Adult Flounder Research Articles

Effect of creatine administration on locomotor activity and stress response in olive flounder (Paralichthys olivaceus)

The creatine kinase system is crucial for maintaining cellular energy homeostasis and plays a role in regulating locomotor behavior in organisms, but its significance in the regulating the motionless behavior in olive flounder is limited. In the first experiment of this study, elevated levels of creatine kinase (CK) activity in the spinal cord were detected in the juvenile group (JG) flounder compared to the adult group (AG) flounder. In the second experiment, to further confirm the involvement of CK in the locomotor behavior, the adult flounder was given an intraperitoneal injection of creatine (150 mg/kg), while the flounder in the control group received a saline solution. After one week post-injection, the behavioral analysis revealed that the flounder in the creatine-treated group displayed higher levels of locomotor activity and a greater number of escape attempts in response to external stimuli when compared to the control group. However, the acute stress response, induced by intraperitoneal injection and characterized by tail beating, was significantly alleviated in the flounder in the creatine-treated group. Additionally, there was an upregulation of the UII and AchR genes in the spinal cord, as well as increased levels of UII and AchR in the muscle tissues of the creatine-treated flounder. However, a reduction in UI mRNA levels was observed in the brain of the flounder. Collectively, our data provide the evidence that the elevated enzyme activity and gene expression of creatine kinase play important roles in off-bottom swimming behavior in the JG flounder. Furthermore, administration of creatine improved the locomotor activity and alleviated the stress response in flounder, which is associated with regulation of the locomotor- and stress-related gene in the brain, spinal cord, and muscle.

Role of Thyroid Hormone in Dynamic Variation of gdf6a Gene during Metamorphosis of Paralichthys olivaceus.

The Japanese flounder (Paralichthys olivaceus) is a marine fish that undergoes a dramatic postembryonic metamorphosis, with the right eye shifting to the left and its lifestyle transitioning from planktonic to benthic. As the light environment of the habitat changes from bright to dim, its photoreceptor system also undergoes adaptive change. Growth differentiation factor 6a (Gdf6a) is a member of the BMP family, which plays a key role in regulating the dorsal-ventral pattern of the retina and photoreceptor fate, and the differentiation of different photoreceptors is also modulated by a thyroid hormone (TH) binding its receptor (TR). However, the relationship between gdf6a and TH and its role in the regulation of photoreceptors during flounder metamorphosis is still poorly understood. In this study, bioinformatics analysis showed that Gdf6a had a conserved TGFB structural domain and clusters with fishes. The expression analysis showed that the expression of gdf6a was highest in the eye tissue of adult flounder and tended to increase and then decrease during metamorphosis, reaching its highest levels at the peak of metamorphosis. Moreover, the expression of gdf6a increased in the early stages of metamorphosis after exogenous TH treatment, while it was inhibited after exogenous thiourea (a TH inhibitor, TU) treatment. To further investigate the targeting role of TH and gdf6a in the metamorphosis of flounder, the results of the Dual-Luciferase revealed that triiodothyronine (T3) may regulate the expression of gdf6a through TRβ. In conclusion, we speculate that TH influences the development of cone photoreceptors during the metamorphosis of the flounder by regulating the expression of gdf6a.

Diet–egg relationships and equilibration of fatty acids in eggs of southern flounder, Paralichthys lethostigma

There is increasing interest in the transfer of nutrients from broodstock diets to eggs due to the consequences for egg and larval quality. Analyses were conducted with southern flounder Paralichthys lethostigma to identify which fatty acids (FAs) in eggs are influenced by broodstock diet and how long it takes for the FA composition of eggs to equilibrate to the broodstock diet. Data were obtained from a laboratory experiment and from hatchery production operations. For each source, two populations of adult flounder were given one or the other of two diet treatments. In the laboratory experiment, the diets differed markedly in FA composition. In the hatchery, a base diet was supplemented on two different schedules. Each of the four diet treatments produced eggs with significantly different mean FA compositions. Of the 27 FAs measured, 19 differed significantly between diets and had corresponding significant differences in eggs (direct diet–egg relationships). Combining results with data from a prior study showed that equilibration of eggs to a change in adult diet takes 8–16 weeks for southern flounder. These findings will be useful for improving broodstock diets and feeding protocols for captive flounder being used by stock-enhancement programs that are designed to mitigate the long-term and widespread population declines of this species.

