Greenback Flounder Rhombosolea Tapirina Research Articles

Complex movement patterns of greenback flounder (Rhombosolea tapirina) in the Murray River estuary and Coorong, Australia

The greenback flounder Rhombosolea tapirina is a commercially-important flatfish species in southern Australia and New Zealand, whose population dynamics are poorly understood. Acoustic telemetry was used to assess movement patterns and area use for R. tapirina in the Murray River estuary and Coorong, South Australia. Twenty fish (221–313mm total length) equipped with acoustic transmitters were monitored for up to seven months during a period of high freshwater inflow. Fish were detected over a large part of the system, but showed a strong preference for brackish and near-marine conditions in the inner estuary. Tagged fish exhibited complex movement patterns that differed among individuals, including: (1) within estuary movements; (2) dispersal from the estuary to the sea; and (3) return migrations between the estuary and the sea. A diurnal shift in fine-scale area use was observed in the part of the estuary where residency was highest, with individuals occupying deeper habitats during the day and shallower areas during the night. The results demonstrate the individualistic and often highly transient behaviour of this species and its ability to undertake regular movements over the spatial scale of 10s of km. Understanding such movement patterns can improve effective management of estuarine flatfish populations and ecosystems.

In vitro effect of vitellogenin on steroid production by ovarian follicles of greenback flounder Rhombosolea tapirina

Ovarian follicles from vitellogenic greenback flounder ( Rhombosolea tapirina) were incubated in L15 medium alone, or containing human chorionic gonadotropin (hCG), dibutyryl cyclic AMP (dbcAMP) or the steroid precursors testosterone (T), 17-hydroxyprogesterone (17P) and androstenedione (A) in the presence of vitellogenin (Vtg) at 0.1–5.0 mg mL − 1. Medium concentrations of 17β−estradiol (E 2) and T were measured by radioimmunoassay. HCG generally stimulated follicular E 2 but not T production, whereas 17P, A and T stimulated production of E 2, T, and E 2 respectively. Treatment of follicles with dbcAMP inhibited follicular E 2 production, but increased follicular T production at high doses. The effect of low concentrations of Vtg on follicular steroid production was variable; however, higher doses of Vtg significantly suppressed basal, hCG-, dbcAMP- and steroid precursor-stimulated follicular E 2 and T production. The results of this study show that high concentrations of Vtg may suppress follicular steroid production by interfering in the steroidogenic pathway. This suggests that Vtg may regulate its own production by limiting the ovarian production of E 2.

Effect of turbidity, prey density and culture history on prey consumption by greenback flounder Rhombosolea tapirina larvae

Fish larvae cultured in turbid “green water” conditions commonly show improved feeding, growth and survival, however, the underlying mechanisms remain unclear. Greenback flounder Rhombosolea tapirina (Günther) larvae, reared in either green water ( Tetraselmis suecica, 5 NTU) or clear water tanks were used in short duration feeding trials to investigate the effect of larval culture history, live prey density (0.01–5 prey ml − 1 ), and turbidity level (0–40 NTU) on feeding performance. Prey consumption was density-dependent at prey densities below 0.1 ml − 1 and 0.05 ml − 1 for feeding on rotifers and Artemia, respectively. Green water reared larvae fed in green water consumed more rotifers across the range of prey densities tested compared with clear water reared larvae fed in clear water. At low prey density, where absolute performance capabilities of the larvae are tested, green water provided immediate improvement to rotifer intake at turbidity levels of 5–20 NTU for larvae with experience of either a clear or green water environment. However, larvae with experience of a green water environment outperformed larvae with experience of a clear water environment. Thus mechanisms that operate over the short term, such as contrast enhancement and chemical stimulation of feeding, as well as mechanisms that operate over the longer term, such as possible differences in retinal development, improvements in handling times and experience, are likely responsible for improved early larval performance in green water.

