Experimental challenges of juvenile and adult Manila clams with the protozoan Perkinsus olseni at different temperatures

Infection with Perkinsus species, primarily P. olseni, is thought to be a major cause of the decline of Manila clam populations in Japan since the 1980s. However, the pathogenicity of the infection has not been sufficiently evaluated to estimate the impact of infection on wild Manila clam populations. We experimentally challenged juvenile (3- to 6-mm shell length) and adult (18- to 22-mm shell length) Manila clams with P. olseni at 18, 23, 28, and 30 °C. Mortality was significantly higher in challenged groups than in control groups. The difference in mean mortality between the challenged and control groups (all life stages and temperatures) was only significant above a threshold of infection intensity ~10⁶ cells/g soft wet tissue (SWT). As temperature increased, the onset of mortality occurred more rapidly. The increase in mortality occurred earlier in juveniles than adults at 28 °C and lower. Our results suggest that the pathogenicity of P. olseni is higher in juveniles than in adults and at higher water temperatures. Given the infection intensities (ca. 10⁶ cells/g SWT) previously reported in wild Manila clams, the parasite likely has considerable impact on wild Manila clam populations, particularly juveniles during periods of high temperature.     

Growth Improvement of Shell Length in the Orange Strain of Manila Clam,Ruditapes philippinarum

Mass selection for shell length was conducted in the orange strain of the Manila clam, Ruditapes philippinarum, in this study. Based on the size‐frequency distribution, the clams constituting the largest 10% of the distribution were selected as parents for the selected line. An equal number of clams of the orange strain were randomly chosen as parents for the control line prior to the size selection. The responses to selection and realized heritability were 2.37 and 0.61, respectively. In addition, 97.3% of the progenies inherited the orange shell color. Our results suggest that mass selection for fast growth of shell length in this orange strain of the Manila clam is effective.     

Growth of the hard clam Meretrix lusoria in Taiwan

The growth of hard clam Meretrix lusoria in Taiwan was observed as it was grown at six different stocking rates (55, 109, 172, 244, 344 and 455 clams/m²), from November 1979 to September 1980. It was found that the stocking rate had more effect on the increase of the total weight of the clam than on the shell length. Under the environmental condition of the study site, the optimum stocking rate was 244 clams/m². When the net production of the clam was lower than 1103 g/m², the growth of the individual clam was not affected by the stocking rate. But when the net production was higher than 1589 g/m², the growth of the individual clam seemed to be retarded. The average pH of the sea water in the clam culture area was between 7.9 and 8.4. The average dissolved oxygen content was between 4.2 and 11.2 mg/l. And the average water salinity was between 30‰ and 35‰. All these three factors did not seem to have direct relationship with the growth of the clams. Sea water temperature, however, showed great effects on the growth of the clams. It is shown that the growth of the hard clam was slow at a water temperature of 15–18°C, increased at 20–22°C, and accelerated at 25–32°C.     

胶州湾移植底播菲律宾蛤仔的生长和死亡特性

2004年5月至2005年4月对胶州湾菲律宾蛤仔（Rudimpes philippinarum）底播增殖区进行了逐月定点采样，将样品带回放于实验室内水族箱暂养，每站随机取样，共对3269个个体进行了基本生物学特性测定，研究了移植底播菲律宾蛤仔的生长、死亡等渔业生物学特性。结果表明，底播增殖菲律宾蛤仔苗种的平均个体质量0．26g、平均壳长11．1mm；底播蛤仔与野生蛤仔在生长规律上基本一致，春末至秋初（4～9月）是其主要生长期；1～3龄期间个体生长速度较快，1～2龄、2～3龄，个体质量分别增长3．88g和4．02g；1～2龄生物量增长最快，2～3龄由于死亡率增大，其生物量增长缓慢；1～3龄蛤仔软体部的生长速度快于贝壳，5～6月是菲律宾蛤仔的繁殖肥育期。根据个体生长特性，3龄为最佳采捕年龄；根据目前的养殖状况，2龄蛤仔已达到商品规格，从生物量上分析，采捕2龄蛤仔收益最高；6月是最佳的捕获时期。蛤仔生长具有明显的季节变化，水温是影响菲律宾蛤仔生长的主要环境因子。[中国水产科学，2006，13（4）：642—649]     