Spatial and temporal patterns of gonadal maturation and spawning in European flounder Platichthys flesus at its southern continental edge

The European flounder (Platichthys flesus) is a marine flatfish with high plasticity in several life-history traits, including spawning sites and habitat use patterns. With the overarching goal of understanding the use of estuaries in connection with maturation and reproduction in adult flounder, this study investigates its reproductive cycle in the Douro Estuary (NW Portugal), at the southern edge of its geographical distribution. Adults were caught by monthly trawling from November 2016 to November 2017. Sex ratio and length-weight relationships were determined. The annual reproductive cycle was described using a combination of gonadosomatic (GSI), hepatosomatic (HSI), and general fish condition (Kn) indices, seasonal gonad development staging (macroscopic and histological), and quantification of plasma sexual steroids. Total sex ratio was 1 M:3 F, and females presented negative allometric growth, while males’ growth was isometric. The various indices consistently indicated that the greatest reproductive activity occurred between January and March, peaking in February. However, some females at advanced reproductive stages (late vitellogenesis), high GSI values and steroid hormone (E2) levels were observed as late as May. Females in spawning condition, i.e., undergoing full oocyte maturation and ovulation, were never found, unlike males who exhibited spermiation within the estuary. Sexual differences in migration patterns were also suggested: males seem to leave earlier and stay longer in offshore spawning grounds, while females return to the estuary shortly after spawning. Overall, there was a good correlation between GSI, seasonal hormone cycling, and sexual maturity of female and male flounder, supporting the use of circulating sex steroids as a less invasive tool to assess maturity status.

Expression characteristics of Hsd3b7 in the gonads of Paralichthys olivaceus.

Steroidogenesis is an important biological process for gonadal differentiation and development. In mammals, 3β-hydroxysteroid dehydrogenase 7 (HSD3B7) could convert 3β-hydroxy of 7α-hydroxycholesterol into a ketone and form 7α-hydroxy-4-cholesten-3-one, which may affect steroidogenesis. However, in fish, the study of Hsd3b7 is still lacking. In this study, Hsd3b7 was identified in the olive flounder Paralichthys olivaceus, an important mariculture fish. According to bioinformatics analysis, Hsd3b7 belongs to a Rossmann-fold NAD(P)(+)-binding protein and can interact in a predictable manner with Hsd17b2, -3, and - 4, which play a role in steroidogenesis. In the adult flounder, Hsd3b7 was expressed in various tissues, at particularly high level in male muscle. The expression levels of Hsd3b7 at gonadal development stages I-V initially increased and then decreased, with an inflection point in the ovary at stage III and in the testis at stage IV. At stage III, the expression level of Hsd3b7 was significantly higher in the ovary than in the testis (P < 0.01). The results of in situ hybridization (ISH) revealed that it was mainly expressed in oocytes of phases I-IV or around oocytes of phases IV-V in the ovaries and around spermatid lobules at stages IV-V in the testes. Three regulatory sites of SRY-box transcription factor 9 (Sox9), a transcription factor involved in steroidogenesis and gonadal differentiation, were predicted in the promoter of Hsd3b7. After intraperitoneal injection with the recombination flounder Sox9a, the expression of Hsd3b7 was significantly up-regulated (P < 0.01). During the flounder gonadal differentiation, 17β-estradiol (E2, 5 μg/g feed) and 17α-methyltestosterone (T, 5 μg/g feed) were used to obtain the phenotypic female or male flounder, and the results showed that in the E2 group, Hsd3b7 expression was highest at 2 cm TL, the primordial gonad stage, which was significantly higher than that at 12 cm TL (P < 0.05). In the T group, Hsd3b7 expression level was also highest at 2 cm TL and significantly higher than at 10 and 12 cm TL (P < 0.05). Moreover, Hsd3b7 was detected to be localized mainly around oogonia and spermatogonia during the differentiated gonads with ISH. These findings first introduce the expression characteristics of Hsd3b7 and the effect of Sox9a on its expression, which contribute to our understanding of the function of Hsd3b7 in fish gonads.

