Preliminary studies to culture mangrove oysters, Crassostrea tulipa, in Sierra Leone

Large quantities of wild mangrove oysters Crassostrea tulipa exist in the intertidal zone of the rivers and estuaries of Sierra Leone. Typically, the wild mangrove oysters are small due to crowding and stress induced by the high ambient temperatures and lack of food at low tide when the oysters are out of water. The wild oysters measure on the average 2.5 cm and weigh 1.5 g for a 2-year growing period.Preliminary studies have been made since 1974 to identify viable culture techniques that will increase the yield (growth rate and size) of mangrove oysters. Suspended culture (rack and raft) was found to be most promising with rafts producing an average 7-cm and 7-g oyster in 7 months. Biological work summarized includes plankton, fouling and bacteriological studies. Some cost-benefit studies are also included.     

Influence of gamete density,salinity and temperature on the normal embryonic development of the mangrove oyster Crassostrea rhizophorae Guilding, 1828

Laboratory experiments were undertaken to determine the optimal environmental conditions and some of the other factors concerned in the development of Crassostrea rhizophorae embryos.Critical variables such as the number of spermatozoa per ovocyte during fertilization, the time of fertilization after gamete liberation, egg density, temperature and salinity were related to the proportion of normal D-larvae of C. rhizophorae in the resulting broods.The highest proportion of normal D-larvae was obtained at concentrations of 500–5000 spermatozoa/ovocyte, under conditions of 25‰ salinity at 25 ± 1°C. The optimal density of eggs, for the production of normal D-larvae, was 10⁴?4 × 10⁴ ovocytes/l. If fertilization was delayed for more than 45 min after liberation of spermatozoa the proportion of normal D-larvae was greatly reduced. The experiments demonstrated that the temperature for developing embryos should be below 30°C. At 20 and 25°C there was a high proportion of normal D-larvae 24 h after fertilization. The ideal salinity for embryonic development in C. rhizophorae was 25–37‰. Below a salinity of 16‰, less than 2.5% of the D-larvae were normal.     

Advantages in tray cultivation of pacific oysters (Crassostrea gigas) in Strangford Lough,N. Ireland

The production of unattached hatchery-reared oyster seed has encouraged the initiation of oyster cultivation in new areas throughout the British Isles, and has led to innovations in methods of handling and growing-on oysters. Strangford Lough, Northern Ireland is an area with no traditional oyster fishery, and the introduction of the fast growing pacific oyster has led to consideration of the most feasible method of cultivating this species. Although raft culture to market size has known advantages, it is normally associated with oysters attached to cultch. However, off-bottom culture by growing unattached oysters intertidally in trays held off the sea-bed is an alternative, which provides higher survival, better growth, higher stocking densities and complete cropping compared with traditional sea-bed culture. Considering these advantages, tray cultivation may be economically justifiable, especially in an area without an existing oyster fishery.     

Effects of mass and position of artificial fouling added to the upper valve of the mangrove oyster Crassostrea rhizophorae on its growth and survival

We ran an experiment on mangrove oysters Crassostrea rhizophorae to evaluate the effects of adding different masses of artificial fouling to the upper valve, either to the umbo region or the ventral edge of the shell. Growth and survival were quantified after a 30 d period in suspended culture in the La Restinga Lagoon, Venezuela. The artificial fouling was cement weighing 0.5, 1, 2 or 3 fold the mass of the upper valve. No fouling was added to a control group. Fouling mass, but not the position of the artificial fouling, affected growth in shell length. However, only the heaviest fouling (3 times the mass of the upper valve) had a significant effect. In contrast, there was no affect of either fouling mass or position on tissue growth. Finally, our data indicated that mortality could be affected by the position where we added artificial fouling (greater mortality when fouling was added to the ventral edge of the shell), but not by fouling mass. Our study indicates it is unlikely that the levels of natural fouling that develop on oysters in suspended culture would be sufficient to affect either growth or survival.     

