Conditioning of the European flat oyster (Ostrea edulis,Linnaeus 1758): effect of food ration

To establish a reliable protocol for the broodstock conditioning of the oyster Ostrea edulis (Linnaeus 1758) in hatcheries is important to obtain the best number of viable larvae all year even outside the natural season for maturation and spawning. Hatchery production of bivalves depends on their initial gonadal maturation state, which is linked to season and several other factors, such as temperature, daylight, salinity and food supply. Ration and diet composition are important in broodstock conditioning because food availability affects fecundity and gametogenic development. In this work, the effect of food ration on O. edulis conditioning was studied in autumn and winter. The results of this study showed that the broodstock conditioned with a food ration of 6% and 9% produces the highest number of larvae in autumn and winter. These food rations produced rapid gonadal development and larval release in a short period with a similar larval production. The oysters conditioned with a food ration of 3% delayed larval release and produced a significantly lower number of larvae in autumn. In the 9% ration treatment, the percentages of mortality were significantly higher in both autumn and winter. Thus, a 6% food ration is likely to be the most suitable for flat oyster conditioning combined with a gradient of temperature (14–18°C) and a gradient of daylight (8–16 h).     

Effects of temperature and photoperiod on the conditioning of the flat oyster (Ostrea edulis L.) in autumn

The production of the flat oyster Ostrea edulis (L.) natural spat in Europe has decreased almost by 60% in the past ten years. Thus, the importance of the production of oyster spat in hatcheries is evident. One of the critical steps in hatchery production is broodstock conditioning, especially difficult in autumn, when gonadal development is in resting period. Conditioning is influence by temperature, photoperiod and nutrition. In this work, the effects of two temperature and three photoperiod regimes on the conditioning of O. edulis were studied for three years by stereological analyses and registering number and dates of spawning and larval yield. Temperature had a positive effect on the gonadal development of O. edulis during conditioning. The percentages of germinal cells in oysters conditioned with a gradient of temperature (14–18°C) were double compared to oysters conditioned at 15°C. Oysters conditioned with longer photoperiods showed higher percentages of germinal cells. There was no interaction between temperature and photoperiod. Spawning was observed in the oysters treated with daylight (8–16 h) ten weeks from the beginning of conditioning. Flat oysters conditioned with 8 h and 8–12 h of daylight delayed the first spawning for a month. Total larval production was higher in the oysters treated with the longest daylight gradient. Gonadal and gametogenic development was a non‐synchronic process and the spawning extended for around two months. A protocol for flat oyster broodstock conditioning in autumn by using both a gradient of temperature (14–18°C) and daylight (8–16 h) is proposed.     

Infection dynamics of Bonamia exitiosa on intertidal Ostrea angasi farms

Bonamia spp. cause epizootics in oysters worldwide. In southern Australia, Bonamia exitiosa Hine, Cochennac and Berthe, 2001 threatens aquaculture of Ostrea angasi Sowerby, 1871. Bonamia spp. infections can display strong seasonality, but seasonal dynamics of B. exitiosa–O. angasi are unknown. Ostrea angasi naïve to B. exitiosa infection were stocked onto farms in three growing regions, and B. exitiosa was monitored seasonally for one year. Environmental parameters we measured did not correlate with B. exitiosa prevalence or infection intensities. Extreme temperatures suggest O. angasi culture systems need development. Bonamia exitiosa prevalence increased over time. After three months, O. angasi had B. exitiosa prevalence of 0.08–0.4, and after one year, the prevalence was 0.57–0.88. At some sites, O. angasi had >0.5 B. exitiosa prevalence in >6 months, but at other sites, >9 months passed before prevalence was >0.5. Bonamia exitiosa infection intensities were low with no seasonal pattern but were affected by the interaction of site, season and oyster meat:shell ratio. Understanding infection and initiating a breeding programme for resistance would provide benefits for O. angasi industry expansion.     