クドア食中毒

A myxozoan parasite Kudoa septempunctata causing the food poisoning of humans which consume raw olive flounder Paralichthys olivaceus has become a new threat to public health. ​This review focuses the recent occurrence of K. septempunctata-induced food poisoning in Japan for 10 years, the typical symptoms of the foodborne disease in humans, parasitology, diagnoses, countermeasures and the remaining issues. ​The numbers of K. septempunctata-induced cases and patients have peaked in 2014, followed by a gradual decrease in recent years. ​The symptoms of humans are characterized by transient vomiting and diarrhea. ​Also, onset of the disease is dose-dependent of fresh spores (> 1 × 107). ​The host fish for K. septempunctata is mainly limited to P. olivaceus. ​The geographical distribution is Japan and Korea. ​The two locality isolates are easily differentiated by a ratio of spores possessing 7 polar capsules. ​The light-microscopy, conventional or real-time PCRs, LAMP, immunochromatography have been developed for the diagnostic tools. ​To prevent the infection with K. septempunctata in juvenile stage of cultured flounder, a sand-filtration combined with ultraviolet irradiation (> 46 mJ/cm2) is effective. ​At the time of shipment of adult flounder, light-microscopic examination for spores is recommended to remove the infected fish group. ​The remaining problems include the disease mechanisms in humans, the host range, and the harmful Kudoa species other than K. septempunctata.

Impacts of chemical stress, season, and climate change on the flounder population of the highly anthropised Seine estuary (France).

The main objective of this study was to improve our knowledge on the responses of fish populations to multistress (diffuse pollution and warming waters) in estuaries. Adult flounders were caught in two estuaries in the Eastern English Channel: the heavily polluted Seine estuary vs the moderately contaminated Canche estuary. Fish samplings were conducted in January just before the reproduction period, and in July when gonads were at rest. The overall rise in coastal winter water temperatures detected over the Channel impairs the flounder's phenology of reproduction in the two estuaries, inducing a delay of maturation process and probably also spawning. The higher liver histopathology index in Seine vs Canche could be the consequence of the fish exposition to a complex cocktail of contaminants in a strongly industrialized estuary. Higher levels of neurotoxicity, gill lipid peroxidation, and liver EROD activity were observed in Seine vs Canche. Furthermore, a possible impairment in mitochondrial metabolism was suggested in the Seine flounder population. We confirmed in this study the potential role of two membrane lipids (sphingomyelin and phosphatidylserine) in the resistance towards oxidative stress in Seine and Canche. Finally, we suggest that the Seine flounder population (and possibly the connected Eastern English Channel flounder populations over the French Coast) could be seriously impacted in the future by multistress: higher winter temperatures and chemical contamination.