Fishery of the mangrove oyster, Crassostrea gasar, Adanson (1757), in the Lagos area,Nigeria

The mangrove oyster, Crassostrea gasar, constitutes an important source of animal protein among some communities in the Lagos area. This paper summarises present knowledge of the fishery of the mangrove oyster in the Lagos area, highlights its prospects, and suggests how oyster production for consumption can be improved.     

Pacific oyster (Crassostrea gigas) spatfall prediction at Hiroshima,Japan, 1977

The spatfall prediction techniques used in Hiroshima Bay, Japan, are described. Data are given for the 1977 season. Plankton hauls from seven stations showed successive high peaks of small, medium and large larvae during early August. Small larvae (< 150 μm diameter) peaked at an average of 1,782 larvae per tonne of sea water (m³) on 10 August; medium larvae (151–250 μm) at 1,258/m³ peaked on 12 August as did large settling larvae (251–300 μm) at 350/m³. A small spatfall occurred on 14, 16 and 18 August when an average of 65, 64 and 65 spat per shell, respectively, attached on collectors during 2 days' immersion.A second, much bigger peak of small larvae built up from 16 August to reach an average maximum of 4,702 larvae/m³ on 24 August. Medium larvae peaked at 754 larvae/m³ on 28 August. A red tide then appeared consisting of vast numbers of a dinoflagellate tentatively identified as Gymnodinium simplex, and the anticipated main spatfall did not occur. The effects of red tides are discussed. By 17 September water conditions had improved and an estimated 50–100 spat were reported attached per shell on the collectors. This is about half the 200 spat per shell considered by the industry to be a good set.     

Comparative effects of diets consisting of one or two algal species upon assimilation efficiencies and growth of juvenile oysters, Crassostrea virginica (Gmelin)

Juvenile oysters, Crassostrea virginica, were fed one of six diets consisting of three species of algae fed singly or in two-species combinations. Dietary components were Isochrysis galbana (Diet I), Platymonas suecica (Diet P), and Thalassiosira pseudonana (Diet T). High (Diet TI), intermediate (Diets T, I, PT, PI), and low (Diet P) growth responses were observed. Intermediate and low growth responses were explained in terms of differing levels of ingestion and assimilation. The high growth response with Diet TI was not explained by measured values of ingestion and assimilation.     

Effects of genotype × environment interactions on the selection of broadly adapted Pacific oysters (Crassostrea gigas)

Broadly adapted genotypes are required if a single line of selection is to improve Pacific oyster (Crassostrea gigas) production throughout the heterogeneous growing environments found in the Pacific Northwest region of the United States. An experiment was conducted to determine if the relative rankings of average family performance (i.e. average individual weight, survival and yield) remain stable across a wide range of growing environments. Twenty-four full-sib oyster families were each planted both intertidally and subtidally in Yaquina Bay, Oregon, and Dabob Bay, Washington. Each family was represented by up to 12 replicate growout bags (stocked with 60 spat each) or 12 replicate lantern nets compartments (stocked with 40 spat each). Oysters were planted in Fall 2002 and yield (kg live weight replicate^− 1), average individual weight (g) and survival (%) were measured in Spring and Fall 2003 and at harvest in Summer 2004. Harvest average individual weight, survival, and yield were all significantly affected by family, environment, and family × environment interactions (P < 0.01). Family, environment, and family × environment interaction, accounted for 5%, 80%, and 2% of the total phenotypic variation in harvest average individual weight, respectively, 36%, 16%, and 7% of the total phenotypic variation in harvest survival, respectively, and 14%, 62%, and 5% of the total phenotypic variation in harvest yield, respectively. Although the effects of family × environment interactions were statistically significant, correlations among average family yields between all growout environments were significantly greater than 0 (r_P > 0.397; P < 0.04) indicating high-yielding families in one environment tended to be high-yielding in other environments. Results of this study suggest that the effect of G×E interactions on average individual weight, survival, and yield, while significant, are not large enough to prevent selection in a limited number of well chosen environments from resulting in favorable correlated gains in other environments in the Pacific Northwest.     