Heritability of larval and juvenile growth for two stocks of Manila clam Ruditapes philippinarum

The heritability of larval and juvenile growth in natural and cultured populations of the Manila clam Ruditapes philippinarum was estimated using an unbalanced nested design and an artificial fertilization technique. A total of 33 full‐sib families and 10 half‐sib families were obtained from the natural population in Shihe (Dalian, Liaoning Province in the north‐eastern part of China), whereas 11 half‐sib families and 33 full‐sib families were generated from the cultured population in Putian (Fujian Province in the south‐eastern part of China). The offspring from each family were reared under the same hatchery and nursery conditions. Results from these studies led to estimates indicating a significantly greater influence due to the dam component compared with the sire component for both natural and cultured populations, which was mainly due to maternal effect. We found that the heritability estimates obtained from the intra‐group correlation of sire half‐sib were precise and unbiased. The narrow‐sense heritabilities of shell length for the Manila clam in the natural population (NP) at larval (at the age of 9 days) and juvenile (at the age of 30 days) stages were 0.22 ± 0.11 and 0.39 ± 0.14, respectively, whereas those in the cultured population (CP) were 0.17 ± 0.09 and 0.87 ± 0.24 respectively. The heritabilities of both NP and CP at 3 days were not significantly different from zero, and were ?0.03 for NP and ?0.14 for CP. These results suggest that selection should be highly effective in Manila clam, and that selecting either a natural or a cultured population for producing faster growing larvae and juveniles should be successful. Selective breeding could thus be used to develop high‐quality seed that would facilitate the development of a shorter culture cycle and higher yields.     

The effect of temperature on the energy budget of the Manila clam, <Emphasis Type="Italic">Ruditapes philippinarum</Emphasis>

In this study, the energy budget of the Manila clam, Ruditapes philippinarum, was evaluated after one-week acclimation periods at 5, 10, 15, 20, and 25°C. Small clams (151 ± 12 mg DW) and large clams (353 ± 16 mg DW) were fed with the microalgae, Isochrysis galbana. Filtration rate, ingestion rate, assimilation efficiency, oxygen-consumption rate, and ammonia excretion rate were measured. Both filtration rate and ingestion rate of small and large clams were found to be related to temperature. The highest Q ₁₀ values were measured in the range 15–20°C for both small and large clams. Assimilation efficiency of both small and large clams was not significantly influenced by temperature, although the maximum mean values were detected at 20°C. Oxygen consumption rate and ammonia excretion rate of small and large clams were found to be related directly to temperature over the entire range, with a maximum being detected at 25°C. The highest Q ₁₀ value was estimated in the range 10–15°C with regard to oxygen consumption rate, and in the range of 15–20°C with regard to ammonia excretion rate. Scope for growth (SFG) was positive at all temperatures, achieving a maximum value at 20°C in both small and large clams, primarily as a consequence of the enhanced ingestion rate which offset the concomitant elevation in the metabolic rate. In this study we have estimated the thermal optimum for this species at 20°C.     

Growth,mortality and reproduction of the transplanted Manila clam (Ruditapes philippinarum Adams & Reeve 1850) in Jiaozhou Bay

Samples of Manila clam (Ruditapes philippinarum Adams & Reeve 1850) were collected from May 2004 to April 2005 monthly, and plankton net trawling of planktonic larvae and bottom sediment sampling surveys were further conducted from May to October 2006 in Jiaozhou Bay. Based on the data collected, growth, mortality and reproduction of the transplanted Manila clam and the environmental effects were examined. The results showed that the enhanced clams grew well and showed a growth trend similar to the local wild ones. The main growth periods lasted from April to September, with the water temperature being the main factor affecting the growth, which was the same as that of the wild clams. There were also two reproduction cycles for the farmed Manila clams each year in Jiaozhou Bay and the main breeding period was from May to June. The phenomenon of delayed metamorphosis was quite common through larval development. The farmed clams could spawn when they reached sexual maturity, but they could not perform effective recruitment as many planktonic larvae died during metamorphosis and settlement. A preliminary study indicated that sediment perturbation and marine environment pollution were the main factors causing the death of larvae in the development process.     