Rapid transmission of Bonamia exitiosa by cohabitation causes mortality in Ostrea angasi

The haplosporidian Bonamia was first detected in Australian shellfish in 1991. Australian isolates in Ostrea angasi Sowerby, 1871 were identified as Bonamia exitiosa Hine, Cochennac and Berthe, 2001, which threatens development of an O. angasi aquaculture industry. European field data suggest that Bonamia ostreae Pichot, Comps, Tigé, Grizel and Rabouin, 1980 infections in Ostrea edulis Linnaeus, 1758 build slowly, but infection dynamics of B. exitiosa in O. angasi are unknown. We investigated B. exitiosa infection in O. angasi by cohabiting uninfected juvenile O. angasi with adults infected with B. exitiosa. Oysters were sampled at 10, 21 and 40 days after cohabitation, and B. exitiosa prevalence and intensity were assessed. Bonamia exitiosa rapidly infected and caused disease in O. angasi. Mortalities began at 12 days, with ˜50% mortality by day 21 and >85% mortality by day 40. Mortalities displayed pathology consistent with clinical B. exitiosa infection. Time to first infection is likely influenced by a combination of parasite infectivity, host exposure and host immune capacity. Host death is not required for transmission, but probably facilitates release of parasites from decaying tissue. Understanding B. exitiosa transmission informs design and interpretation of field studies and aids development of management strategies for oyster aquaculture.     

Bonamia in Ostrea angasi: Diagnostic performance,field prevalence and intensity

Bonamia spp. parasites threaten flat oyster (Ostrea spp.) farming worldwide. Understanding test performance is important for designing surveillance and interpreting diagnostic results. Following a pilot survey which found low Bonamia sp. intensity in farmed Ostrea angasi, we tested further oysters (n = 100–150) from each of three farms for Bonamia sp. using heart smear, histology and qPCR. We used a Bayesian Latent Class Model to assess diagnostic sensitivity (DSe) and specificity (DSp) of these tests individually or in combination, and to assess prevalence. Histology was the best individual test (DSe 0.76, DSp 0.93) compared to quantitative polymerase chain reaction (qPCR) (DSe 0.69, DSp 0.93) and heart smear (DSe 0.61, DSp 0.60). Histology combined with qPCR and defining a positive from either test as an infected case maximized test performance (DSe 0.91, DSp 0.88). Prevalence was higher at two farms in a high‐density oyster growing region than at a farm cultivating oysters at lower density. Parasite intensities were lower than in New Zealand and European studies, and this is probably contributed to differences in the performance of test when compared to other studies. Understanding diagnostic test performance in different populations can support the development of improved Bonamia surveillance programs.     

Determining culture strategies in populations of the European flat oyster, Ostrea edulis, affected by bonamiosis

Bonamiosis affects the viability of oyster culture in Galicia (NW Spain). This study shows the effects of the disease on three oyster populations from hatcheries in different culture areas (Cambados and Bueu) and different origin: stock A [F₁]_, stock B [F₂]_, and stock C [F₂]. These stocks were cultured between November 1998 and November 2000 in Cambados and Bueu. In the Cambados area, bonamiosis is considered to be endemic, while the disease does not generally affect Bueu area. Sampling was conducted at 3-month intervals to study prevalence of Bonamia, mortality and growth rates. In the Cambados area, bonamiosis was detected 12 months after beginning culture but in the Bueu area, it was not detected until 24 months after initiating cultures. Our results suggest that there is a relationship between presence of Bonamia ostreae and oyster mortalities, which were observed in the three stocks from Cambados. These mortalities were associated with the culture area (Cambados) and not with stock of oysters. Growth was satisfactory in both areas as oysters reached commercial size (60 mm) 15 months after beginning the cultures. In an area where Bonamia is present, such as Cambados, the oysters should be marketed promptly (after approximately 15–18 months of culture) but in areas where Bonamia is not present, such as Bueu, the oyster farmer can culture oysters over a long period of time, prior to harvest, if necessary.     

