banks (NAFO Divisions 3LNOPs).

First Autonomous Underwater Vehicle Observations of a Potential Petrale Sole (Eopsetta jordani) Spawning Aggregation Off the US West Coast

Understanding the timing and reproductive behavior of commercial fish species is a key part of well-informed stock assessments and fishery management, but this information is often limited, particularly for species that spawn in deep water. Petrale sole (Eopsetta jordani) is one of the most commercially important flatfish species in the US and is known to spawn off the West Coast during winter months. A number of spawning grounds have been identified using catch data and tagging studies, but to our knowledge there have been no direct visual observations of aggregating petrale sole. In 2018, we observed unusually high densities of petrale sole in autonomous underwater vehicle (AUV) imagery collected at a feature inshore of Santa Lucia Bank off the California coast. In addition to aggregations of fish, we observed fish in positions and configurations that we believe to be related to spawning behavior including physical contact between individuals and evidence of consistent size differentiation within clusters. We present images of the aggregating petrale sole and information on the physical and environmental conditions at this site. We also compare these observations to the results of AUV surveys carried out in 2005 at the same site. Analysis of commercial catch data from this area supports the hypothesis that this is a seasonal petrale sole aggregation as fishing at this location occurs mainly during winter months with catches dominated by petrale sole. In addition to the potential identification of a spawning aggregation, these observations show how advanced technologies can provide insight into the reproductive behavior of a commercially important species in-situ.

Identification of lncRNA-miRNA-mRNA Network Involved in Sexual Size Dimorphism of Chinese Tongue Sole (Cynoglossus semilaevis)

Chinese tongue sole (Cynoglossus semilaevis) is a flatfish species unique to Northeast Asia, exhibiting the typical female-biased sexual size dimorphism (SSD). To explore the possible regulatory roles of non-coding RNAs (ncRNAs) on this phenomenon, whole transcriptomic analysis was performed by using female, male, and pseudomale C. semilaevis to identify differentially expressed (DE) long ncRNAs (DE lncRNAs), microRNA (DE miRNAs), and differentially expressed genes (DEGs) from the brain, gonad, liver, and muscle tissues. Most of them were concentrated in the gonad and muscle, and the gene expression patterns of pseudomale individuals were similar to male individuals. The association of DE lncRNAs and target messenger RNAs (mRNAs) was predicted based on antisense, cis-, and trans-regulatory mechanisms, with enriched protein digestion and absorption, cyclic adenosine monophosphate (cAMP) signaling pathway, sulfur metabolism, cell cycle, and splicesome (p < 0.05). Furthermore, weighted gene co-expression network analysis (WGCNA) was employed to cluster the expression patterns of DE lncRNA, and two modules (greenyellow and blue) had the highest positive and negative correlations with growth traits, respectively. Importantly, the female-biased expression in the greenyellow module and the male- and pseudomale-biased expression in the blue module were observed in the gonad. The target gene analysis for DE miRNA revealed 3,034 mRNA-miRNA pairs with the opposite expression patterns. Finally, the lncRNA-miRNA-mRNA network, including 385 DE lncRNAs, 138 DE miRNAs, and 456 DEGs, was constructed. Among which, 78 DE lncRNAs, 12 DE miRNAs, and 13 DEGs involved in cell growth and death pathway were related to the SSD of C. semilaevis. This study described the lncRNA-miRNA-mRNA regulatory network in the SSD of C. semilaevis for the first time. The functional prediction analysis suggested that these DE lncRNAs and DE miRNAs might be involved in flatfish SSD by regulating several potential growth-related pathways (e.g., cell cycle, cAMP signaling, and Rap1 signaling). Further studies related to these ncRNAs will enlarge our understanding of the regulatory effects of ncRNAs on fish SSD.