Effect of geographic origin, temperature and timing of broodstock collection on conditioning, spawning success and larval viability of Ruditapes decussatus (Linné, 1758)

Culture of Ruditapes decussatus is clearly limited by the availability of seed, as this production proceeds almost exclusively from natural recruitment. Artificial spawning and larval rearing programs could provide an alternative source of spat. This study was designed to evaluate the effect of different conditioning temperatures on the broodstock maturation, spawning success and larval viability of two geographically (north and south of the Iberian Peninsula) distinct populations of European clam (R. decussatus) collected at different periods of the year in order to create “optimal” artificial spawning and larval rearing programs. Two batches of clams from each population were collected in October and February, and conditioned at 18 ± 1°C, 20 ± 1°C and 22 ± 1°C. Of the three variables analysed the timing of broodstock collection was the most determining factor for gametogenic development, spawning and larval rearing. Geographic origin and conditioning temperature also greatly affected the spawning. The results also showed that the February conditioning was more effective than October and that the best conditioning temperatures were 20 ± 1°C and 22 ± 1°C for the northern and southern populations, respectively. These results suggest that the efficient conditioning temperature for each population of the same species is related to the seasonal temperature regime from their geographic origin. Larval viability and growth performance seemed to be independent of the broodstock conditioning.     

Reproductive cycle of the venerid clam <Emphasis Type="Italic">Meretrix lusoria</Emphasis> in Ariake Sound and Tokyo Bay,Japan

We assessed the reproductive cycle of the venerid clam Meretrix lusoria by histological analysis of the gonads. Individuals for study were collected from natural populations on the Shirakawa tidal flat, Ariake Sound, and from populations that had been transplanted from the Shirakawa flat to the Oi flat in Tokyo Bay. In both study areas, the reproductive cycle was synchronized between sexes. Gonads of the clam started to develop in early spring and matured during the summer. Mass spawning occurred in the late summer/early fall. The clam matured at a shell length of 17–20 mm, which is much smaller than previously considered. While trophic conditions and salinity differed considerably in the two study areas, water temperatures showed similar seasonal changes (12°C during the winter and around 30°C during the summer). Thus, temperature probably controlled gonadal development. The coincidence of the period of spawning with the period of frequent intrusion of hypoxic waters into the tidal flats in Tokyo Bay suggests that such hypoxic events interfere with clam recruitment and are at least partly responsible for the disappearance of the natural population at this location.     

Cryopreservation of trochophore larvae from the hard clam Mercenaria mercenaria: Evaluation of the cryoprotectant toxicity,cooling rate and thawing temperature

Aquaculture of hard clams Mercenaria mercenaria is a $65 million industry along the east coast and Gulf of Mexico coast in the United States. The goal of this study was to develop a preliminary protocol to cryopreserve trochophore larvae of hard clams. The objectives were to evaluate the: 1) toxicity of cryoprotectants, including dimethyl sulfoxide (DMSO), propylene glycol, ethylene glycol and glycerol, at 5, 10, 15 and 20% for exposure time of 1, 15, 30, 45, 60 and 75 min; 2) effects of cooling rates (5, 10, 20 and 30°C/min for the first trial; and 1, 3 and 5°C/min for second trial from 4 to ?80°C), thawing temperature (30, 40 and 50°C) and their interactions on post‐thaw viability. A basic protocol was concluded as: 15‐hr trochophore larvae mixed with DMSO or propylene glycol (5, 10%), equilibrated for 15 min, cooled in a programmable freezer from 4 to ?80°C at a cooling rate of 5°C/min and thawed at 50°C for 6 s. With this protocol, the immediate post‐thaw trochophore survival was 23 ± 14%, and survival to D‐stage was 27 ± 14%. This is the first report on larval cryopreservation in the hard clam and would have application for genetic breeding and seed production.     