Characteristics of spawning behaviour in cultured greenback flounder Rhombosolea tapirina

Cultured stocks of 2nd generation 2+ year old greenback flounder Rhombosolea tapirina were maintained at low density and under conditions of minimal disturbance over 4 reproductive seasons. Natural in-tank spawning began in the second year after introduction in 2 separate tank systems and continued for 3 successive seasons until the experiment was terminated. Video records showed spawning on 5 out of 34 days on which filming occurred, with spawning behaviour consisting of approach and courting of an ovulated female by a male, followed by a circular paired swim in mid-water culminating in egg release. Spawning was concentrated in austral winter and spring in all 3 study years. In the first year, spawning episodes were strongly correlated with lunar phase (new moon), whereas in years 2 and 3, initiation of substantive spawning for the season coincided with the new moon but there was little evidence of lunar synchronisation thereafter. Volumes of eggs produced and egg fertility were highly variable and not clearly related to season. Most spawning events occurred between midnight and 07:30 h, with the majority between 04:00 and 06:00 h (∼2 h before sunrise). The results of the present study further emphasize the utility of low disturbance maintenance for the development of naturally spawning cultured broodstock, and suggest that wild stocks are likely to show pre-dawn spawning in winter and spring, possibly synchronised to lunar phase.

Patterns of oocyte growth, vitellogenin and gonadal steroid concentrations in greenback flounder

The pattern of oocyte development in association with changes of plasma concentrations of vitellogenin (Vtg), 17β‐oestradiol (E2) and testosterone (T) was investigated in maturing female greenback flounder Rhombosolea tapirina over the first part of a reproductive season (February to June). Examination of oocyte size‐frequency distributions showed that the oocyte developmental pattern in R. tapirina is multiple group synchrony, and that reproductively mature fish were present at all sampling times. There were no significant temporal variations in the gonado‐somatic index (IG), hepato‐somatic index (IH), or plasma concentrations of Vtg, E2 and T during the sampling period, which indicates that reproductive development is not synchronized within the population. Significant increases in IG, IH and plasma concentrations of Vtg, E2 and T, however, were observed in vitellogenic fish, and in fish undergoing final maturation. A positive relationship was also found between the growth of oocytes and plasma concentrations of Vtg, E2 and T, although the patterns of increase were different for each variable. Plasma concentrations of Vtg and E2 rose steadily across oocyte sizes from 100 to 450 μm, but the rate of increase of plasma E2 was slower than that of Vtg, and both reached a saturated concentration at oocyte sizes of c. 450 μm. In contrast, plasma concentrations of T showed no marked increase until oocytes grew beyond 400 μm.

Prey selection by greenback flounder Rhombosolea tapirina (Günther) larvae

The potential for greenback flounder ( Rhombosolea tapirina), as a new species for aquaculture in temperate Australia, is currently under investigation. Isolation of an endemic temperate rotifer, Testudinella sp., provided the opportunity to assess this genus against standard live prey species used in aquaculture. Greenback flounder larvae were offered a range of rotifer prey species ( Testudinella sp., Brachionus rotundiformis, Brachionus plicatilis) and the brine shrimp Artemia, either in isolation or combination, in order to test the hypothesis that factors besides prey size affect prey selection. From first-feeding to approximately day 13 post-hatching, greenback flounder larvae preferentially consumed the rotifer Testudinella sp. over the larger width rotifer B. plicatilis and to a lesser extent the rotifer of similar width dimension, B. rotundiformis. From day 15 post-hatching, consumption (number of prey consumed per larva per unit time interval) and selection ( α) of B. plicatilis was significantly higher than of Testudinella sp. and Artemia, but by day 18 post-hatching, there was a shift of feeding preference to Artemia. This indicated that size of prey ingested increased with age, likely reflecting an ontogenetic increase in larval sensory function, mouth gape and prey capture and handling ability. Significant selection of Testudinella sp. over the larger rotifer B. plicatilis occurred despite greenback flounder larvae being able to consume B. plicatilis. This along with significant selection (day 11 post-hatching) and consumption (days 11 and 12 post-hatching) of Testudinella sp. over the rotifer of similar width B. rotundiformis, indicates that prey size is not the sole determinant of prey consumption and suggests that greenback flounder larvae exhibit both species-specific and size-specific prey selection. Understanding the basis of prey selection by larval fish predators is essential for enhancing prey consumption in culture, especially in the context of ontogenetic shifts in prey species preference.

