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Cryopreservation of sea bass (Dicentrarchus labrax) spermatozoa in experimental and production simulating conditions

Abstract A sperm cryopreservation protocol adapted from turbot, was tested on sea bass using either 250-μL straws or 1.5-mL cryovials. A dilution to 1/3 in Mounib s extender and a cooling rate of −65 °C·min −1 allowed frozen sperm to recover an initial motility similar to that of fresh sperm at thawing; however, significant differences in motility ( P n = 10 fish semen) were observed at further post-activation times, the motility decrease being faster in thawed sperm. At the experimental scale, triplicate inseminations of 2-mL aliquots (approximately 2 000 eggs) showed a significant fertility decay of thawed sperm compared to that of fresh sperm ( P n = 12 fish semen) when a discriminating 35·10 3 spermatozoa to egg ratio was applied. When 70·10 3 and 200·10 3 spermatozoa per egg were provided in the same experimental conditions, no significant difference appeared between the fertilisation rates of fresh and thawed sperm. In order to validate the procedure for production or cryobank purpose, a scaled-up protocol was established. Two and 50 mL batches of eggs (approximately 2·10 3 and 50·10 3 eggs, respectively) were inseminated in triplicate using either fresh or thawed individual sperms of 5 males with 200·10 3 spermatozoa per egg. The mean fertility decreased by 23.5 % due to cryopreservation. This decline was explained by the loss of fertility of only one sperm, and only in large-volume conditions, probably due to the delay of use after thawing.

Sexual growth dimorphism in sea bass Dicentrarchus labrax

Abstract The growth of 100 sea bass feminized by estradiol treatment and 200 individually tagged controls was followed from the age of 10 months [27±1.7 g, mean±confidence interval (95%)] to 45 months (1160±46 g) in order to analyze the evolution of sexual dimorphism as a function of the age and sexual maturity. One hundred percent females were found in the treated group ( n =94) versus 55% in the control nontreated group ( n =171). Males remained significantly smaller than both females and feminized fish from the beginning of the experiment. The resulting weight advantage [(female mean weight−male mean weight)/male mean weight, expressed as a percentage] for females was much more pronounced during the early stages of development (67% at 10 months of age), decreased in the second year of life and then became stabilized around 25%. Females and feminized fish retained the same growth, suggesting that growth is related to phenotypic sex in the sea bass. The growth rate was influenced by sex in the fourth year of life, period during which the females grew faster in terms of both weight and length. Compared to the males, they had a relatively higher ratio of digestive tract to body weight (+26%), but lower ratios of visceral fat to body weight (−49%) and muscle lipid content (−16%) at slaughtering, at which time they had finished their second vitellogenic cycle (45 months).

Effects of rearing density, size grading and parental factors on sex ratios of the sea bass (Dicentrarchus labrax L.) in intensive aquaculture

Abstract The effects of rearing density and size grading on sex ratios in 30 families of sea bass reared in the same tank from the fertilization stage onwards were investigated. Two extreme density treatments (high versus low) were applied in quadruplicates throughout the labile period for sex determination. An excess of males (88%) was found in all groups. Growth was faster under low density from 49 to 191 days post fertilization (p.f.) (27–10% relative difference in length) and was then equivalent between the two groups until the end of the treatments (414 days p.f.). Density had no effect on sex ratio, suggesting that the high densities usually applied in aquaculture are not involved in the systematic excess of males reported in farmed populations. The fish were genotyped at three to six microsatellite loci. This allowed them to be assigned to the individual breeders used in the mating design. The percentage of females was significantly influenced by both the dam and the sire, highlighting the existence of genetic variation of sex ratio. Repeated size grading performed from 84 to 199 days p.f. had no effect on the sex ratio of the overall population studied showing that sex determination had not been affected by the treatment. However, the highest percentages of females were found in the lead classes from 84 days p.f. Most of the fish in this lead group were descended from one dam and the percentage of females found in the individual families were higher in this class than in the non-graded group. These results show that females start growing faster than males long before morphological sexual differentiation occurs.