Spatial contraction of demersal fish populations in a large marine ecosystem

AbstractAimThe interdependencies between trophic interactions, environmental factors and anthropogenic forcing determine how species distributions change over time. Large changes in species distributions have occurred as a result of climate change. The objective of this study was to analyse how the spatial distribution of cod and flounder has changed in the Baltic Sea during the past four decades characterized by large hydrological changes.LocationBaltic SeaTaxonCod (Gadus morhua) and flounder (Platichthys flesus).MethodsCatch per unit of effort (CPUE) data for adult and juvenile cod and for adult flounder were modelled using Delta‐Generalized additive models including environmental and geographical variables between 1979 and 2016. From the annual CPUE predictions for each species, yearly distribution maps and depth distribution curves were obtained. Mean depth and the depth range were estimated to provide an indication on preferred depth and habitat occupancy.ResultsAdult and juvenile cod showed a contraction in their distribution in the southern areas of the Baltic Sea. Flounder, instead, showed an expansion in its distribution with an increase in abundance in the northern areas. The depth distributions showed a progressive shift of the mean depth of occurrence towards shallower waters for adult cod and flounder and towards deeper waters for juvenile cod, as well as a contraction of the species depth ranges, evident mainly from the late 1980s.Main conclusionsOur study illustrates large changes in the spatial distribution of cod and flounder in the Baltic Sea. The changes in depth distribution occurred from the late 1980s are probably due to a combination of expanded areas of hypoxia in deep waters and an increase in predation risk in shallow waters. The net effect of these changes is an increased spatial overlap between life stages and species, which may amplify cod cannibalism and the interaction strength between cod and flounder.

Assessing the effects of internal and external acoustic tagging methods on European flounder Platichthys flesus

Fish telemetry studies give unique information about fish movements (migrations), but there are still several methodological constraints. A critical step is the choice of an appropriate tag and tagging method, essential to guaranteeing that the tagged fish is representative of the general wild population. The effects of two tagging methods with acoustic transmitters on the behavior, survival, condition, and growth of the European flounder Platichthys flesus (L.) were evaluated in tank experiments. Two 33-day assays were conducted: one with adult flounder to test intracoelomic (internal) and external tagging with Vemco V7-2 L dummy tags, and a second assay with juveniles using only external tags. Intracoelomic implantation resulted in very low survival rate (10%), fish lethargy, and a lack of response to food. External tagging did not affect behavior but had negative effects on the final condition of adult flounder, and on the specific growth rates (SGR) of smaller-sized fish. The external tagging method is the most appropriate for P. flesus because of the technique’s simplicity, expedience, and fewer negative effects upon the fish.

Characterizing and predicting the distribution of Baltic Sea flounder (Platichthys flesus) during the spawning season

Identification of essential fish habitats (EFH), such as spawning habitats, is important for nature conservation, sustainable fisheries management and marine spatial planning. Two sympatric flounder (Platichthys flesus) ecotypes are present in the Baltic Sea, pelagic and demersal spawning flounder, both displaying ecological and physiological adaptations to the low-salinity environment of this young inland sea. In this study we have addressed three main research questions: 1) What environmental conditions characterize the spatial distribution and abundance of adult flounder during the spawning season? 2) What are the main factors defining the habitats of the two flounder ecotypes during the spawning season? 3) Where are the potential spawning areas of flounder? We modelled catch per unit of effort (CPUE) of flounder from gillnet surveys conducted over the southern and central Baltic Sea in the spring of 2014 and 2015 using generalized additive models. A general model included all the stations fished during the survey while two other models, one for the demersal and one for the pelagic spawning flounder, included only the stations where each flounder ecotype should dominate. The general model captured distinct ecotype-specific signals as it identified dual salinity and water depth responses. The model for the demersal spawning flounder revealed a negative relation with the abundance of round goby (Neogobius melanostomus) and a positive relation with Secchi depth and cod abundance. Vegetation and substrate did not play an important role in the choice of habitat for the demersal ecotype. The model for the pelagic spawning flounder showed a negative relation with temperature and bottom current and a positive relation with salinity. Spatial predictions of potential spawning areas of flounder showed a decrease in habitat availability for the pelagic spawning flounder over the last 20years in the central part of the Baltic Sea, which may explain part of the observed changes in populations' biomass. We conclude that spatiotemporal modelling of habitat availability can improve our understanding of fish stock dynamics and may provide necessary biological knowledge for the development of marine spatial plans.