Development of an enzyme-linked immunosorbent assay (ELISA) for vitellogenin measurement in greenback flounder Rhombosolea tapirina

Vitellogenin (Vtg) was purified from the plasma of 17β-estradiol (E2)-injected male greenback flounder,Rhombosolea tapirina. The molecular weight of the native Vtg was estimated by gel filtration as ∼ 540 kD. SDS-PAGE and Western blotting analyses indicated that this protein consisted of three bands with molecular weights of 155, 104, 79 kD, respectively. A polyclonal antibody against the highest molecular weight band of putative Vtg was generated in sheep and an indirect antibody-capture competitive enzyme-linked immunosorbent assay (ELISA) was developed. The assay was validatedfor plasma Vtg measurement in greenback flounder. Serial dilutions of plasma from vitellogenic females parallelled the standard Vtg curve, whereas no cross-reaction was observed with the plasma of males in the ELISA. The Vtg ELISA was used to assess the induction of Vtg by E2in vivo in males. The induction of Vtg in greenback flounder showed a time- and dose-dependent response as in other species. In E2-treated fish, detectable levels of Vtg were first found at 48 h, and reached a peak at 96 h post-injection. Plasma levels of Vtg increased as the E2 dose increased with a threshold of ∼ 0.1 mg kg−1.

Correlation between oocyte development and plasma concentrations of steroids and vitellogenin in greenback flounder Rhombosolea tapirina

The development of oocytes in association with changes in plasma concentrations of vitellogenin (Vtg), 17β-estradiol (E2) and testosterone (T) was investigated in maturing female greenback flounder Rhombosolea tapirina over the first part of a reproductive season. During vitellogenesis, increasing oocyte size was accompanied by the sustained elevation of plasma Vtg and E2. There was a marked increase in T towards the end of vitellogenesis.

Histological, growth and 7-ethoxyresorufin O-deethylase (EROD) activity responses of greenback flounder Rhombosolea tapirina to contaminated marine sediment and diet

Pathological abnormalities and mixed function oxygenase (MFO) enzyme changes are frequently used as indicators of anthropogenic contaminant exposure and effect. However, there is a paucity of research investigating the effects of contaminated sediment on native Australian benthic teleosts. As part of an ecotoxicological assessment of contaminated marine sediments in northern Tasmania, CYP1A induction, histological and growth response of the greenback flounder, Rhombosolea tapirina, exposed to contaminated marine sediments were examined. Hatchery reared flounder were exposed to reference sediment, contaminated sediment or contaminated sediment and diet for 6 weeks. CYP1A induction, using the ethoxyresorufin- O-deethylase (EROD) assay, and the histological and growth response in the flounder were examined on cessation of the exposure trial. Significant differences were found between treatments in histological, growth and EROD response. Exposure to contaminated sediment and diet elicited a multi-organ histological response: principally partial and total epidermal erosion and multifocal necrosis of the liver. The prevalence of total epidermal erosion was greatest with exposure to disturbed contaminated sediment (66.65±16.65%). The prevalence of multifocal necrosis of the liver was greatest with exposure to contaminanted sediment and diet (66.65±16.65%). Growth reduction, measured as percentage growth inhibition, was evident in flounder exposed to contaminated sediment and diet (18.2±11.99%). Additionally, exposure to contaminated sediment and diet elicited elevated induction of the EROD liver detoxification enzyme (139.65±24.22 pmol/min/mg protein) compared to exposure to contaminated sediment and non-contaminated diet (6.25±0.81 pmol/min/mg) indicating the presence and potential bioavailability of xenobiotics via food. Further, more inhibited growth and histological alteration associated with exposure to contaminated sediment and diet suggest contaminants in Deceitful Cove sediment are cytotoxic.

The effect of meal size and meal duration on food anticipatory activity in greenback flounder

Latency of food anticipatory activity (FAA) in greenback flounder Rhombosolea tapirina was about 21 days. Fish fed at meal sizes of 0·25 and 0·5%W day−1 exhibited FAA under meal durations of 1, 3 and 7 h. Fish fed at 1·5%W day−1 showed FAA only at a meal duration of 1 h. At each meal size, FAA was shorter and lower the longer the duration of the meal. The mean durations of FAA and post‐feeding activity were correlated positively (r=0·87; P<0·01; n=7). FAA persisted for <3 days during food deprivation. It is suggested that greenback flounder was capable of evaluating the energetic and temporal impacts of a single daily meal.

The effect of feeding regime on growth, locomotor activity pattern and the development of food anticipatory activity in greenback flounder

The mean final weight of juvenile greenback flounder Rhombosolea tapirina fed a single daily meal during the scotophase was lower than that of groups fed continuously or fed twelve, six, three or one meal during the photophase. Final weight did not differ among the latter five groups. Size variation in all groups of fish decreased during the course of the trial over 120 days. Groups assigned to a higher frequency (and hence a longer total daily meal duration) of daily meals were less active during mealtime. A distinct food anticipatory effect was observed in the groups of fish receiving a single daily meal during either photophase or scotophase, and also developed before at least one meal in the group of fish fed three meals per day.