Influence of feeding regime and temperature on development and settlement of oyster Ostrea edulis (Linnaeus, 1758) larvae

Under controlled conditions of food density and temperature, larval performances (ingestion, growth, survival and settlement success) of the flat oyster, Ostrea edulis, were investigated using a flow‐through rearing system. In the first experiment, oyster larvae were reared at five different phytoplankton densities (70, 500, 1500, 2500 and 3500 μm³ μL^?1: ≈1, 8, 25, 42 and 58 cells μL^?1 equivalent TCg), and in the second, larvae were grown at four different temperatures (15, 20, 25 and 30°C). Overall, larvae survived a wide range of food density and temperature, with high survival recorded at the end of the experiments. Microalgae concentration and temperature both impacted significantly larval development and settlement success. A mixed diet of Chaetoceros neogracile and Tisochrysis lutea (1:1 cell volume) maintained throughout the whole larval life at a concentration of 1500 μm³ μL^?1 allowed the best larval development of O. edulis at 25°C with high survival (98%), good growth (16 μm day^?1) and high settlement success (68%). In addition, optimum larval development (survival ≥97%; growth ≥17 μm day^?1) and settlement (≥78%) were achieved at 25 and 30°C, at microalgae concentrations of 1500 μm³ μL^?1. In contrast, temperature of 20°C led to lower development (≤10 μm day^?1) and weaker settlement (≤27%), whereas at 15°C, no settlement occurred. The design experiments allowed the estimation of the maximum surface‐area‐specific ingestion rate  = 120 ± 4 μm³ day^?1 μm^?2, the half saturation coefficient {X_K} = 537 ± 142 μm³ μL^?1 and the Arrhenius temperature T_A = 8355 K. This contribution put a tangible basis for a future O. edulis Dynamic Energy Budget (DEB) larval growth model.     

The effects of stressors on the dynamics of Bonamia exitiosus Hine, Cochennec-Laureau & Berthe, infections in flat oysters Ostrea chilensis (Philippi)

Oysters, Ostrea chilensis, infected with Bonamia exitiosus were held under stressful conditions for 14 days, and then under normal conditions for 3.5 months, to determine the affects of stress on B.exitiosus infection levels. The stressors used were (1) air exposure for 8 h daily; (2) hot water (25–26 °C), or (3) cold water (7 °C) for 1 h daily; (4) hyposaline (15‰), or (5) hypersaline (39–40‰) water; (6) starvation in filtered sea water; and (7) vigorous stirring four times a day. A control tank held oysters in static sea water, which was changed daily. Oysters were also (8) kept in a trough among heavily infected oysters (trough exposure), or (9) in a trough without other oysters (control), for 4 months. Oysters in hyposaline conditions all died within 3 weeks, but this was apparently unrelated to Bonamia infection. Otherwise, cumulative mortalities were highest in trough exposure, cold, hypersaline and hot treatments. There was a significant difference in prevalence between treatments, and the stir, hot, cold, hypersalinity and trough exposure treatments had significantly higher intensities of infection than controls. The mean intensity of Bonamia infection was significantly higher among female and spent oysters than in male and hermaphrodite oysters.     

Assessment of temperature or salinity effects on larval development by catecholamine‐induced metamorphosis of hatchery‐reared flat oyster,Ostrea angasi (Sowerby 1871) larvae

A need to improve larval rearing techniques led to the development of protocols for catecholamine‐induced settlement of flat oyster, Ostrea angasi, larvae. To further refine these techniques and optimize settlement percentages, the influence of salinity or temperature on development of O. angasi larvae was assessed using epinephrine‐induced metamorphosis. Larvae were reared between salinities of 15–35 and temperatures between 14.5 and 31°C. The greatest percentage survival, growth, development occurred when larvae were reared between 26 and 29°C and between salinities of 30 and 35. Larvae reared outside this salinity and temperature range exhibited reduced growth, survival and/or delayed development. Short‐term (1 h) reduction in larval rearing temperature from 26°C to 23.5°C significantly increased larval metamorphosis without affecting larval survival. Short‐term (1 h) increase in larval rearing temperature from 26°C to 29 and 31°C decreased larval survival and metamorphosis. To ensure repeatability in outcomes, tests showed that larvae sourced from different estuaries did not vary significantly in their metamorphic response to short‐term temperature manipulation and epinephrine‐induced metamorphosis.     

Impact of varying cultivation depths on growth rate and survival of the European flat oyster Ostrea Edulis, L.