Cultivation of the Pacific oyster (Crassostrea gigas) in a non-tidal hypersaline pond

The Pacific Oyster (Crassostrea gigas) has been grown in a shallow hypersaline pond, approximately 3 km² in area. The pond is non-tidal and is the primary one of a series used in the manufacture of salt by evaporation.In the past, similar areas have been considered unsuitable for oyster cultivation: however, the growth rates recorded for C. gigas in this experiment were very high (maximum of 22.4 g/month live weight) and mortalities were less than 30% over the period of study (12 months).The successful cultivation of C. gigas in the pond appears to depend on water circulation caused by wind action and it is likely that similar non-tidal areas could be used for oyster farming.     

The effect of intertidal exposure on the growth and survival of hatchery-reared Pacific oysters (Crassostrea gigas Thunberg) kept in trays during their first ongrowing season

In a number of trials during 1971 and 1972 small populations of hatchery-reared Pacific oyster spat (Crassostrea gigas Thunberg) 0.07–4.1 g mean live weight, were grown intertidally in trays to measure the effect of exposure to air on growth and survival. In the Menai Straits, N. Wales, and at Paglesham Pool, R. Roach, Essex, several stations were selected above low water of spring tides (LWST) to cover the range of 0–30% exposure to air. Live weight, dry meat weight and dry shell weight were reduced with increased exposure to air, but the ratio of dry meat weight to dry shell weight (shell condition index), the gross biochemical composition, and survival were not affected. By extrapolation the point of no growth was found to be at 36 and 47% exposure to air at the sites in the Menai Straits and R. Roach, respectively.     

Growth trials of Crassostrea gigas and Ostrea edulis in inshore waters of Malta (Central Mediterranean)

To investigate the possibility of establishing oyster culture in Malta, growth trials with C. gigas and O. edulis were undertaken. The oysters were placed in floating cages at depths of 3 and 8 m at Marsaxlokk bay, Mistra bay and Rinella creek. These oysters were sampled at two-month intervals for their growth, meat-shell index and survival. The temperature, salinity, oxygen tension, chlorophyll-a levels and the levels of particulate matter in suspension in the water at the test sites were monitored concurrently. A programme of qualitative and quantitative analyses of the natural phytoplankton population was also carried out.O. edulis suffered catastrophic summer mortalities at all locations when the ambient temperature of the water reached about 25°C; this virtually rules out any possibility of starting any flat oyster industry using this system. As for C. gigas, Marsaxlokk bay 3 m appears to hold some promise for its culture. At this location, this species showed reasonable growth, relatively low mortality and normal shell growth. Although at Rinella overall growth of C. gigas was comparable to that at Marsaxlokk at 3 m, the Rinella oysters accumulated copper and exhibited abnormal and thickened shell growth with the result that the oysters were of a much inferior condition. Growth at Mistra (both depths) and Marsaxlokk 8 m was stunted.No clear correlation could be established between the performance of the oysters and the environmental parameters monitored. Several factors that may have been responsible for the observed differences in oyster performance are discussed. In particular, the nature of the water movements is thought to have played a key role in dictating the growth of the oysters.     

Setting of a procedure for experimental incubation of Pacific oyster (Crassostrea gigas) embryos