Characterization of parameters for in vitro culture of isolated ovarian follicles of greenback flounder Rhombosolea tapirina

Isolated ovarian follicles of greenback flounder Rhombosolea tapirina were incubated with a variety of gonadotropins (GtHs) and steroid precursors for periods of up to 42 h, and levels of free and glucuronated testosterone (T) and 17β-estradiol (E 2) in the medium, and free T and E 2 from inside follicles were measured by RIA. Short incubations (6 h) generated increases in T and E 2 in response to steroid precursors, but not human chorionic GtH (hCG), or salmon or carp GtH. At incubation times of 18 h, all GtHs stimulated T and, or E 2 production, whereas after 42-h incubation, GtH effects on E 2 production had disappeared. Steroid precursors remained effective at 18 and 42 h. T and E 2 glucuronides were formed in small quantities but did not account for loss of treatment effects at long incubation times. Instead, this could be explained by accumulation of E 2 in controls as a result of continued basal steroid production. Follicles absorbed substantial amounts of both endogenous and exogenous steroid from the medium, however, this did not appear to have any influence on changes in treatment effects with incubation time. Flounder follicles were most sensitive to hCG, followed by salmon and carp GtH at approximately 10-fold higher concentrations. Ovarian segments were not sensitive to any GtH but did convert exogenous steroid precursors indicating that tissue access by GtH may be a limiting factor under certain in vitro conditions. HCG augmented the conversion of 17-hydroxyprogesterone (17P) to T but not T to E 2, consistent with the relative GtH-insensitivity of aromatase in other species. Follicles converted a range of steroid precursors with equal competence, indicating that no step in the cleavage pathway is strongly rate-limited, and that choice of precursor is unlikely to affect the assessment of steroidogenic activity.

Milt characteristics and plasma levels of gonadal steroids in greenback flounder Rhombosolea tapirina following treatment with exogenous hormones

Fish were treated with exogenous hormones, and milt and blood samples were collected for up to 96 h post‐treatment. Blood plasma samples were assayed for the gonadal steroids testosterone (T), 11‐ketotestosterone (11KT) and 17,20ß‐dihydroxy‐4‐pregnen‐3‐one (17.20ßP). Milt volume, spermatocrit and sperm motility were measured from milt samples. Non‐spermiated fish showed increased plasma T and 11KT in response to human chorionic gonadotropin (hCG) but not luteinising hormone releasing hormone analogue (LHRHa). Fish did not become spermiated in response to treatment with hCG, LHRHa, 11KT, 17‐hydroxyprogesterone (17P) or 17,20ßP. Spermiated fish showed an increase in milt volume in response to hCG and LHRHa but not exogenous steroids. Sperm motility declined to zero over 120 s and was not affected by hormone treatment or sampling time. Increased milt volume was accompanied by increased plasma T and 11KT, but not 17.20ßP levels. In a separate experiment, LHRHa delivered by injection or pellet was equally effective at increasing milt volume but had no effect on plasma steroid levels. Spermatocrit declined with stripping but was not affected by hormone treatment, nor was sperm motility. Co‐treatment of fish with 17P plus LHRHa had no additive effect on plasma steroid concentrations or milt volume. The results suggest that as in other teleosts, gonadotropin mediates milt production in greenback flounder.

Immune response of greenback flounder Rhombosolea tapirina after exposure to contaminated marine sediment and diet

Non-specific immune response of greenback flounder, Rhombosolea tapirina, exposed to contaminated marine sediments was examined. Reference sediments from Port Sorell and contaminated sediments from Deceitful Cove, Tasmania, Australia were investigated. Hatchery-reared flounder were exposed to reference sediment, contaminated sediment or contaminated sediment and diet for 6 weeks. Phagocytic capacity and lysozyme response in flounder were examined on cessation of exposure trial. Significant differences were found in phagocytic capacity and lysozyme response between treatments. Exposure to contaminated sediment, irrespective of diet or benthic disturbance elicited inhibition of phagocytic efficiency in flounder. Disturbance of contaminated sediment stimulated lysozyme activity. The immune response in flounder indicates potential immunotoxicity of sediment from Deceitful Cove.