Hatchery production of diploid and triploid clams, Tapes dorsatus (Lamarck 1818): a potential new species for aquaculture

To assess the potential for hatchery production of the venerid clam, Tapes dorsatus, diploid and triploid clams were produced and ongrown until the spat reached 5–8 mm in size. Triploidy percentages at metamorphosis that ranged from 56 to 85% were induced using a 15-min exposure to 1 mg/l cytochalasin B. No differences were observed in the growth rate of sibling diploid and triploid T. dorsatus larvae. Survival of diploid clams to pediveliger stage was however higher. Post-metamorphic growth and survival of diploid and triploid clams remained the same until the clams were removed from the hatchery to outdoor nurseries. Throughout the larval and early spat phases of production, T. dorsatus growth was similar to that reported for the Manila clam, Tapes philippinarum. Based upon its ease of culture, rapid growth and marketability, T. dorsatus are thought to have considerable aquaculture potential.     

The breeding potential and biochemical composition of triploid Manila clams, Tapes philippinarum Adams and Reeve

In 1989 and 1990. triploid Manila clam, Tapes philippinarum Adams and Reeve, seed were reared to 15-20 mm at the Fisheries Laboratory, Conwy, and planted out in the Menai Strait, North Wales. In each of the summers of 1992 and 1993, three of these batches, at 2, 3 or 4 years old, were returned to the laboratory to assess ploidy, size, spawning potential and biochemical composition. Percentage triploidy at this time was similar to that in the seed. After 6 and 8 weeks of warmwater conditioning. Only 45 out of l21 triploids (37%) were induced to spawn by thermal shock, with only one spawning as a male. By comparison, 75% of diploid clams spawned with a 1:1 ratio of males to females. Mean fecundity of triploids was significantly lower than that for diploids, at 0.497 compared with 1.54 million eggs per female. Compared with eggs from diploid females, eggs from triploids were larger and significantly fewer of them developed into D-larvae when fertilized by sperm from diploid males. Triploid clams were heavier and had a higher condition index and carbohydrate content than diploids of the same age, but lipid levels were similar. Potential advantages of producing and cultivating 100% triploid batches of Manila clam seed are discussed.     

Culture of Juvenile Atlantic Surfclams, Spisula solidissima solidissima and Spisula solidissima similis, in Forced-Flow Upwellers in a Bivalve Hatchery in Coastal Georgia

Four experimental growth studies for juvenile Atlantic surfclams, Spisula solidissima solidissima and Spisula solidissima similis , under hatchery conditions were conducted in 10-cm diameter forced-flow upweller units. Experiments were designed to determine optimum food ration (2%, 5%, and 8% g dry weight of algae/g wet weight of clam), water culture temperature (20, 25, and 30°C), flow rate (2, 4, and 6 Lpm), and stocking density (7, 14, and 21 g wet weight of clams) for rearing juvenile clams in upweller units. Each experimental trial was performed for a 14-d period. Nested Analysis of Variance showed that juvenile surfclams Spisula solidissima similis grew significantly faster at a temperature of 20°C and food rations of 5 % and 8%. In all four experiments, clam mortalities were not observed except in the temperature experiment, where total mortality occurred for clams cultured at 30°C. Clam growth in biomass and size decreased significantly with increases in culture temperature. For Spisula solidissima solidissima , clam growth was significantly reduced in terms of size and biomass at a flow rate of 6 Lpm. No difference in size or biomass was determined between clams grown at a flow rate of 2 or 4 Lpm. Surfclams at the lowest stocking density of 7 g wet weight significantly increased more in net biomass compared to those at higher stocking densities. Clams were significantly larger in shell length when grown at the two lower stocking densities compared to those at the highest stocking density. Hatchery-rearing protocol for juvenile surfclams is discussed.     

Effect of tooth spacing on the contact selection and available selection of a dredge for the equilateral Venus clam <Emphasis Type="Italic">Gomphina melanaegis</Emphasis>

ABSTRACT:   The effect of various tooth spacing on the contact selection and the net-mouth available selection of a dredge for the equilateral Venus clam Gomphina melanaegis is evaluated by applying a contact-probability model based on the SELECT method to the data of paired-gear tests with a control dredge of 12-mm tooth spacing and four test dredges of tooth spacing 16, 20, 24 and 35 mm. A master selection curve was estimated to be s ( R ) = exp(−11.23 R  + 4.799)/(1 + exp[−11.23 R  + 4.799]) where, R is l / d , the ratio of the shell length, l , to the tooth spacing, d . The shell width of 50% retention was slightly smaller than the tooth spacing, which means that size selectivity of teeth was a type of contact selection which was dependent on the tooth spacing. Contact probability, δ, of the clams coming into contact with the teeth were 0.625, 0.467, 0.563, 0.670 and 0.976 for tooth spacing 12, 16, 20, 24 and 35 mm, respectively. This indicates that some clams entered the dredge bagnet without contacting the teeth as the sand clogging the tooth spacing carried them into the bagnet. Available selection curve of the dredge net-mouth with teeth was obtained as δ s ( R ) + 1 −  δ , and examined in terms of the legal landing size of the Venus clam.     

