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ISSN : 1225-3480
Reproductive cycle, first sexual maturity, spawning amount related with the size and spawning interval in female Ruditapes philippinarum were investigated by histological observation and the analysis of morphometric data during artificial spawning induction. Ruditapes philippinarum is dioecious and oviparous. The reproductive cycle of this species can be subdivided into five successive stages: early active stage (January to March), late active stage (February to May), ripe stage (April to August), partially spawned stage (May to October), and spent/inactive stage (August to February). The spawning period was once a year between May and early October, and the main spawning occurred between July and August when seawater temperature was approximately <TEX>$20^{\circ}C$</TEX>. Percentages of first sexual maturity of female clam of 15.1-20.0 mm in shell length were 56.3%, and 100% for the clams > 25.1 mm. The mean number of the spawned eggs increased with the increase of size classes (shell length). In case of spawning induction by the same size class, the number of spawned eggs were gradually decreased with the increase of spawning frequencies (the first, second, and third spawnings). In the experiments of artificial spawning induction during the spawning season, the interval of each spawning was estimated to be 15-17 days (average 16.5 days).
In this study, the influence of water temperature and salinity on sand ejection of Manila clams, Ruditapes philippinarum, was investigated. The result showed that, under different water temperature, the highest quantity of sand ejection was at <TEX>$20^{\circ}C$</TEX>, the value of which was 0.091 g, while the smallest one was only 0.058 g at 0<TEX>$^{\circ}C$</TEX>. The highest releasing rate, 95.79%, could be seen in <TEX>$20^{\circ}C$</TEX> group, while <TEX>$0^{\circ}C$</TEX> group was the lowest one, 85.29%. Under different salinity, 30 psu group had the highest value, 0.057 g, and 0 psu (freshwater) group was the lowest one, only 0.026 g. At the same time, the highest releasing rate appeared at 25 psu, which was 90%. 0 psu group was the lowest one in releasing rate, 60.47%. According to these results, we recommend that the clams should be placed into the clear seawater with about 25 psu of salinity at near <TEX>$20^{\circ}C$</TEX> to let the clams perform the self-depuration. Better quality of clams containing less impurity in the aquaculture and commerce could be obtained from this pre-treatment.
This study was undertaken with the intent to describe the influence of water temperature and food concentration on the filtration rates of the Asiatic clam, Corbicula fluminea. The clams were collected at Lake Geumho near Yeongsan river, during March 2001. Food organism, Scenedesmus sp. (KMCC FC-34), was indoor-cultured in f/2 medium, and was used to measure the filtration rate of the clams. Filtration rate of the clams was measured by indirect method. Cell concentrations of food organisms were determined by direct counting cells using the hemacytometer under the light microscope. The filtration rate of the clams increased with water temperature up to circa <TEX>$25^{\circ}C$</TEX>. Above this temperature, the filtration rate decreased rapidly. The minimal filtration rate of the clams was recorded at <TEX>$5^{\circ}C$</TEX>. Thermal coefficient, <TEX>$Q_10$</TEX> values at low temperature range were much higher than those at high temperature range. These results indicate the asiatic clam is more sensitive in cold water like most of marine bivalves. There was a strong reversed correlation between filtration rate and food concentration. Filtration rate of the clams was reduced as food concentration was increased.
The embryos of marine bivalves have been commonly used in bioassays for the quality assessment of marine environments. Although several standard protocols for developmental bioassay with bivalves have been already proposed, there have been few trials for applying these protocols in environmental assessment, or for developing new protocol with Korean species. So, there is a strong need to establish the standard bioassay protocols using bivalves commonly found in Korean waters. Prior to developing a new protocol, it is essential to know the optimum conditions for the reliable bioassay procedures. Here, we established the purpose of this study to determine the optimum bioassay conditions for successful development of a common mussel, Mytilus galloprovincialis. The conditions considered as critical for developmental bioassay, and determined in this study were; (1) temperature, (2) salinity, and (3) initial density of embryo. The optimal temperature for developmental bioassay of M. galloprovincialis was determined as <TEX>$15^{\circ}C$</TEX>. At this temperature, the required time for the embryo to become veliger larva was 48 hr. The acceptable range of salinity for the embryotoxicity test using M. galloprivincialis was from 30 to 35 psu, which was narrower than that of the natural habitat of adult populations. The optimum density of embryo at the beginning of bioassay was 100 embryos/ml. Over this density, the proportion of normally developed larvae decreased significantly. The results obtained in this study will serve as a basis for preparation of the standard bioassay protocol using embryo of M. galloprovincialis.