Genetic diversity and signatures of selection in the mito-gynogenetic olive flounder Paralichthys olivaceus revealed by genome-wide SNP markers

The artificial gynogenesis technique has been proven to be an effective genetic tool and has been widely used in fish breeding. The evaluation of the genetic basis of the artificial gynogenetic population is essential for breeding and farming, and the detection of the signatures of selection is potential to elucidate genes and mutations associated with important phenotypic traits. Olive flounder Paralichthys olivaceus is a commercially important flatfish species cultured in China, Japan, and Korea, and artificial gynogenesis has been used for its genetic breeding. In this study, the genetic variation of the artificial mito-gynogenetic population and common population of the flounder were analyzed using SNP markers with 2b-RAD genotyping technique. A total of 162 mito-gynogenetic and 90 common flounders were used, and 39,018 high-quality genome-wide single nucleotide polymorphisms (SNPs) were identified across two populations. The average observed heterozygosity (Ho) and average expected heterozygosity (He) of SNP were 0.1895 and 0.2413, and 0.0222 and 0.2974 in the common and mito-gynogenetic populations, respectively. Most loci significantly deviated from Hardy-Weinberg equilibrium (HWE) after Bonferroni correction in the former population. However, the genetic distance between the two populations suggested no significant genetic differentiation. Furthermore, analysis of run of homozygosity (ROH) showed low level of genomic inbreeding in the common population (inbreed coefficient was 0.2171) compared to higher inbreeding in the mito-gynogenetic population (inbreed coefficient was 0.9227). Short ROHs were more frequent in the common population, while long ROHs were more frequent in the mito-gynogenetic population, providing insight in the developmental history of the flounder. A total of 580 genes with putative selection signatures were identified across the two populations based on selective sweeps. These genes were involved in 39 KEGG pathways associated with some important traits, such as reproduction (e.g., sox8 and sox9) and immune response (e.g., dctn5 and socs-1). These results would be helpful for molecular marker-assisted breeding and conservation of genetic resources in the olive flounder and other marine culture fish.

Benthic food web structures as an explanation for prolonged ecological half-life of 137Cs in flatfish species in the Fukushima coastal area

After the accident at the Fukushima Daiichi Nuclear Power Plant (FDNPP), Japan, in March 2011, 137Cs in demersal fish had, between 2011 and 2015, a prolonged ecological half-life when compared to pelagic fish. Using stable isotope mixing models combined with gut content analysis and 137Cs activity concentrations, this study investigated the hypothesis that an unexplored food web structure could be a contributing factor explaining the ecological half-life of 137Cs in benthic flatfish. Benthic invertebrates and demersal fish species sampled in 2015 still showed 137Cs activity concentrations higher than pre-accident. The mixing models of stable N and C isotopes and gut content analysis identified deposit, suspension and filter feeders to be the main flatfish food items in the benthos. There was a significant correlation between 137Cs activity concentrations in specific flatfish species and benthos, and between 137Cs activity concentrations in benthos and surface sediment. The results of this study partially explained the 137Cs activity concentrations found in the analysed demersal fish, suggesting that the benthos can be a continuous source of 137Cs for the demersal fish during this period of time. Extending monitoring programmes to include invertebrates that are not food species for humans would greatly improve our ability to understand the role of trophic transfer pathways and take appropriate management actions.

Reducing cod bycatch in flatfish fisheries

Management measures to facilitate the recovery of fish stocks can lead to shifts in traditional fishing patterns and target species. In the Baltic Sea, drastic reductions in catch quota for cod (Gadus morhua) force mixed demersal trawl fisheries to avoid cod bycatch and focus on flatfish species. This study developed and tested a simple selection concept that aims to avoid cod bycatch in flatfish-directed trawl fisheries by removing a section of the top panel from the extension piece of the trawl (roofless concept). Sea trials testing the performance of a baseline roofless design, and two designs intended to enhance escape reactions of cod were conducted during two sea cruises. Analysis of the resulting catch data revealed that applying the baseline roofless design consistently reduced cod bycatch by ∼75%. Catches of the target species plaice (Pleuronectes platessa) and flounder (Platichthys flesus) were reduced by less than 15%; however, we estimated that catch losses of the two flatfish species could be balanced by increasing fishing effort to ∼8% and ∼12%, respectively. None of the two roofless designs, intended to enhance escape reactions on cod, improved the performance of the baseline design. Under the scenario of fishery choke caused by limited cod quotas, we estimated that the use of the baseline roofless concept could increase fishing possibilities for flatfish by more than 300%.