Effect of Luteinising Hormone-Releasing Hormone Analogue and Human Chorionic Gonadotropin on Ovulation, Plasma and Ovarian Levels of Gonadal Steroids in Greenback Flounder Rhombosolea tapirina

Female greenback flounder Rhombosolea tapirina were treated with either des Gly10 [D-Ala6] LHRH ethylamide (LHRH-a) at 50 or 100 pg/kg intraperitoneal injection (ipi), LHRH-a cholesterol pellet (LHRH-a pellet) 100 μg/kg implanted intraperitoneally, or human chorionic gonadotropin (hCG) 1,000 IU/kg ipi. Treatment with hCG, LHRH-a (50 μg/kg) ipi and LHRH-a pellet increased the total number of ovulations above control levels. LHRH-a (100 μg/kg) ipi was not consistently more effective than the control and both LHRH-a (50 μg/kg) ipi and LHRH-a pellet induced more ovulations than LHRH-a (100 μg/kg) ipi. Of those fish that ovulated, most ovulated more than twice, and most ovulations occurred at daily intervals. Oocyte diameters increased and oocyte stage advanced significantly in response to all exogenous hormone treatments. This was accompanied by increases in plasma and ovarian levels of 17β-estradiol (E2, and in most cases, plasma and ovarian levels of testosterone (T). Plasma and ovarian levels of 17,20β-dihydroxy-4-pregnen-3-one (17,20βP) were not consistently elevated in association with reproductive events.

Feeding behaviour of greenback flounder larvae, Rhombosolea tapirina (Günther) with differing exposure histories to live prey

This study examined the feeding performance of cultured larvae of the greenback flounder Rhombosolea tapirina, using the live feed organisms, Brachionus plicatilis and Artemia sp., to determine the primary sensory modality involved in feeding, and the effect of previous exposure to prey on subsequent prey selection. The proportion of larvae that fed on rotifers in the light (400–700 nm daylight fluorescent tubes — light intensity of 5–6 μmol s −1 m −2), increased significantly from 66% to 96% from day 12 to day 27 post-hatching, respectively. In comparison, the proportion of larvae that fed on rotifers in total darkness, never exceeded 5% during the same period. This indicates that greenback flounder larvae are primarily dependent upon vision to feed. Prior feeding experience of greenback flounder larvae with either rotifers only (R-treatment) or a mixed diet of Artemia and rotifers (A&R-treatment) significantly affected subsequent prey selection when larvae were offered a mixed diet of Artemia and rotifers. It did not, however, affect the temporal onset of selection of the novel prey species ( Artemia) by R-treatment larvae. Therefore, the differences in prey selection by larvae, with or without prior exposure to Artemia prey, were not due to the inability of larvae to handle and ingest a novel prey species, but they reflect positive selection for familiar prey. The latter indicates a learned component in the feeding behaviour of fish larvae. This has implications for the timing of the introduction of new live prey species during intensive culture of marine fish larvae.

A comparison of sex steroid binding protein (SBP) in four species of teleost fish.

A sex steroid binding protein (SBP) binding E2 with high affinity has been detected in a pleuronectid (greenback flounder Rhombosolea tapirina), two sparids (black bream Acanthopagrus butcheri and snapper Pagrus auratus), and its presence has been confirmed in a salmonid (rainbow trout Oncorhynchus mykiss). SBP binding characteristics were measured using a hot saturation assay for trout, bream and snapper, and a cold saturation assay for flounder. Bound and unbound steroid were separated by incubation with dextran-coated charcoal (DCC). Affinity for E2 was highest in trout (kD=0.44 nM), followed by bream (kD=3.39 nM) and snapper (kD=10.7 nM). The lowest affinity was found in flounder (kD=84.7 nM). Binding capacity, however, was greatest in flounder (Bmax=164 nM), followed by trout (Bmax=92 nM), and then bream and snapper (Bmax=50 and 39 nM, respectively). Binding of E2 to SBP had a very rapid rate of association, and most dissociation occurred within 5 min. To confirm that the plasma protein measured here was SBP, the relative binding affinities of SBP for a range of steroids were measured. In trout, bream and snapper, SBP bound E2 with the highest affinity, followed by T. In contrast, the relative affinity of T for flounder SBP was more than twice that of E2. The rank orders of affinity of binding indicate the importance of an unhindered 17β-hydroxyl group, and a 3-hydroxyl or 3-ketone group for high affinity binding to SBP. These requirements for high affinity binding are present in most animals possessing SBP and indicate conservation of the SBP molecule through evolution.