Occurrence and prevalence of Marteilioides chungmueasis have been reported in several waters around Tongyeong but no report has been made for Pukman bay. Therefore, we investigated that the prevalence and infection intensities in Pukman Bay at the inside and the outside areas which are hydrographically divided by tidal current. Furthermore, various environmental parameters were investigated in order to elucidate effective parameter for parasitic infection. Infection rates of Marteilioides chungmuensis in adult oysters were ranged 3.3-20.0% at the inside area during September 2002 through January 2003, and 3.3-30.9% at the outside area during August 2002 through January 2003. External manifestation of infected oyster consisted of abnormal egg masses with nodular appearance in the soft tissue. Histopathological symptoms included massive hemocytic infiltration within or around the follicle wall and atrophic epithelium of digestive diverticula. For the environmental parameters, comparative study made differences between two side of the Bay during the infection period: inside > outside for SS while inside < outside for chlorophyll-a. A positive relationship was observed between chlorophyll-a and infection period, which might indicate the difference in food availability between two areas. The prevalence of ovarian parasite Marteilioides chungmuensis, therefore, was highly associated with food availability. Pearson's correlation analysis was made between environmental parameters and infection prevalence. Significance was observed in water temperature (p < 0.05), suspended solids (p < 0.01) and chlorophyll-a (p < 0.05). A principle component analysis showed that infection of the ovarian parasite, Marteilioides chungmuensis, exhibited effects of seasonality (component I = 55.2%) and chemical/physical environmental factors (component II = 24.4%). These results clearly indicate that the infection of ovarian parasite, M. chungmuensis in the Pacific oyster Crassostrea gigas is closely associated with seasonality and food availability.
A series of radiotracer studies were conducted to evaluate the influence of water temperature and/or body size on the clearance rates and uptake rates of Cd, Se and Zn in the Asiatic clam, Corbicula fluminea, Asian clam Potamocorbula amurensis and Balthic clam, Macoma balthica. Uptake rates of Cd, Se and Zn were estimated simultaneously with clearance rate of clams under 3 different water temperature conditions (5, 13 and <TEX>$21^{\circ}C$</TEX>). The weight specific clearance and metal uptake rates of P. amurensiswere increased with temperature, however, no consistent temperature effect was observed for the other clams. The variation of uptake rates of Cd, Se and Zn along with temperature or body size in each clam species was well associated with clearance rates. The inter-species as well as the intra-species difference of metal uptake rates could be well explained by the variation of clearance rates of clam individuals.
A series of radiotracer experiments were employed to quantitatively compare the biokinetics of uptake from the dissolved phase (influx rates), uptake from the various types of food source (assimilation efficiency), and loss (efflux) of Ag between Potamocorbula amurensis and Macoma balthica. Simultaneously, influx rates of dissolved Cd in both clams were determined to compare with those of Ag. Effects of salinity on influx rates were evaluated in these 2 euryhaline species, as were effects of clam size. Influx rate of Ag and Cd (<TEX>${\mu}g g^{-1}$</TEX> [dry wt.] <TEX>$d^{-1}$</TEX>) increased linearly with metal concentrations. Influx rates of Ag in both clams were 3 to 4 times those of Cd. Absolute influx rates of the 2 metals were 4 to 5 times greater in P. amurensis than M. balthica, probably because of differences in biological attributes (i.e. clearance rate or gill surface area). As salinity was reduced from 20 to 2.5 psu, the influx rate of Cd in P. amurensis increased 4-fold and that of Ag increased 6-fold, consistent with expected changes in speciation. Weight-specific metal influx rates (<TEX>${\mu}g g^{-1}$</TEX> [dry wt.] <TEX>$d^{-1}$</TEX>) were negatively correlated with the tissue dry weight of the clams, but most rate constants determining physiological turnover of assimilated metals were not affected by clam size.