Standardized experimental protocols designed to study Pacific oyster (Crassostrea gigas) gamete quality have not been previously published. Gamete sampling variations and confounding effect of interacting factors result in large variations between replicates, decreasing the effects of the studied factors. This work aims at defining a standardized procedure for incubation of oyster embryos designed for experimental purposes.In a first phase, four experiments were developed to improve embryo sampling and handling. They showed that a minimum of 50 embryos must be counted to decrease the variation between counting replicates. For reliable results, sampling must be carried out from 0 to 7 min after a careful agitation of seawater containing embryos (salinity: 35.6‰). Compared to control and a 30 cm one, the D-larval yield of embryos submitted to a 1 m fall was significantly reduced, showing the limited effect of mechanical disturbance on Pacific oyster oocytes survival. Then, maintaining 2.2 million embryos in 250 ml seawater for a two hours period resulted in a decrease of D-larval yield which could not be explained by a decrease of O₂ content and pH of seawater.The second phase of this work included a set of five experiments, defining experimental incubation conditions. A higher larval yield was observed using 1.8 l beakers and 150 l tanks compared to 0.3 ml microtiter plates and 1 l fish egg incubators. Higher larval yields were recorded when embryo density ranged from 5 to 100 ml^− 1, compared to values between 500 and 2000 ml^− 1. Compared to controls (no antibiotic or no presence of light), no changes of D-larval yield were observed by adding 20 ppm chloramphenicol or by maintaining embryos in total darkness. Then, a significant decrease of pH and O₂ content was observed during the incubation period. However, these changes could not be considered as limiting for Pacific oyster embryo development.In conclusion, experimental incubation conditions have been defined in this study: 30 embryos ml^− 1 incubated in 1.8 l beakers without antibiotic and regardless of light intensity, when not higher than 500 lux. The mean coefficients of variation observed between tank replicates ranged from 13.1 to 16.5%. The standardized incubation procedure described in this work will help to study quality variations of Pacific oyster gametes.     

Monthly variation of condition index,energy reserves and antibacterial activity in Pacific oysters,Crassostrea gigas,in Stansbury (South Australia)

This paper investigates the temporal responses of 2-year old Crassostrea gigas to environmental changes in Stansbury, South Australia from September 2005 to October 2006. A total of 360 oysters were grown in one-line baskets on the farm using six replicates that were sampled monthly. A range of environmental parameters were assessed and correlated against biological indicators for oyster condition, metabolism and antimicrobial activity. Food availability by chlorophyll a, was low throughout the study period (0.5–1.5 µg L^? 1) and was significantly correlated to phosphorus concentrations. The condition index and shell weight of oysters significantly increased over the year, with the condition index dropping after spawning but then recovering within one month. Significant temporal variation in energy storage and utilization were observed in different tissues over the year. Glycogen in the mantle tissue was influenced by reproduction and correlated to chlorophyll a levels, but not in the gill or adductor muscle. The mantle glycogen and gill protein reached the lowest level in February when spawning occurred and presented evidence for seasonal variation in oyster metabolic activity. However, mantle and adductor muscle proteins did not drop after spawning indicating that these proteins contribute little to gametogenesis. Hemolymph protein was negatively correlated to water temperature and chlorophyll a, reaching the lowest level during summer. Hemolymph antibacterial activity significantly decreased after spawning, implying that the period of post-spawning is critical for oyster health. This study revealed trade-offs in the energy budget between immune resistance, growth, and reproduction. The results indicate that in a lean water environment, spawning events significantly regulate metabolic and immune capacities of oysters and a second year of rearing increased meat and shell weight but not the shell length. These findings are applicable to the management and development of oyster aquaculture within temperate southern hemisphere.     

Molecular identification and distribution of mangrove oysters ( Crassostrea) in an estuarine ecosystem in Southeast Brazil: implications for aquaculture and fisheries management

This study investigated the existing species of Crassostrea in the natural environment, farming systems and artificial spat collectors at a protected estuarine area in Brazil, through PCR‐RFLP of mitochondrial 16S rRNA. Among 450 samples collected in the natural environment, 303 were C. brasiliana and 147 C. rhizophorae. Oysters present in the rocky subtidal zone were C. brasiliana. However, both species occurred on mangrove roots in the intertidal zone. Farm‐raised samples included only C. brasiliana. It was observed that attached specimens in commercial collectors had a banding pattern distinct from C. brasiliana and C. rhizophorae, indicating the presence of a third species in the estuary. The 16S rRNA sequence analysis showed that these specimens are clustered with the oysters from Pacific and Indian Oceans, and genetically close to the oysters of Beihai, China (0.3% genetic distance). Oysters obtained from the seed capture showed 17.8% distance of in relation to C. brasiliana, 17.6% for C. rhizophorae and 10.3% for C. gigas, demonstrating high genetic divergence from these species. The occurrence of an exotic species in the Cananéia estuary may have strong ecological and economic implications which require new guidelines for farming, conservation and sustainable fisheries management for the native oyster species.     