Identification of Potential Blind-Side Hypermelanosis-Related lncRNA-miRNA-mRNA Regulatory Network in a Flatfish Species, Chinese Tongue Sole (Cynoglossus semilaevis).

Blind-side hypermelanosis has emerged as a major concern in commercial rearing environments of the flatfish aquaculture industry. To date, the underlying molecular mechanisms are not well understood. To fill this gap, in this study, whole transcriptomic sequencing and analyses were performed using normal skins and hypermelanic skins of the blind side of Chinese tongue sole (Cynoglossus semilaevis). Differentially expressed long non-coding RNAs (DElncRNAs), miRNAs (DEmiRNAs), and differentially expressed genes as well as their competing endogenous RNA (ceRNA) networks were identified. A total of 34 DElncRNAs, 226 DEmiRNAs, and 610 DEGs were identified. Finally, lncRNA–miRNA–mRNA regulatory networks (involving 29 DElncRNAs, 106 DEmiRNAs, and 162 DEGs) associated with blind-side hypermelanosis were constructed. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analyses of 162 DEGs in ceRNA networks identified DEGs (e.g., oca2, mc1r, and ihhb) in pigmentation-related biological processes and DEGs (e.g., ca4, glul, and fut9) in nitrogen metabolism, glycosphingolipid biosynthesis, and folate biosynthesis pathways, as well as their corresponding DElncRNAs and DEmiRNAs to potentially play key regulatory roles in blind-side hypermelanosis. In conclusion, this is the first study on the ceRNA regulatory network associated with blind-side hypermelanosis in flatfish. These new findings expand the spectrum of non-coding regulatory mechanisms underpinning blind-side hypermelanosis, which facilitates the further exploration of molecular regulatory mechanisms of malpigmentation in flatfish.

Short-term exposure to pharmaceuticals negatively impacts marine flatfish species: Histological, biochemical and molecular clues for an integrated ecosystem risk assessment

The marine habitat and its biodiversity can be impacted by released pharmaceuticals. The short-term (7 days) effect of 3 commonly used drugs – warfarin, dexamethasone and imidazole – on Senegalese sole (Solea senegalensis) juveniles was investigated. Occurrence of hemorrhages, histopathological alterations, antioxidant status, activity of antioxidant enzymes and expression of genes involved in the xenobiotic response (pxr, abcb1 and cyp1a), were evaluated. The results showed a time and drug-dependent effect. Warfarin exposure induced hemorrhages, hepatocyte vacuolar degeneration, and altered the activity of glutathione peroxidase (GPx) and the expression of all the studied genes. Dexamethasone exposure increased liver glycogen content, altered antioxidant status, GPx and superoxide dismutase activities, as well as abcb1 and cyp1a expression. Imidazole induced hepatocyte vacuolar degeneration and ballooning, and altered the antioxidant status and expression of the tested genes. The present work anticipates a deeper impact of pharmaceuticals on the aquatic environment than previously reported, thus underlining the urgent need for an integrated risk assessment.

Experimental challenge of flatfishes (Pleuronectidae) with salmonid alphavirus (SAV): Observations on tissue tropism and pathology in common dab Limanda limanda L.