European oysters, Ostrea edulis, L. were kept in plastic mesh cages at three depths at two commercial oyster farm sites within Mali Ston Bay, Croatia, between April 2004 and August 2005. Oysters from each cultivation depth were sampled at both sites at approximately 3‐month intervals for growth parameters, mortality rate control and condition index calculation. The growth parameters of experimental groups from both sites only revealed a considerable distinction for shell length growth (P=0.011). Total weight gain and soft tissue weight gain were the highest for groups cultivated in the middle of the water column as compared with the top and bottom positions for both studied sites. Survival rates were very similar in all experimental groups, situated at different depths of both experimental sites, and differences in the mortality peak during the summer months were not significant. Condition indices fluctuated throughout the trial period and were the highest in spring and summer, with a reduction during the winter months and improvement again in the spring.     

A blue carbon model for the European flat oyster (Ostrea edulis) and its application in environmental restoration

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Dietary influence on growth and development of flat oyster,Ostrea angasi (Sowerby, 1871), larvae

The influence of algal diet on survival, growth and development of hatchery reared flat oyster, Ostrea angasi, larvae was investigated in a series of uni, binary and ternary algal diet trials. Early stage larvae (140–230 μm shell length) generally grew faster than late‐stage larvae (230–340 μm shell length) when fed unialgal diets. Of the 24 algal diets evaluated, larvae fed unialgal diets of Isochrysis sp. (T. Iso), Nannochloropsis oculata, Tetraselmis chuii or Pavlova lutheri; a binary diet of T. chuii+T. Iso; or ternary diets of T. chuii+T. Iso combined with P. lutheri or N. oculata had the greatest larval growth, survival, development and metamorphosis, in the respective trials. The correlation between growth rate and spat produced in late‐stage larvae was stronger when fed unialgal diets (r = 0.75) than when larvae were fed either binary or ternary diets (r = 0.44 and r = 0.45 respectively). Marked differences in proportion of spat produced (24 h post metamorphosis) were evident among diets producing similar growth rates. For hatchery production of O. angasi larvae, ternary diets of T. chuii+T. Iso combined with either P. lutheri or N. oculata were the best diets to maximize larval growth rate, development and survival 24 h post metamorphosis.     

Use of neuroactive catecholamines to chemically induce metamorphosis of hatchery-reared flat oyster, Ostrea angasi, larvae

Low numbers and unreliable wild catch of the native flat oyster, Ostrea angasi , spat has resulted in the NSW flat oyster industry being reliant on hatchery-produced spat. The need to produce culchless spat in the hatchery stimulated investigation of several catecholamines to induce metamorphosis in O. angasi larvae. Larvae were treated with one of four neuroactive catecholamines (epinephrine, epinephrine bitartrate, l -Dopa and GABA) at one of four concentrations (10⁻³, 10⁻⁴, 10⁻⁵ or 10⁻⁶  m ) for one of three treatment durations (0.5, 1–2 h) to determine morphogenic action for culchless spat production. Epinephrine bitartrate at 10⁻³ and 10⁻⁴  m and epinephrine at 10⁻³, 10⁻⁴ and 10⁻⁵  m , for a treatment duration of 1–2 h, produced significantly greater numbers of spat and culchless spat, compared with any other treatment combination. The other catecholamines tested did not induce a significant increase in the total number of spat or culchless spat, over untreated controls. Separate trials found that long-term treatment (24 h) with epinephrine bitartrate and epinephrine at morphogenic concentrations inhibited metamorphosis. Consecutive daily use of epinephrine bitartrate increased the numbers of spat and culchless spat produced, but did not affect larval or short-term post-larval survival. Treatment with 10⁻³  m epinephrine bitartrate or 10⁻⁴  m epinephrine for 1 h is recommended for routine commercial production of culchless flat oyster spat.     

Settlement of Ostrea edulis is determined by the availability of hard substrata rather than by its nature: Implications for stock recovery and restoration of the European oyster

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Detection of Herpesvirus anguillae (AngHV‐1) in European eel Anguilla anguilla (L.) originating from northern Poland—assessment of suitability of selected diagnostic methods

The Community Action Plan requests EU member states to implement measures that ensure the recovery of the severely depleted European eel stocks. One of the main threats is posed by Anguillid herpesvirus 1 (AngHV‐1) leading to increased mortality in both wild and farmed eels. Following recommendations of the OIE to minimize the risk of obtaining false‐negative results, the main aim of the study was to optimize diagnostic methods for AngHV‐1 detection using conventional PCR, nested PCR and in situ hybridization assay. While 53.3% of the individual organ samples were tested positive for AngHV‐1 by PCR, the additional virus analysis via nested PCR revealed that the actual prevalence was 93.3%. In the cell cultivation passages, a cytopathic effect was hardly found in the first two rounds. In the third passage onto cell cultures, a lytic CPE was detected. The identification and confirmation of the viruses obtained from cell cultures as well as directly from the organ tissues were proceeded by PCR, nested PCR and sequencing of the PCR products. While no positive signal was detectable in the first round by PCR using samples from the third cell culture passages, the nested PCR provided weak but visible positive signals.     