Production of meiotic gynogenetic and triploidsea bass, Dicentrarchus labrax L. 1. Performances, maturation and carcass quality

Abstract Meiotic gynogenetic and triploid sea bass were produced by pressure shocks according to a previously published protocol. Pressure-treated groups did not survive as well as controls during early development and larval rearing. Performances, sexual maturation and carcass quality were examined over a period of 34–45 months. At the age of 34 months, growth of the gynogenetic fish was comparable to that of the control but inferior in the triploid fish. A predominance of male fish was found within the triploid groups, while diploid and meiotic gynogenetic fish showed equal proportions of the sexes. Gonadal maturation in triploid fish was significantly impaired, particularly in the females that showed rudimentary ovaries. Triploid males exhibited primary maturation but proved to be gametically sterile. Pressure-induced triploids did not grow as well as diploids, but these results might be ascribed to specific on-growing conditions (communal rearing). The performance of gynogenetic sea bass was comparable to that of control. The superiority of diploid fish over their triploid counterparts was confirmed during the final growing period and more clearly so in females. Performances of triploids varied according to their maternal origin. Overall, striking qualitative differences between diploid and triploid fish were found at the age of 34 and 45 months, although the results varied in a gender-specific manner. A strong maternal effect was also observed. The potential advantages of triploid sea bass for aquaculture purposes are discussed.

Sexual differentiation and juvenile intersexuality in the European sea bass (Dicentrarchus labrax)

Sexual differentiation was studied at the histological level using a mixture of 30 families of sea bass Dicentrarchus labrax. Most of the fish (93%) differentiated into males as usually observed in farmed populations. All testes were differentiated when the males reached 12 cm and no more undifferentiated fish were found from 419 days post-fertilization (p.f.). In 28% of the males, among the biggest, sexual differentiation had already begun at 168 days p.f. (8.3–9.5 cm) and these fish started spermatogenesis in their first year of life. The other males differentiated later and remained immature at the end of their first year of life. Ovaries could be identified at the histological level from the age of 168 days p.f. (7.9–9.0 cm) and the females became significantly longer than the males from the age of 191 days p.f., i.e. during the process of ovarian differentiation. In the studied group, 62% of the males developed intratesticular oocytes. Such intersexuality had no consequence on growth rate. Intratesticular oocytes were also recorded in testes of wild males originating from Atlantic (Britain and Gulf of Gascogne) and West Mediterranean showing that juvenile intersexuality is not restricted to farmed populations but is a widespread phenomenon in sea bass.

Saline preferendum for the European sea bass, Dicentrarchus labrax, larvae and juveniles: effect of salinity on early development and sex determination

Abstract The sea bass, Dicentrarchus labrax , is an eurhyaline marine fish. Juveniles of this species are known to frequent estuaries and lagoons where salinity is lower than in the open sea. Sex determination occurs during this phase of fish life and has been shown labile and sensitive to environmental factors. In this work, the effect of rearing salinity on sex determination and early development of the sea bass was investigated. An excess of males (87%) was found in all groups and the salinity level [(natural sea water salinity, mean: 37‰) vs. (15‰)], when maintained constant, had no effect on the sex-ratio. The transfer from low to high salinity at 93 days post-fertilization (p.f.) increased the percentage of males (93%) suggesting that sexual differentiation in this species may be influenced by such an osmotic stress. Growth was improved by a 15‰ salinity at the beginning of larval rearing (14 days p.f.) and at the end of pre-growing (234–458 days p.f.), periods during which low temperatures were applied. Survival during larval rearing and nursery were also improved in the groups reared at low salinity and so was swimbladder inflation. These results show that sea bass juveniles have a low saline preferendum, a finding that corresponds to the conditions they actually encounter in the wild during their juvenile ecophase; sex determination is not directly modulated by the salinity level but seems to be subjected to complex environmental regulations.

Viral infection associated with rotifer mortalities in mass culture

Abstract Viral lesions were observed in rotifers, Brachionus plicatilis, during their mass culture as living food for seabream and turbot larvae reared in a French experimental hatchery. Isolated and purified virions, called RBV, exhibiting a buoyant density of 1.29–1.30, were non-enveloped, icosahedral in shape, and about 60 nm in diameter. When observed in thin sections or negatively stained, they exhibited the morphological characters of the family Birnaviridae. Because one virus of this family (IPNV) is considered highly pathogenic for salmonid fishes, and one other of this type (SBV) has been associated with seabass larvae mortalities, the possibilities of transmission to fish fed with such infected rotifers is discussed.

Infectious diseases affecting mass production of the marine rotifer Brachionus plicatilis

During recent years, investigations on decrease of rotifer density in culture tanks from several hatcheries have shown that infectious diseases might be associated with abnormal mortality. The first reported infection was caused by an unusual birnavirus referred as rotifer birnavirus (RBV). Viral lesions appeared in cells of gastric glands and spread to other epithelia. Virus particles were also found within ovarian cells. Highly infected rotifers exhibited abnormal behavior and died. The second important pathogen found in Brachionus plicatilis has been provisionally related to the Lagenidiaceae. The fungus developed in up to 85% of rotifers and caused sudden, precipitous decrease in animal density. The occurrence of these diseases shows that in the future infectious agents will have to be considered as important factors capable of reducing the productivity of rotifer mass cultures.