Salmonid alphavirus (SAV) is the aetiological agent of pancreas disease (PD), a serious viral disease in salmonids. For several decades, SAV was known to infect salmonid species only, until SAV was detected using real-time PCR in several species of wild-caught flatfishes in Scotland in 2010. The presence of SAV in wild flatfishes has been confirmed by further surveys from Ireland and Scotland. The role of flatfishes in SAV-spread and epizootiology has not been elucidated, and no experimental challenges have been conducted to examine virus tissue tropism, virulence and pathology in flatfishes. Wild-caught flatfishes (common dab; Limanda limanda, European plaice; Pleuronectes platessa, European flounder; Platichthys flesus and lemon sole; Microstomus kitt) were either intramuscularly (i.m.) or intraperitoneally (i.p.) challenged with SAV3 or exposed to SAV3 through cohabitation with i.p. injected salmon. SAV-infections were seen in i.m. and i.p. injected dab and i.p. injected salmon but did not result in a transmissible infection in dab although several routes of entry were assessed (oral route not tested). SAV was detected in several tissues of eight common dab (not from cohabitants), with high SAV-levels in pancreas. No viraemia was detected in the SAV-positive common dab and no virus shedding were detected in the tanks. However, pathology in exocrine pancreas and hearts consistent with SAV-replication were seen. This is the first study reporting SAV-induced pathology in a non-salmonid species. The results from the present challenge study supports evidence for common dab being susceptible hosts for SAV. The study also demonstrates that flatfishes are less susceptible to SAV3-infection than salmon.

Changes in functioning of the largest coastal North Sea flatfish nursery, the Wadden Sea, over the past half century

The international Wadden Sea is an important flatfish nursery. Information from the Dutch Wadden Sea indicates that the flatfish nursery function of the area has been affected during the last decades. Increased seawater temperature has affected settling, habitat suitability for and growth performance of the various flatfish species. Settling of plaice, flounder and to a lesser extent sole larvae occurs earlier nowadays. In the 1960s, 0-, I-, II- and III-group plaice were present, but since 2000, II-group has disappeared and densities of I-group have decreased. For juvenile flounder, II-group almost disappeared, and for dab, a decline in densities of all age groups was observed from the 1990s onwards. Summer temperatures exceed the optimum for the cold-water species (plaice, flounder and dab) with increasing frequency, level and duration. Only for 0-group sole, the period with optimal growth conditions has become longer and has resulted in increased growth. Mortality rates in 0-group plaice have increased, coinciding with an increase in water temperatures and an increase in the abundance of predators. The decrease in density of juvenile plaice and dab in the Wadden Sea has not affected recruitment to North Sea stocks, suggesting that other areas have taken over part of the nursery function. The predicted increase in seawater temperature in the next decades will continue to improve the conditions for sole. The temperature tolerance of plaice and dab and to a lesser extent flounder will further reduce their scope for growth and may ultimately result in their disappearance from the Wadden Sea.

Identifying persistent biomass areas: The case study of the common sole in the northern Iberian waters

The common sole (Solea solea) is a species of flatfish of great commercial relevance in the northern Atlantic Iberian waters. Although many studies investigated the early life-stage of this species, little is known on the adult phases and the size structure of the population. Here we provide an analysis of the spatiotemporal distribution of adult’s common sole occurrence, conditional-to-presence biomass and median length in northern Atlantic Iberian waters using a Bayesian approach and three different spatiotemporal model structures that characterize the fundamental distributional patterns of fish as either persistent, opportunistic or progressive. Results highlight that the distribution of common sole adults in the area has been persistent over the years and that is partially affect by the bathymetry. Three main abundance hotspots were identified coinciding with spatial closure areas restricted to the fishing activity. Additionally, results show that small specimens tend to aggregate in shallower waters, while larger specimens inhabit deeper waters. Understanding the spatiotemporal distribution of the common sole population could improve our knowledge of this species and help to design more accurate biomass index that could be used to calibrate stock assessment models.

Chromosome-level assembly reveals a putative Y-autosomal fusion in the sex determination system of the Greenland Halibut (Reinhardtius hippoglossoides).