Direct and indirect responses to selection on individual body weight in the Pacific oyster (Crassostrea gigas)

Three experiments were performed to examine the heritability of body weight among adult Pacific oysters (Crassostrea gigas) evaluated in Yaquina Bay, Oregon, USA, and to determine if selection on individual body weight could result in changes in offspring survival and yield. The first two experiments utilized midparent-offspring regressions to estimate the heritability (h²) of adult oyster body weight and the coheritability (h_x,y) between adult midparent body weight and offspring performance, including juvenile average body weight, survival and yield as well as adult survival and yield. In Experiment 1 both parents and offspring were evaluated in an “upriver” environment in Yaquina Bay. In Experiment 2 parents were evaluated in a “downriver” environment, while offspring were evaluated in an “upriver” environment. Experiment 3 contrasted average body weight, survival, and yield of offspring (evaluated upriver) derived from three large sires and three small sires mated to the same five females that were chosen at random (all parents evaluated downriver). In Spring 2002, 12 full-sib families from Experiment 1, 19 families from Experiment 2, and 26 families from Experiment 3 were stocked into lantern nets and suspended in Yaquina Bay. Measurements of yield (kg tier^− 1), average body weight (g), and survival (%) were recorded after one and two growing seasons in the field. Heritability estimates for adult body weight at harvest ranged from 0.313 (± 0.083) in Experiment 1 to 0.003 (± 0.073) in Experiment 2. In Experiment 3, average body weight did not differ between offspring derived from large sires and offspring derived from small sires (P = 0.47). Significant negative coheritability estimates were observed between adult midparent body weight and offspring survival in both Experiment 1 and Experiment 2. Significant negative coheritability estimates between adult midparent body weight and offspring yield were observed in Experiment 2 but not in Experiment 1. In Experiment 3, offspring derived from large sires had significantly lower survival and yield than offspring derived from small sires. These results show adult oyster body weight to be heritable but also subject to site-specific adaptation such that selection in the downriver Yaquina Bay environment was ineffective at changing average body weight in the upriver environment. Negative coheritability estimates between performance traits indicate that adult oyster body weight may be a poor indirect measure of oyster yield potential, and that selection solely for increased body weight could lead to a decrease in offspring yield.     

Seasonal variations in biochemical composition and condition index of cultured mussels (Mytilus galloprovincialis Lmk) in the Lagoon of Venice (North Adriatic)

Seasonal changes in the main biochemical components and in the condition index of cultured mussels (Mytilus galloprovincialis Lmk) from the Venice Lagoon have been followed during the period October 1979–August 1981.Carbohydrates (2–32%) show winter minima and summer maxima, while proteins (30–52%) have a rather irregular pattern. Lipids (2–13%) and condition index exhibit an annual cycle with two minima (late spring; autumn-winter) and two maxima (early spring; summer).Seasonal variations of biochemical composition and condition index are discussed in connection with the reproductive cycle and some environmental parameters such as temperature and phytoplankton availability.The comparison between the present data and those from other geographic areas confirms the peculiarity of the environment of the Venice Lagoon.     

Impact of a native predatory whelk on cultivated oysters (Crassostrea gigas) crops in San Quintin Bay, Mexico

San Quintin Bay (Baja, California, Mexico) is extensively utilized by artisanal farmers to cultivate Pacific oysters ( Crassostrea gigas ) using a rope/rack system. A large juvenile oyster mortality event was significantly correlated with the presence of native predatory whelks, Macron trochlea . Surveys, field and lab experiments were performed to identify the predatory behaviour of the whelk, determine factors affecting oyster mortality, and quantify the impact on oyster farmers. Macron trochlea was found to be a voracious, active predator, which can consume ∼4 juvenile oysters day⁻¹. Juvenile oysters suffer significantly higher mortality in the presence of whelks. Macron trochlea has the largest impact on small oysters. Once oysters grow to >30 mm they reach a size refuge above which there is much lower mortality. At average growth rates, an oyster can escape predation after ∼3 months. But, in areas of highest whelk densities, at average predation rates, whelks could consume the standing crop of juvenile oysters in approximately 43 days. For the artisanal oyster farmers of San Quintin this represents a substantial economic loss, which was unaccounted for previously. Farmers can reduce loss to whelk predation by seeding juvenile oysters in low whelk density areas until oysters reach the size refuge.     

