Larval Turbot Scophthalmus Maximus Research Articles

The ontogeny of acidic digestive tract in larval turbot Scophthalmus maximus based on H+/K+-ATPase and pepsinogen

The ontogeny of acidic digestive tract in larval turbot Scophthalmus maximus based on H+/K+-ATPase and pepsinogen

Digestion of copepod eggs by larval turbot Scophthalmus maximus and egg viability following gut passage

Between 20.5 and 93.6 % of the subitaneous eggs of 6 species of egg-carrying copepods passed undigested through the digestive tracts of larval and early postlarval turbot Scophthalmus maximus. Viability of the eggs of Eurytemora affinis, E. velox and Euterpina acutifrons remained high on egestion (67.0 to 91.7 %), Pseudocalanus elongatus and Oncaea venusta eggs had low viability (1.1 to 1.5 %), while all Corycaeus anglicus eggs were rendered inviable. The indigestibility of the eggs denies the turbot larvae a potentially valuable food resource, while retention of high egg viability in certain species reduces the effect of predation.

Influence of rearing temperature on the distribution of muscle fibre types in the turbot Scophthalmus maximus at metamorphosis

Larval turbot Scophthalmus maximus L. which were undergoing metamorphosis were divided into two groups and reared at either 17 or 22°C for 1 month. Fish in the two groups had a similar standard length and body mass at the end of the experimental period. Three muscle fibre types were distinguished in posterior myotomes using histochemical techniques. Tonic fibres which stained weakly for myofibrillar ATPase activity (MATPase) and succinic dehydrogenase activity (SDHase) formed a scattered superficial layer on the dorsal and ventral surfaces. The number of tonic muscle fibres/myotome was 43% higher in fish reared at 17°C (63) than at 22°C (44) ( P < 0.01). Superficial red fibres, which stained heavily for SDHase, constituted 10.7% of the myotomal muscle cross-sectional area in 17°C-acclimated turbot compared with only 7.8% in 22°C-acclimated fish ( P < 0.01). Red fibres showed an intermediate staining for myofibrillar ATPase at pH 9.4 relative to tonic and white fibres, and were the most stable fibre type following alkaline preincubation (pH 10.4). In this respect the red fibres of juvenile turbot resemble the fast red or intermediate muscle fibre types of adult teleosts. Red and white fibres stained much more heavily for glycogen in turbot reared at 17 than 22°C. It is concluded that relatively small differences in rearing temperature are sufficient to affect the distribution of muscle fibre types and energy storage levels in larval and juvenile fish.

Metabolism of larval turbot Scophthalmus maximus (L.) and uptake of amino acids from seawater studied by autoradiographic and radiochemical methods

The ability of larval turbot Scophthalmus maximus (L.) to ingest labelled 14C-alanine seawater during their yolk-sac stages was investigated by means of autoradiographic and microscopic methods. Autoradiograms showed that after 24 h of exposure to 14C-alanine virtually all of the label was located in the intestinal lumen. Larval metabolism was estimated by measuring changes in dry wt, RNA:DNA ratio and C:N ratio. Dry wt decreased by ≈20% during yolk-sac absorption at 14°C and 20‰. No significant difference was observed in the RNA:DNA ratio in the larval body during this period. The C:N ratio decreased during the study. Catabolic and anabolic metabolism of the assimilated labelled amino acid was evaluated by measuring the production of 14CO 2 and the incorporation of radioactivity into molecules insoluble in TCA. Yolk-sac larvae were incubated for 24 h in labelled 14C-alanine seawater and then transferred to unlabelled media; release of labelled CO 2 by the larvae was low, but the amount of tracer incorporated into the TCA-insoluble fraction of the larval tissues was large. The data strongly suggest drinking activity and a concomitant intake of amino acids in turbot larvae during the yolk-sac stage. The results indicate that the labelled amino acid presumably contributes to the general metabolism of the larvae.