不同近交系数蛤仔橙色品系的生长和成活研究

以不同遗传背景的蛤仔（Ruditapes philippinarum）橙色品系为材料,采用建立半同胞家系子一代H1（F=0.125）、全同胞家系子一代F1（F=0.250）、全同胞家系子四代F4（F=0.594）及自然对照组C1（F=0）的方法,进行了不同近交系数蛤仔橙色品系生长和成活的研究.结果表明,在整个养殖周期中F1组的壳长生长最慢,在稚贝期和养成期均显著小于F4和C1组（P＜0.05）,H1、F4和C1组之间的壳长生长差异不显著（P＞0.05）.C1组的成活率在整个养殖周期中为最小,除幼虫期3日龄外,其他各日龄均显著小于其他试验组（P＜0.05）,而H1、F1和F4组之间的成活率则差异不显著（P＞0.05）.试验组F1壳长生长的衰退率为2.92％ ～ 18.33％,平均值为（12.05±6.54）％,试验组H1在稚贝期、养成期显现出一定的衰退现象,而试验组F4在整个养殖周期中均未表现出近交衰退现象;各试验组的成活性状在各日龄均未出现近交衰退.该研究为橙色品系的选育奠定一定的理论基础.     

不同地理群体菲律宾蛤仔表型性状的相关性与通径分析

随机选取繁殖时期的南方群体北方养殖菲律宾蛤仔(SN)、北方土著品种(NN)和南方群体南方养殖蛤仔(SS),测量其壳长(SL)、壳高(SH)、壳宽(SW)、活体质量(mL)、软体质量(mM)和烘干后软体部干质量(mF),采用相关与通径分析方法分析不同地理群体的菲律宾蛤仔表型性状对软体质量的作用效果,为菲律宾蛤仔的选择育种提供理论依据和测度指标。试验结果表明,不同地理群体菲律宾蛤仔的形态学指标无显著差异(P>0.05);不同地理群体各数量性状之间的相关关系均达到极显著水平(P<0.01);决定系数分析表明,不同地理群体菲律宾蛤仔影响软体质量的主要表型性状并不相同,南方群体北方养殖蛤仔群体的壳宽直接作用最大,壳宽通过壳高的间接作用是次要因素;北方土著品种群体壳长是主要因素,壳长通过壳高的间接作用是次要因素;南方群体南方养殖蛤仔群体壳高的直接作用是主要因素,壳长通过壳高的间接作用是次要因素。对通径系数检验不显著的自变量进行删除,利用逐步回归的方法,建立不同地理群体菲律宾蛤仔软体质量的最优回归方程:SN群体,mM1=-4.276+0.211SH+0.116SW;NN群体,mM2=-2.806+0.088SL+0.087SH;SS群体,mM3=-3.101+0.290SH。回归方程分析表明回归关系均达到极显著水平(P<0.01)。     

干露及淡水浸泡对菲律宾蛤仔稚贝生长和存活的影响

于2004年3—5月研究了干露及淡水浸泡对菲律宾蛤仔稚贝生长及存活的影响。试验结果表明,(1)同种规格稚贝耐干露和淡水浸泡能力随着温度升高而下降;相同温度下,不同规格稚贝耐干露能力随规格的增加而增大,耐淡水浸泡能力在稚贝壳长9.5mm时,随规格增大而增大,当稚贝规格9.5mm时,耐淡水浸泡能力随规格的增大而减小。(2)经干露和淡水浸泡的稚贝在开始7d内生长缓慢,生长速度远不及未经处理的稚贝,7d后各种规格的稚贝生长明显加快,达到、甚至超过未经处理稚贝的生长速度,存在补偿生长现象。     

Larval and post‐larval growth,spat production and off‐bottom cultivation of the smooth venus clam Chionista fluctifraga