Movement patterns of Chilean flounder (Paralichthys adspersus) inside Tongoy Bay (central northern Chile): Observations using passive acoustic telemetry

The movement patterns of juvenile and adult Chilean flounder (P. adpersus) were investigated inside Tongoy Bay using ultrasound signal acoustic receivers from June 2012 to March, 2013. Flounder landings in Tongoy Bay and Puerto Aldea from December 2011 to March 2013 were examined. Multiple regression analysis indicated that the Catch per Unit of Effort of Chilean flounder was significantly and negatively related to temperature and depth. Analyses of site- and time-specific length-frequency distributions indicated movement of Chilean flounder on the time scale of weeks, which was likely due to emigration of fish >30 cm in total length. A mark-recapture study was performed. Visible elastomer paint was used to tag 7,510 Chilean flounder. A total of 12 Chilean flounder individuals of different lengths were tagged with an ultrasound transmission device to monitor their movement inside Tongoy Bay. Adults flounder showed increased activity inside Tongoy Bay during the study period, likely due of the differences in length among the released individuals. Although differences were detected in the area occupied by juvenile and adult flounders in Tongoy Bay, it was also noticed that the smaller sized individuals exhibited changes in behavior after implanting the transmitters that resulted in impaired capacity to move freely. Key words: Chilean flounder, acoustic telemetry, movement patters, landing, mark-recapture.

Molecular cloning and sexually dimorphic expression of wnt4 in olive flounder (Paralichthys olivaceus).

WNT4 (wingless-type MMTV integration site family, member 4) is regarded as a key regulator of gonad differentiation in mammalians. However, the potential role of wnt4 in teleosts during gonad differentiation and development is still unclear. The full-length cDNA sequence of wnt4 in olive flounder (Paralichthys olivaceus) was obtained using RACE (rapid amplification of cDNA ends) technique. The wnt4 ORF contains 1059 nucleotides, encoding a protein with a signal peptide domain and a wnt family domain. Expression in tissues of adult flounders was analyzed by real-time RT-PCR. The results showed that wnt4 was widely expressed in multiple tissues of flounders, and the expression level was significantly higher in ovary than in testis. Then wnt4 expression pattern was investigated during gonadal differentiation period and at gonadal development stages (I-V). The results showed the expression levels were significantly higher in testis than in ovary during gonadal differentiation. Notably, wnt4 expression had a very significant increase before testis differentiation. At gonad different developmental stages, there was no expression signal at stage I or stage II, and the expression of wnt4 was much stronger in ovary than in testis at stage III and stage IV, followed by a faint expression in stage V in both sexes. Our results imply that cloned wnt4 could be wnt4a. It is a sex-related gene and its expression pattern in gonadal differentiation period of flounder is different from that in mammalians or other teleosts. Flounder wnt4 might play more important role in testis than in ovary during gonadal differentiation.

Molecular cloning and sexually dimorphic expression patterns of nr0b1 and nr5a2 in olive flounder, Paralichthys olivaceus.

The nr0b1 and nr5a2 genes, members of the nuclear receptor superfamily, are strong candidate genes involved in gonadal differentiation in several vertebrate species. In this study, an nr0b1 complementary DNA (cDNA) of 1446 bp, which encodes a predicted 298 amino acid protein, and an nr5a2 cDNA of 2425 bp, which encodes a deduced 523 amino acid protein, were obtained from olive flounder Paralichthys olivaceus. Both genes were expressed in multiple organ tissues of adult flounder, with a higher expression in ovary than in testis. Quantitative real-time RT-PCR was performed to investigate their temporal expression profiles in gonads during differentiation and at five development stages. Results indicated that nr0b1 and nr5a2 were expressed in primitive gonad and in the ensuing gonadal differentiation periods. In general, both genes were more highly expressed in ovary than in testis at all observed development stages. The expression level of cyp19a correlated with the nr5a2/nr0b1 ratio over the course of flounder gonadal differentiation; hence, nr0b1 and nr5a2 genes may be involved in flounder ovarian differentiation by regulating the expression of cyp19a.

