| The Pacific oysters Crassostrea gigas adapt temperature and salinity widely andhave many advantages such as rich nutrient, delicious taste, fast growth and higheconomic benefits. Oysters have been one of the most important commercial shellfishin the world. China produces about million tons of cultured oysters every year, notonly for domestic consumption but also for exportation.In China, the scale and production of coastal aquaculture industry increase with thedevelopment of economy. But at the same time, coastal water area is contaminatedand resulting to the occurence of pathogenic bacteria and water eutrophication whichoften cause harmful algae blooms. Shellfish are filter feeders that can bioaccumulatehigh levels pathogenic microorganism and biotoxin from overlying waters, theypresent a health risk when consumed raw or only lightly cooked. On this background,we studied the shellfish depuration technology, and the effect of harmful algaeAlexandrium catenella on the Pacific oysters.1Uptake and elimination of Escherichia coli by Pacific Oyster Crassostrea gigasPacific oysters were placed in seawater inoculated with different concentrations ofEscherichia coli, then depurated the oysters. Individual oysters and different tissueswere analyzed to research the uptake and elimination kinetics, and the tissuedistribution of E. coli. After accumulation, oysters were found to contain about twolog values higher than that in the water; when depurating for36hours, the numbers ofE. coli decreased come to3log values. Oysters accumulated E. coli very fast in thefirst1hour, then at a slower rate and came to the highest uptake value, at last with aslight fluctuate. The elimination kinetic was similar to it. It was found that E. coli concentrations were various in different tissues, and the highest of E. coli was locatedin digestive glands after accumulation. During elimination, the concentration of E.coli in digestive glands and gonad decreased very fast and retained the lowest levelcompared with the other tissues. The results obtained in this research indicated thatthe depuration effect was associated with the initial concentration, the contaminatedoysters with concentration of E. coli no higher than5log values could be depurated tosafe after a36-hour depuration period. The tissue distribution of E. coli suggested thatthe uptake and elimination of microorganism of oysters may be related to the feedingand digesting physiological activities.2Effects of different factors on depuration of Escherichia coli from CrassostreagigasSamples of C. gigas, maintained in a circulation system using ultraviolet light, werecontaminated with E. coli at5log MPN/100mL for bioaccumulation in still seawater.Turn on the depuration system, we performed an experiment to determine the effectsof different factors on depuration of E. coli from C. gigas. The E. coli decreasedsharply during the first6hours of depuration, then at a slower rate within the left time.After36hours, the numbers of E. coli decreased come to2or3log values. Theresults of the experiment suggest that: when shellfish weight to water weight is1:6,water exchange rate is3times per hour, temperature is15oC or25oC, and the basketslocate on the first level, the efficiency of depuration is the best.3Effects of toxic dinoflagellate Alexandrium catenella as a diet on sexualmaturation and reproductive output in the Pacific oyster Crassostrea gigasThis study assessed the effects of the toxin-producing dinoflagellate,Alexandrium catenella, on sexual maturation and reproductive output in the Pacificoyster, Crassostrea gigas. Oysters were exposed to A. catenella from April to June2012, and compared to a control batch of oysters fed with Isochrysis galbana. Duringthe exposure, clearance rates, histological observations, biochemical composition (glycogen, lipid, protein and nucleic acids) as well as reproductive output of oysterswere researched. The results indicated that A. catenella could be a food source, andinhibits the clearance rate of I. galbana. Significant pathological changes in the formof degeneration in adductor muscles, mantle and ovary and several inflammatoryresponses were observed in oysters when exposed to the harmful microalga. Theexposure to A. catenella also had negative effects on assimilation, biochemicalcomposition and the reproduction of oysters. The results obtained in the studydemonstrate that the toxic microalga, affecting quality of eggs, can have consequenceson fertility, embryo and larval output.4Effects of the dinoflagellate Alexandrium catenella on early development of thePacific oyster Crassostrea gigasThe impact of the dinoflagellate A. catenella on the early development of thePacific oysters were investigated by exposing embryos or larvae todifferent concentrations of A. catenella. The embryonic and newly hatched larvaldevelopments were inhibited after exposure to A. catenella at a concentration of100cells/mL. In particular, the trochophore larvae showed a retarded metamorphosis tothe D-shaped larval stage, and could not finish the hatching process at concentrationsexceeding1000cells/mL. Adverse effects of A. catenella on the development offeeding larvae and settlement rate were shown at concentrations of more than300cells/mL. The survival rate of embryos and trochophore larvae were sensitive to A.catenella and it decreased significantly at a concentration of300cells/mL, thesurvival rate of D-shaped larvae was reduced significantly at a concentration of600cells/mL, whereas the umbo larvae and eye-spot larvae were affected significantly at ahigh concentration of2000cells/mL only. The results obtained in this studydemonstrate that A. catenella severely affects the early stage of development of C.gigas, but the effects of harmful algae on early development of oysters are variedduring different stages. 5Effects of diets containing the dinoflagellate Alexandrium catenella on earlydevelopment of the Pacific oyster Crassostrea gigasMarine shellfish often have a planktonic larval stage and feed with microalgae insurrounding water as food. Harmful algae usually blooms at this stage, and in thiscase, early development of the Pacific oyster C. gigas may be exposed to the harmfulalgae. The toxic dinoflagellate A. catenella was used as a diet to investigate the effectof A. catenella as a single diet on the early development of C. gigas. In this study,when fed A. catenella as a mono-alga diet, the harmful algae had obvious negativeeffects on larvae and juveniles of oysters, especially on D-shaped larvae. Fed A.catenella as a mixed diet, the harmful algae inhibited the survival rate of D-shapedlarvae and umbo larvae as same as the settlement of eye-spot larvae when thedinoflagellate proportion came to50%. The adverse effect became stronger with theincreased proportion of dinoflagellate. The results indicated that shellfish larvae couldprey on dinoflagellate algae as a food but could not digest them efficiently. Thenutritional reserves were significantly affected, the larvae had a delayed developmentand a smaller size. This study also suggested that the effects of A. catenella wasvarious on different developmental stages. The D-shaped larvae was the mostsensitive response to harmful algae, and this stage may be destroyed if harmful algaebloomed at this stage and may have lasting legacies for juveniles and adults. |
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