Harvesting practices of the clam Chionista fluctifraga show a decline in commercial size and densities, but no strategies have been developed to maintain clam beds. Aquaculture represents an alternative for preserving this resource. Adult clams from commercial grounds were used as broodstock. Conditioning, induction of spawning, cultivation of larvae, settlement of eyed larvae and nursing of postlarvae were performed in the hatchery for producing spat. Larvae and postlarvae were used to measure increase in shell height and data were fitted to exponential growth models. Spat were placed in floating trays and maintained in off‐bottom cultivation for 9 months. Samples of clams and tissues were collected monthly to measure absolute growth, shell height increase and a condition index. Larvae, postlarvae and juveniles showed exponential growth patterns. Mean shell height increased about 0.030 mm day^?1 during larval and post‐larval stages and 0.049 mm day^?1 during field cultivation. Pediveligers (height 215 ± 83 μm) entered metamorphosis at days 9–13 after fertilization, and postlarvae reached 3011.7 ± 325.5 μm (height) at day 60. After field cultivation, survival was about 95%; juvenile shell height was 20.6 ± 2.2 mm, and total weight was 5.3 ± 0.7 g. Growth rates were superior to natural conditions and the condition index was high throughout the study. Our results show that spat of C. fluctifraga can be produced in the hatchery, and that field production can be maintained in off‐bottom trays until reaching commercial size. Aquaculture activities for this species need to be established and evaluated.     

EXPERIMENTAL CULTURE OF THE OCEAN QUAHOG,Arctica islandica

Larvae and early postlarvae of the ocean quahog, Arctica islandica, were reared under experimental hatchery conditions. Mature eggs were stripped from ripe adults and exposed to a dilute solution of ammonium hydroxide for various lengths of time prior to addition of stripped sperm. The larval clams were reared through settlement and metamorphosis using the Wells-Glancy (centrifuged, incubated seawater) method of algal culture and/or modifications of standard hatchery techniques developed by Loosanoff and Davis. Experimental cultures were maintained at various temperatures ranging from 8.5° to 14.5°C. At temperatures of approximately 13°C, the minimum time to settlement was 32 days, while settlement was not observed in a culture maintained between 8.5° and 10.0°C until approximately 55 days after fertilization. Larval growth rates were significantly lower in the culture maintained at 8.5–10.0°C than in cultures maintained at 11.0–14.5°C. An optical micrograph sequence of larval stages from the straight-hinge stage through metamorphosis is presented to facilitate identification of Arctica islandica specimens isolated from plankton samples. While various workers have reported exceedingly low growth rates of juvenile and adult Arctica, growth rates of larval Arctica appear to be fairly “typical” of rates encountered within the class Bivalvia.     

Intertidal Growth and Survival of Northern Quahogs Mercenaria mercenaria (Linnaeus 1758) and Atlantic Surf Clams Spisula solidissima (Dillwyn 1817) in Georgia

This work reports on the effects of intertidal planting heights upon the growth and survival of the northern quahog, Mereenaria mercenaria (L.) and the Atlantic sod clam, Spisula solidissima (Dillwyn), in the castal waters of Georgia. Quahogs (N = 100 per cage) at 19.5 mm in shell length were planted in replicate cages (N = 3) placed at the spring (SLW), mean (MLW), one (+1), two (+2), and three (+3) hours above mean low water mark. Surf clams (N = 200 per cage) at 41.5 mm in shell length were placed in replicate cages (N = 3) deployed, as above, in all sites except +3. Quahogs grown at the SLW mark after 15 months were significantly larger than clams planted at other tidal heights. Quahogs planted at the MLW mark were significantly greater in shell length than those planted at the +3, +2, and +1 marks. Clams at the upper three sites were not significantly different in size. An inverse relationship between clam growth and intertidal planting height was evident among surf clams. Surf clams grown at the SLW mark for six months were signplificmtly larger than those at the MLW mark which were larger than those grown at the +1 above MLW mark. No significant differences in quahog survival were detected between intertidal planting sites. There was no significant difference in surf clam survival between SLW (77.5%) and MLW (61%) marks, but there was significantly lower survival at the +1 mark (15%) and no survival at the +2 mark.