長崎県沿岸に生息するヒラメ &lt;i&gt;Paralichthys olivaceus&lt;/i&gt; の遊泳行動の時空間変動

ヒラメの遊泳行動の時空間的な変化を調べるために，大村湾と東シナ海で深度・温度記録計を用いた行動調査を実施した。その結果，ヒラメの活動率は，両海域ともに季節的に変化した。1 月の東シナ海の活動率は大村湾より 39 倍高く，海域による違いも見られた。このような行動の変化は刺網の影響面積に影響を及ぼすことが分かった。ヒラメの遊泳行動に関する知見は資源量の変動を正確に推定する際に役立つものと考えられる。

Cloning of the cDNA encoding adenosine 5′-monophosphate deaminase 1 and its mRNA expression in Japanese flounder Paralichthys olivaceus

AMP deaminase catalyzes the conversion of AMP into IMP and ammonia. In the present study, a full-length cDNA of AMPD1 from skeletal muscle of Japanese flounder Paralichthys olivaceus was cloned and characterized. The 2 526 bp cDNA contains a 5′-UTR of 78 bp, a 3′-UTR of 237 bp and an open reading frame (ORF) of 2 211 bp, which encodes a protein of 736 amino acids. The predicted protein contains a highly conserved AMP deaminase motif (SLSTDDP) and an ATP-binding site sequence (EPLMEEYAIAAQVFK). Phylogenetic analysis showed that the AMPD1 and AMPD3 genes originate from the same branch, but are evolutionarily distant from the AMPD2 gene. RT-PCR showed that the flounder AMPD1 gene was expressed only in skeletal muscle. QRT-PCR analysis revealed a statistically significant 2.54 fold higher level of AMPD1 mRNA in adult muscle (750±40 g) compared with juvenile muscle (7.5±2 g) (P<0.05). HPLC analysis showed that the IMP content in adult muscle (3.35±0.21 mg/g) was also statistically significantly higher than in juvenile muscle (1.08±0.04 mg/g) (P<0.05). There is a direct relationship between the AMPD1 gene expression level and IMP content in the skeletal muscle of juvenile and adult flounders. These results may provide useful information for quality improvement and molecular breeding of aquatic animals.

Response to “Comment on ‘Otolith Microchemistry Reveals Substantial Use of Freshwater by Southern Flounder in the Northern Gulf of Mexico’” by Pedro Morais

Lowe et al. (Estuar Coast 34:630–639, 2011) used Sr/Ca in the otoliths of juvenile southern flounder (Paralichthys lethostigma) to demonstrate the importance of low-salinity habitats in the northern Gulf of Mexico during early life. While otolith cores indicated that most southern flounder originated in high-salinity environments (elevated Sr/Ca in otolith core) and moved into lower salinity habitats (decreased Sr/Ca outside of the otolith core), approximately one third of the sampled individuals had otolith core chemistry that indicated spawning or hatching in or near freshwater habitats (low Sr/Ca throughout the otolith). Morais (Estuar Coast 34:630–639, 2012) proposes a different interpretation: he suggests that the high Sr/Ca found in 68 % of juvenile otolith cores (and low Sr/Ca just outside of the core region) may be due to females hydrating their eggs in offshore (high-salinity) waters, with spawning, hatching, and egg/larval development all actually occurring in low-salinity environments (his “sea-run mother” hypothesis). While there is ample literature supporting the importance of maternal contributions to otolith microchemistry during vitellogenesis, it is our contention that such contributions ultimately depend on the ecology and reproductive physiology of each species. Further, a mass inshore migration of adult southern flounder during their spawning season is counter to the well-documented life history patterns for this species throughout its range. While we appreciate Morais' perspective, we continue to stand by the conclusion of our original paper.