大型底栖动物生态学研究进展

珠江口是广东最大的出海口,在中国河口生态系统中占据重要地位。为揭示珠江口大型底栖动物群落的季节性变化规律,分别于2019年冬季 (12月)、2020年夏季 (8月)、2021年春季 (3月) 和秋季 (10月) 对珠江口海域开展了4个航次的调查。运用多元统计分析方法对采集的大型底栖动物的种类组成、生物量和多样性指数等进行了分析。结果显示：4次调查共发现大型底栖动物138 种,以春季大型底栖动物种类数最高 (45 种),春季栖息密度最高 (590 个·m⁻²),夏季最低。生物量则是冬季最高、秋季最低。光滑倍棘蛇尾 (Amphiopholis laevis) 为春、夏和秋季的共有优势种。单因素方差分析 (One-way ANOVA) 结果表明,Margalef丰富度指数 (D)、Shannon指数 (H') 和Pielou均匀度指数 (J') 均在季节间变化显著 (P<0.05)。Pearson相关性分析显示,大型底栖动物多样性与pH、悬浮物 (Suspended solids, SS) 和化学需氧量 (Chemical oxygen demand, COD) 等多种环境因子呈显著相关性。研究表明大型底栖动物的季节性变化是多种环境因子共同作用的结果,需进一步加强对该海域生物多样性的长期监测与保护。

     

Growth,mortality and reproductive traits of diploid and triploid Pacific oysters (Crassostrea gigas,THUNBERG, 1793) in Southern Brazil

This work aimed to evaluate the whole weight (g), shell height (mm), mortality (%) and reproductive aspects (condition index and histology) of triploid (3N) Pacific oysters (Crassostrea gigas) cultivated in southern Brazil. To accomplish this, a comparative experiment was performed with a diploid (2N) control group in two cultivation sites on Santa Catarina Island: North Bay (NB) and South Bay (SB). Whole weight, height and cumulative mortality were evaluated monthly for eight months and condition index (CI) for six months. Histological analysis was performed every two weeks for four months to investigate reproductive aspects. At the end of the experiment, whole weight and height were similar for 3N and 2N oysters. However, while more growth in whole weight and height was observed in the initial phase of cultivation in NB, such growth was not detected in SB until intermediate and final grow‐out phase. At harvest, cumulative mortality in 3N oysters was similar that in 2N oysters. The results suggest that diploid and triploid oysters grown in southern Brazil have similar developmental patterns based on whole weight and height, as well as reproductive cycle. In addition, cultivation sites have more influence on growth and mortality than ploidy levels.     

长牡蛎3种壳色家系间杂交子代生长和存活比较

为了利用杂种优势培育长牡蛎优良新品种,实验以3种不同壳色长牡蛎家系(白色/W、黑色/B、紫色/P)为材料,采用3×3完全双列杂交法,建立了3个自交组合和6个正反杂交组合,分析了各实验组幼虫期和养成期的生长、存活以及杂交子代的杂种优势。结果表明,浮游幼虫期,杂交组PB表现出显著的生长优势;与自交组相比,各杂交组均有着较高的幼虫存活率;在幼虫存活率方面,所有杂交组均有较高的杂种优势率。在养成期,紫壳色自交组的壳高显著大于白壳色和黑壳色自交组;6个杂交组中,PB的壳高生长最快,BP次之,PW、WP的生长最慢;各杂交组与自交组的成活率差异均不显著;杂交组BP及其反交组PB的壳高、壳长、总重和存活率的杂种优势率分别介于3.71%~15.27%、-2.00%~13.10%、11.23%~41.56%、-2.77%~9.83%,其他4个杂交组在整个养成阶段没有表现出杂种优势。