Encephalomyelitis among turbot associated with a picornavirus-like agent

An outbreak of encephalomyelitis among larval turbot ScophthaLmus maximus caused very heavy mortalities. Bacteriological findings were very unspecific, and parasites were not observed. By electron microscopy large numbers of virus particles were found in the brain and medulla of the diseased fish. Based on the size of the particles and their position in the cytoplasm of the infected cells it is suggested that the particles belong to the picomaviridae. The neurotropic character of the disease further suggests that the virus might be an enterovirus. Parallel with the development of hatching and larval rearing techniques of new species in aquaculture, new disease problems of parasitological, bacterial, and viral origin occur. In turbot production, vibriosis appears to be a common problem, and we have hitherto isolated Vibrio anguillarum serogroups 01, 02a, and 02b as well as members of the Vibrio splendidus/pelagius group. A more complex ulcerative disease has been described in Spain involving 3 facultative pathogens, Vibrio carchariae, Myxobacteria, and a protozoan resembling Cryptocaryon spp. (Devesa et al. 1989). Viral agents demonstrated in turbot include 'Herpesvirus scophthalmi' (Buchanan & Madeley 1978), infectious pancreatic necrosis virus (Castric et al. 1987) and a virus which was suggested to be a member of the Reovirus group (Lupiani et al. 1989). This report deals with an investigation of a disease associated with heavy mortalities among young turbot Scophthalmus maximus in a Norwegian fish farm. Materials and methods. The disease problem appeared in an extensive procruction unit near Bergen, Norway. Turbot larvae were reared in 18 floating bags (50 m3) in a maine lagoon (140 000 m3). The bags were stocked with 80 000 to 150 000 eggs per batch at the Author for correspondence O Inter-ResearchIPrinted in Germany beginning of June 1989. The temperature was 12 to 13 C at the time of stocking, but increased gradually to 17 or 18°C over the next 2 to 3 wk, with surface water temperatures reaching 20 C around noon. In late June the weather changed, and heavy rainfall, accompanied by a drop in surface temperatures to between 14 and 15C, occurred. The larvae were fed zooplankton collected from the lagoon. They seemed to develop normally until the beginning of July, when heavy mortality occurred. In the first batch of larvae, the mortality appeared during weaning to moist pellets (weight 50 to 100 mg). Later, the clinical signs associated with the mortality in the first batch were observed among other batches of larvae in the lagoon. The last batches, stocked in August 1989, died within 3 wk after hatching. Fish transferred to weaning tanks that exhibited mortalities were treated with antibiotics (Nifurazolidone). This did not reduce mortality. The initial clinical signs were reduced feeding activity followed by a darkening in pigmentation. Diseased fish became lethargic, often lying abdomen-up on the bottom. Atypical swimming, such as rotating, spinning, and horizontal looping, was observed when the fish were disturbed. The clinical signs indicated disturbance of the central nervous system and were followed by 100 % mortality. Samples were taken from internal organs (pronephros, liver, spleen) and plated on tryptic soy agar containing 2 % NaCl or marine agar (Difco) with 5 % citrated calf blood, and TCBS agar (Difco). Incubation was at 20°C for 48 h. Resulting isolates were characterized according to West & Colwell (1984) and Larsen (1985). Gills, skin, and intestine were examined for the presence of parasites by light microscopy. 5 k g 5. & ,< a k g b

Use of Endogenous Energy Sources by Larval Turbot Scophthalmus maximus

Yolk-sac larvae of turbot from the Baltic Sea were raised at 15, 18, and 21°C in brackish water (17‰ salinity) and larvae from the North Sea were raised at 15°C in seawater (32‰). Rate of yolk utilization and growth of larvae in length and weight were monitored. Decrease of yolk weight was correlated with time and temperature, whereas the increment of larval tissue reached a peak after 2/3 of the yolk-sac phase. During the last 24 h before the larvae acquired the ability to feed, they suffered from energy deficiencies. North Sea larvae, which floated passively at the surface of full-strength seawater, consumed yolk less rapidly than Baltic Sea larvae, which sank in water of 17‰ salinity and had to swim. Yolk-utilization efficiencies (percentage of yolk energy transformed into tissue energy) were highest in seawater at 15°C (72%) and lowest in brackish waters at 21°C (51%). Efficiencies decreased with time during development under all experimental conditions.

The food of larval turbot Scophthalmus maximus L. from the west central North Sea

An examination was made of the gut contents of the larvae of turbot 3·5–16·9 mm in length, from off the northeast coast of England. All the larvae contained a wide variety of planktonic crustaceans the most important of which were the cladocerans Podon leuckarti and Evadne nordmanni and copepodites of five species of calanoid copepods. Oikopleura dioica was also taken. Phytoplankton was found in the stomachs of some of the smaller larvae. It was estimated that the larvae ate 28·0% of their body weight each day.