Clonagem e avaliação da expressão gênica do sbGnRH em machos juvenis e adultos de linguado, Paralichthys orbignyanus

Este estudo buscou clonar o cDNA do sbGnRH, identificar sua expressão em diferentes tecidos do linguado, bem como avaliar possíveis diferenças no RNA mensageiro (RNAm) desse gene no cérebro de linguados machos juvenis e adultos. Por meio da RT-PCR, demonstrou-se pela primeira vez, a clonagem da região codificadora do sbGnRH contendo 297 nucleotídeos do cérebro do linguado. A expressão do sbGnRH foi detectada em vários tecidos periféricos. Foram detectados níveis mais elevados de RNAm do sbGnRH no hipotálamo dos animais adultos. Estes resultados sugerem que o sbGnRH está envolvido na puberdade do linguado.

The migration patterns of the European flounder Platichthys flesus (Linnaeus, 1758) (Pleuronectidae, Pisces) at the southern limit of its distribution range: Ecological implications and fishery management

This study aims to determine the diversity of migration patterns of the European flounder ( Platichthys flesus (Linnaeus, 1758)) present in the Minho estuary and in the adjacent coastal area (NW-Iberian Peninsula). Assessing the diversity of flounder migration patterns at the southern limit of its distribution allows the determination of characteristics of the species' ecology and provides useful information for fishery managers, since it is a regionally important estuarine fishery. An unexpected result of our study was that flounder appears to spawn in both estuarine and coastal areas and not just in coastal areas as was previously widely accepted. Our interpretation of otolith strontium distribution patterns from flounder specimens collected in the freshwater tidal area of the Minho estuary and in the lower estuary suggested that the flounders hatched in the estuary, while only 6.7% of those captured in the coastal area hatched in the coastal area. Ultimately, studies aimed at collecting larval stages and adult flounders must be made to confirm that flounders spawn in the estuary and to define new and better scientifically supported fishing policies, or simply to confirm the existing ones regarding temporal and spatial closures for each gear used in the Minho estuary.

Quantitative comparison of the expression of myogenic regulatory factors in flounder (Paralichthys olivaceus) embryos and adult tissues

MyoD, Myf5, and myogenin are myogenic regulatory factors that play important roles during myogenesis. It is thought that MyoD and Myf5 are required for myogenic determination, while myogenin is important for terminal differentiation and lineage maintenance. To better understand the function of myogenic regulatory factors in muscle development of flounder, an important economic fish in Asia, real-time quantitative RT-PCR was used to characterize the expression patterns of MyoD, Myf5, and myogenin at early stages of embryo development, and in different tissues of the adult flounder. The results show that, Myf5 is the first gene to be expressed during the early stages of flounder development, followed by MyoD and myogenin. The expressions of Myf5, yoD, and myogenin at the early stages have a common characteristic: expression gradually increased to a peak level, and then gradually decreased to an extremely low level. In the adult flounder, the expression of the three genes in muscle is much higher than that in other tissues, indicating that they are important for muscle growth and maintenance of grown fish. During embryonic stages, the expression level of MyoD might serve an important role in the balance between muscle cell differentiation and proliferation. When the MyoD expression is over 30% of its highest level, the muscle cells enter the differentiation stage.

High-frequency depth recording reveals the vertical movement of flounder in the Tsugaru Strait of northern Japan

Analysis of high-frequency depth-recording data of adult Japanese flounder Paralichthys olivaceus (Temminck & Schlegel) with depth/temperature logging tags, released in the Tsugaru Strait of northern Japan, has yielded new insights into behavioural differences on vertical movement. Here, we document diel differences in fine-temporal-scale swimming behaviour observed from six flounder released during the no-spawning season. While the flounder remained on the seabed for the majority of the recording period, fish occasionally left the seabed, swam into the water column and then swam back to the seabed. The mean swimming duration per tagged fish ranged from 44 ± 49 to 94 ± 164 s, with a maximum observed swimming duration of 44.5 min. Vertical movements included one or more clear ascent and descent phases highlighted by a distinct peak. Our results reveal that in the no-spawning season nocturnal swimming is more active and frequent than during the day. Our results provide important information about diel differences in swimming behaviour with respect to vertical movement and also show the vulnerability of Japanese flounder to capture by bottom-fishing gear (e.g. trawling) during the day.