Despite the commercial importance of Greenland Halibut (Reinhardtius hippoglossoides), important gaps still persist in our knowledge of this species, including its reproductive biology and sex determination mechanism. Here, we combined single-molecule sequencing of long reads (Pacific Sciences) with chromatin conformation capture sequencing (Hi-C) data to assemble the first chromosome-level reference genome for this species. The high-quality assembly encompassed more than 598 Megabases (Mb) assigned to 1594 scaffolds (scaffold N50 = 25 Mb) with 96% of its total length distributed among 24 chromosomes. Investigation of the syntenic relationship with other economically important flatfish species revealed a high conservation of synteny blocks among members of this phylogenetic clade. Sex determination analysis revealed that similar to other teleost fishes, flatfishes also exhibit a high level of plasticity and turnover in sex determination mechanisms. A low-coverage whole-genome sequence analysis of 198 individuals revealed that Greenland Halibut possesses a male heterogametic XY system and several putative candidate genes implied in the sex determination of this species. Our study also suggests for the first time in flatfishes that a putative Y-autosomal fusion could be associated with a reduction of recombination typical of the early steps of sex chromosome evolution.

Mucus piRNAs profiles of Vibrio harveyi-infected Cynoglossus semilaevis: A hint for fish disease monitoring.

The half-smooth tongue sole, Cynoglossus semilaevis, is an important cultured flatfish species. Vibrio harveyi is a common pathogen to this fish, which may result in great economic loss to C.semilaevis culture industry. piRNAs, a non-coding RNAs with 26-32nt, have been regarded as promising biomarkers for cancer diagnosis and fish diseases. Here, we extracted the RNA from mucus of C.semilaevis and constructed the differential expression profiles of piRNAs between the sick fish (MS) and healthy fish (MC). We identified 45,696 differentially expressed piRNAs including 22,735 up-regulated piRNAs and 22,961 down-regulated piRNAs in MS group compared with MC group. The GO enrichment and KEGG pathway enrichment analyses of the differential piRNAs were carried out. The result showed immunity-related target genes mainly involved in immune system process, response to stimulus, cell killing, immune system, infectious diseases and cell growth and death. The 10 most differentially expressed piRNAs were chosen to perform the qRT-PCR, while only seven piRNAs were consistent with the sequence result. Compared with MC group, the expression levels of piR-mmu-72173>piR-rno-62831>piR-xtr-704880, piR-dme-15546979, piR-mmu-49941660, piR-mmu-29283297 and piR-mmu-1758399 were significantly lower, and piR-gga-10574 and piR-gga-134812 were significantly higher in MS group. These piRNAs may be potential biomarkers during the V.harveyi infection of C.semilaevis. This study could provide a new method to identify the infection status of C.semilaevis and understand better about the innate and adaptive immune system in C.semilaevis during bacterial infection.

Temporal Transcriptome Analysis Reveals Dynamic Expression Profiles of Gametes and Embryonic Development in Japanese Flounder (Paralichthys olivaceus).

The maternal-to-zygotic transition (MZT) is a crucial event in embryo development. While the features of the MZT across species are shared, the stage of this transition is different among species. We characterized MZT in a flatfish species, Japanese flounder (Paralichthys olivaceus). In this study, we analyzed the 551.57 GB transcriptome data of two types of gametes (sperms and eggs) and 10 embryo developmental stages in Japanese flounder. We identified 2512 maternal factor-related genes and found that most of those maternal factor-related genes expression decreased at the low blastula (LB) stage and remained silent in the subsequent embryonic development period. Meanwhile, we verified that the zygotic genome transcription might occur at the 128-cell stage and large-scale transcription began at the LB stage, which indicates the LB stage is the major wave zygotic genome activation (ZGA) occurs. In addition, we indicated that the Wnt signaling pathway, playing a diverse role in embryonic development, was involved in the ZGA and the axis formation. The results reported the list of the maternal genes in Japanese flounder and defined the stage of MZT, contributing to the understanding of the details of MZT during Japanese flounder embryonic development.