The effect of food quality on glycogen content,the fatty acid profile and winter mortality in cultivated oyster spat (Ostrea edulis)

Winter mortality in hatchery reared oyster spat (Ostrea edulis) that received three different diets during the summer period was investigated. Oysters fed a natural type diet had a winter mortality of 18.3 ± 6.3% while oysters fed cultivated algae (a mixture of Tetraselmis suecica, Isochrysis galbana and Chaetoceros muelleri) had a mortality of 73.0 ± 9.7%. A group of oysters fed a mix between the two diets had a mortality of 54.7 ± 10.6%. Tissue samples were taken at the start of the experiment, after the summer period and after the winter period in order to determine growth and the content of glycogen and fatty acids. The glycogen content decreased for all groups during the winter but the decrease was highest in oysters fed the natural diet. This group also contained the largest variety of fatty acids, but there was no difference in the content of the essential fatty acids EPA, DPA and DHA between the groups. It is concluded that transplantation of spat to the sea in spring and early summer may reduce winter mortality since the feeding period on a more varied natural algal diet is prolonged compared to transplantation of spat later in the season.     

Growth of the European flat oyster in the Mediterranean Sea (Murcia, SE Spain)

Growth of the natural European flat oyster (Ostrea edulis L.) spat from the Mar Menor (Murcia, Spain) was studied in the Mediterranean Sea over 18 months. The oysters were cultured in two types of containers, stackable plastic Galician trays and plastic mesh pots, deployed at a depth of 15 m hung from a long-line. Two size classes of spat were used, Class I spat (initially 53 mm in size and 19 g in weight) and class II spat (initially 31 mm, 4 g). The best results were observed in the class I oysters grown in the trays, which yielded a substantial biomass (24.77 kg m^–2), the survival rate was 69% and commercial size (60 mm) was attained by all the oysters within 9 months of the start of the experiment. The observed growth of the class II oysters was poor, attaining a mean of not more than 50 mm and 18 g; the survival rate ranged between 25–74% and only between 8–13% of the class II spat attained commercial size.     

Temperature‐dependent feed requirements in farmed blue mussels (Mytilus edulis L.) estimated from soft tissue growth and oxygen consumption and ammonia‐N excretion

Feed requirements were estimated from specific growth rates in standardized soft tissue dry weight (SGR_DW’) and atomic O:N ratios for mussels fed seven rations of microalgae (5–735 μg C h^?1 ind^?1) at 7 and 14°C respectively. The mean oxygen consumption and ammonia‐N excretion rates were significantly higher at 14°C (0.29 μg O² and 27.3 μg N ind^?1 h¹) compared with those at 7°C (0.16 μg O² and 11.4 μg N ind^?1 h^?1) (P < 0.05), resulting in O:N ratios between 3 and 45 at 7°C and 7 and 28 at 14°C. Low O:N ratios indicate protein catabolism and an unfavourable condition, whereas high ratios indicate that carbohydrate is the primary energy source. The measured SGR_DW’ suggests minimum feed requirements of ~240 and ~570 μg C ind^?1 h^?1 for weight maintenance at 7 and 14°C, with corresponding O:N ratios of 24 and 16, respectively, indicating a more stressed condition at 14°C. A 0.5% SGR_DW’ day^?1 was obtained by ~565 (O:N = 29) and ~680 (O:N = 23) μg C ind^?1 h^?1 at 7 and 14°C respectively. A positive and significantly higher SGR_DW’, with the lowest feed ration at 7°C compared with a negative SGR_DW’ at 14°C (P < 0.05), indicated that storage time can also possibly be prolonged at low temperatures if the mussels are not fed.