The combined effect of feeding time and ration on growth performance and nitrogen metabolism of greenback flounder

Greenback flounder Rhombosolea tapirina (c. 2 g) fed to satiation had significantly (P <0·01) higher feed consumption in the evening than in the morning whereas there was no difference between feeding times for flounder fed restricted rations (1 or 2% body weight per day) because they consumed all of the ration. Differences in growth performance were due to feeding time and ration. Carcass moisture, lipid and energy content were significantly (P <0·001) different between rations; length gain was significantly affected by feeding time (P <0·05) and ration (P <0·001); weight gain showed a significant (P <0·001) interaction between feeding time and ration. The relationship between feed consumption and specific growth rate showed that the exponential gradient was significantly higher (P<0·01) for the evening fed fish and indicated feed efficiency for evening fed fish increased as feed consumption increased. Urea excretion increased from 12–20 to 58–63% of total nitrogen excretion at the 1 and 3% rations, respectively. Ammonia and urea excretion were significantly affected by ration (P <0·001) and feeding time (P <0·05). Fish fed the 2% ration in the evening had higher growth efficiency and significantly (P <0·01) lower rates of urea excretion than fish fed 2 or 3% ration in the morning. It is suggested that the higher energetic costs associated with differences in ammonia and urea excretion contributed to differences in growth efficiency.

Effect of processing soybean meal on the apparent digestibility of practical diets for the greenback flounder Rhombosolea tapirina (Günther)

Aquaculture ResearchVolume 30, Issue 9 p. 719-723 Effect of processing soybean meal on the apparent digestibility of practical diets for the greenback flounder Rhombosolea tapirina (Günther) M. P. Bransden, M. P. Bransden School of Aquaculture and Tasmanian Aquaculture and Fisheries Institute, University of Tasmania, Launceston, Tasmania, AustraliaSearch for more papers by this authorC. G. Carter, C. G. Carter School of Aquaculture and Tasmanian Aquaculture and Fisheries Institute, University of Tasmania, Launceston, Tasmania, AustraliaSearch for more papers by this author M. P. Bransden, M. P. Bransden School of Aquaculture and Tasmanian Aquaculture and Fisheries Institute, University of Tasmania, Launceston, Tasmania, AustraliaSearch for more papers by this authorC. G. Carter, C. G. Carter School of Aquaculture and Tasmanian Aquaculture and Fisheries Institute, University of Tasmania, Launceston, Tasmania, AustraliaSearch for more papers by this author First published: 09 October 2008 https://doi.org/10.1046/j.1365-2109.1999.00367.xCitations: 11 Dr C G Carter, School of Aquaculture, University of Tasmania, PO Box 1214, Launceston, Tasmania 7250, Australia AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Citing Literature Volume30, Issue9September 1999Pages 719-723 RelatedInformation

Diel feeding rhythms of greenback flounderRhombosolea tapirina(Günther 1862): the role of light-dark cycles and food deprivation

The role of light–dark (LD) cycles and food deprivation in the regulation of diel feeding rhythms in greenback flounder Rhombosolea tapirina (Günther 1862) was examined. A computerized monitoring system was developed to record uneaten pellets, and food intake was estimated as the difference between pellets presented and those uneaten. Three groups of five fish each (75–281 g) were exposed sequentially to the LD cycles: LD 12:12, LD 12:12 (scotophase advanced by 9 h), LD 6:18, DD, LD 6:6:6:6, LL and LD 12:12 (return to the initial cycle). Illumination was 120 lux at the water surface (complete darkness at scotophase), and water temperature was 15 ± 1 °C. Fish fed mainly during the light hours under LD 12:12 and resynchronized to the scotophase-advanced LD 12:12 regime after about 12 h. Feeding started at the onset of light and extended to the early scotophase under LD 6:18. A circadian feeding rhythm was detected in fish under DD and LD 6:6:6:6 regimes. Under the LL regime, two groups of fish displayed arrhythmic feeding patterns and did not resynchronize to LD 12:12 for at least 13 days. In contrast, the third group of fish exhibited a circadian feeding rhythm under the LL regime and immediately re-entrained to LD 12:12. Fish were deprived of food for a single period of 26–57 h once feeding patterns had been established under LD 12:12, LD 12 : 12 (scotophase advanced by 9 h) and DD regimes, and feed delivery was resumed during the scotophase or subjective night; the timing of feeding rhythms was shifted by the reintroduction of feed, but progressively resynchronized to the LD cycles. The results suggest that the greenback flounder is a diurnal feeder, that a LD 12:12 cycle is a potent environmental cue to entrain circadian feeding rhythms, and that a biological clock is involved in the timing of feeding.