Unraveling cryptic diversity among shallow-water tonguefishes (Pleuronectiformes: Cynoglossidae: Symphurus) from the Indo-West Pacific region, with descriptions of five new species.

Species of Symphurus (Pleuronectiformes: Cynoglossidae) are relatively small-sized tonguefishes occurring worldwide in tropical, subtropical, and warm-temperate seas. In the Indo-West Pacific Ocean, species of Symphurus inhabiting waters shallower than 200 m are rarely reported; only five have been described, S. microrhynchus (Weber, 1913), S. holothuriae Chabanaud, 1948, S. monostigmus Munroe, 2006, S. leucochilus Lee et al. 2014, and S. longirostris Lee et al. 2016. Examination of museum and recently collected specimens yielded over 100+ Symphurus captured in relatively shallow waters off Japan, Papua New Guinea, the Philippines, Taiwan, and Vietnam. All of these specimens, except S. monostigmus (with 14 caudal-fin rays), were originally tentatively identified as S. microrhynchus because of shared similarities (small size, low meristic values, 12 caudal-fin rays, shared pigmentation traits). Detailed comparisons revealed that, although similar, specimens from allopatric locations have small differences in meristic, morphometric and pigmentation features. In previous literature, these small differences were thought to represent intraspecific variation among populations of a widespread species, S. microrhynchus. However, further study, including molecular data, has revealed that such minor differences among specimens from allopatric locations actually represent interspecific, and not population-level, variations. Where available, molecular differences among these allopatric populations, in contrast to the morphological features, were significantly different (9.0 to 26.3%), providing additional strong support for the hypothesis that more than one species is represented among fishes examined. Combined data from morphological and molecular characters, and species delimitation analysis, reveal that five, undescribed, cryptic species should be recognized: S. brachycephalus n. sp. from Vietnam, S. hongae n. sp. from Taiwan, S. leptosomus n. sp. from the Philippines, S. polylepis n. sp. from Papua New Guinea, and S. robustus n. sp. from Japan. Also, based on new information, the previous decision to place S. holothuriae Chabanaud in the synonymy of S. microrhynchus was determined to be premature. This species should be recognized as valid until additional specimens are captured and the taxonomic status of this nominal species re-evaluated. At least 10 species of Indo-West Pacific shallow-water Symphurus are now known. Eight are members of the Symphurus microrhynchus species complex with hypothesized closer relationship to each other than to the other two species of shallow-water tonguefishes. Included in this study are redescriptions of S. microrhynchus and S. holothuriae based on their holotypes, including an expanded number of morphological characters not previously used to diagnose these species; redescriptions are also provided for comparative purposes of three other shallow-water species; five new cryptic species are described; and lastly, detailed comparisons and an identification key to all 10 species of shallow-water Symphurus occurring in the Indo-West Pacific Ocean are provided. Two additional populations are also identified that likely represent other undescribed taxa belonging to the S. microrhynchus species complex. Adequate specimens are not available at this time to formally describe these nominal species. This study contributes further understanding about species diversity within Symphurus inhabiting shallow waters of the Indo-West Pacific Ocean. Several other nominal species of small-sized cynoglossid and soleid flatfishes are currently considered to have widespread distributions in the Indo-West Pacific. Many of these species also have junior synonyms available based on nominal species described from allopatric sites within their geographic ranges. How many of these presumed populations of widespread species will be resurrected from synonymy once additional specimens and their genetic information becomes available remains an interesting question for future study.