Samples of Corbicula japonica Prime of Jujin estuary in Gochang were collected from July 2000 to September 2001. Age of C. japonica was determined from the rings on the shell. The relationship between shell length and ring radius in each ring group was expressed as a regression line. Therefore, there is a correspondence in each ring formation. Based on the monthly variation of the marginal index (MI') of the shell, it is assumed that the ring of this species was formed once a year during the period of February and March. The relationship between shell length (SL; mm) and total weight (TW; g) was expressed by the following equation: TW = 1.0942 <TEX>${\times}10^{-4}SL^{3.3217}$</TEX> (<TEX>$r^2$</TEX> = 0.9905). Shell length (SL) and shell height (SH; mm) was highly correlated with shell height as the following equation: SH = 0.9174 SL - 0.9935 (<TEX>$r^2$</TEX> = 0.9885). The shell length (SL) - shell width (SW) relation was also expressed by the following equation; SW = 0.5925 SL - 1.1706 (<TEX>$r^2$</TEX> = 0.9726). Growth curves for shell length and total weight fitted to the von Bertalanffy's growth curve were expressed as: <TEX>$$SL_t = 46.4861[1-e^{-0.3383(t+0.0958)}]$$</TEX>, <TEX>$$TW_t = 34.54[1-e^{-0.3383(t+0.0958)}]^3.3217$$</TEX>.
The purpose of this study is to determine the appropriate substrate for larval settlement and spat growth in the purple clam, Saxidomus purpuratus in laboratory culture. Larvae were reared with 3 different types of sediments (mud, sand, and mixed) for 46 days in settlement experiment, and settled spats were further grown in 3 types of sediments for 36 weeks in growth experiment. The density of settled spats in muddy sediments was more than 2 times higher than those in mixed or sandy sediments. But, the average size of settled spats in muddy sediments was smaller than those in mixed or sandy sediments. After 36 weeks of growth period, growth rate decreased as shell length increased. When shell length was less than 2 mm, growth rate in mixed sediments was significantly higher than that in sandy sediments. When shell length was more than 2 mm, there was no significant difference in growth rate among different substrates. Sediment type affected growth rate only when the spats were relatively small (less than 2 mm). Muddy sediments seems better for larval settlement, while mixed sediments is best for spat growth. We suggest the laboratory procedure for enhancing seedling production of S. purpuratus.
We investigated the reproductive cycle with ovarian development of Chlamys farreri by histological observations, and seasonal changes in biochemical components of the adductor muscle, digestive diverticula and ovary were studied by biochemical analysis. The reproductive cycle of this species can be classified into five successive stages: early active stage (January to March), late active stage (March to April), ripe stage (April to August), partially spawned stage (June to August) and spent/inactive stage (August to January). According to ANOVA test, there were significant differences (p < 0.05) in total protein, total lipid and glycogen contents among months for all of the adductor muscle, digestive diverticula and ovary. Total protein contents in ovary and digestive diverticula showed significant changes (ANOVA, p < 0.05) during the study period, while that in the adductor muscle did not. Total protein content was highest in the adductor muscle, followed by ovary, and lowest in digestive diverticula. There was no correlation in total protein content between the adductor muscle and digestive diverticula (p = 0.220). But strong positive correlation was found between adductor muscle and ovary (r = 0.450, p = 0.013). ANOVA showed that there were significant differences in total lipid and glycogen contents among months for all of the adductor muscle, ovary, and digestive diverticula (p < 0.05). The monthly changes in total lipid content were highly variable in ovary and digestive gland. High contents of total lipid were found during April and May-June in ovary, while March and June-July in digestive diverticula. There was a strong negative correlation in total lipid content between ovary and digestive diverticula (r = -0.397, p = 0.030). Unlike total protein or total lipid, glycogen content in the adductor muscle was most dynamic. It showed more than 36-fold changes in the adductor muscle (at most 3-fold change in ovary) during the study period. Glycogen content was higher during May-July in the adductor muscle, while it was higher in March and August in digestive diverticula. There was a strong negative correlation in glycogen content between the adductor muscle and digestive diverticula (r = -0.584, p = 0.001).