Biological characteristics of the primitive flatfish Indian halibut (Psettodes erumei) from the tropical northeastern Indian Ocean, including implications of the use of incorrect aging methods on mortality estimates

The Indian halibut (Psettodes erumei) is an important flatfish species in tropical and subtropical waters of the northern Indian Ocean. However, inappropriate aging methods used in previous studies of Indian halibut limited the use of age and growth data from those studies. This study highlights the importance of correct aging methods and identification of sexually dimorphic growth, particularly if growth characteristics are used to estimate mortality rates. Counts of opaque zones in whole otoliths of Indian halibut were almost always fewer than those in their corresponding sections, particularly for older individuals. Multiple validation methods demonstrate opaque zones form annually in otoliths of Indian halibut. Although the frequency of females increased with size, due to sexually dimorphic growth, males attained a greater maximum age (16 versus 11 years). Because of the greater ages and rapid early growth of Indian halibut described in this study, natural mortality estimates calculated by using Pauly’s equation, particularly for males, were higher than estimates derived from the same equation in previous studies, which employed growth parameter estimates based on ages derived from whole otoliths, vertebrae, or monthly length–frequency trends and, typically, did not account for sexually dimorphic growth. In northwestern Australia, spawning in Indian halibut commences when water temperature and day length begin to increase and occurs between early spring and late summer.

Experimental assessment of the effect of net wing spread on relative catch efficiency of four flatfishes by a four seam bottom trawl

The northeast US fishing industry and other stakeholders have considerable interest in the quality of bottom trawl survey results because of their importance in the setting of quotas, especially those directly derived from bottom trawl indices measured in catch per area swept by the survey net. Net spread (also referred to as wing spread) impacts catches and thus indices and, for the federal northeast bottom trawl survey, it is correlated with the depth at which a tow is performed. As part of an ongoing partnership between the Northeast Fisheries Science Center, the New England and Mid-Atlantic Fisheries Management Councils, members of the fishing industry, and academics in the region, an experiment to evaluate the impact of wing spread on the catchability of a number of species was conducted in the Fall of 2019 on the F/V Karen Elizabeth. The experiment targeted four species of flatfishes using a twin trawl rigging to collect simultaneous observations of catch and wing spread relative to a controlled optimal net spread. We analyzed these data to estimate the effect of net spread on relative catch efficiency and to evaluate the hypothesis that relative catch efficiency decreases as net spread departs from the optimal configuration. We found that, while the total catch was impacted by net wing spread (catches increased with increasing swept-area), there was no evidence of a wing spread effect on relative catch efficiency for any of the flatfish species. These results suggest that bottom trawl surveys should account for swept-area of individual tows, but that it is not necessary to incorporate species- or length-specific efficiency conversions based on the width of the net.

A review of the effects of contamination and temperature in Solea solea larvae. Modeling perspectives in the context of climate change

The flatfish species Solea solea has been the subject of research for supporting their management in fisheries, restocking natural populations, domestication in aquaculture, and ecotoxicology research. Soles undergo a metamorphosis with drastic morphological and physiological changes and settlement, processes that make them potentially more vulnerable to pollutants than other fish. Up to date, efforts made to develop its aquaculture production in Europe have been limited. In the context of climate change (CC), this review aims i) to gather research conducted in S. solea larvae that summarizes the effects of increased temperature and contaminant exposures during larval development; and ii) to provide a summarized and synergistic view about its larval development. The review consists of 4 sections. Section 1 justifies the selection of this species from ecological and economic perspectives. Section 2 focuses on larvae ontogenesis, metamorphosis, rearing challenges, and further aquaculture production. Section 3 reviews studies dealing with the effects of temperature change (due to CC) and pollutants on larval development. Finally, section 4 provides a “how to go forward on ecotoxicological research” guideline, in which we highlight the methods that we found promising as tools to study the combined effects of CC and pollution. The section includes a multidisciplinary framework that proposes how existing data coming from different scientific domains can be synthesized to be useful for risk assessors and ecotoxicologists. To benefit from such a framework, it is necessary to reach consensus and nurture team effort from players that operate in different research disciplines.