P. K. S. Shin

Characterization of subpopulations and immune-related parameters of hemocytes in the green-lipped mussel Perna viridis

The green-lipped mussel Perna viridis is distributed widely in the estuarine and coastal areas of the Indo-Pacific region and extensively cultured as an inexpensive protein source. Morphology and immunological activities of hemocytes of P.xa0viridis were investigated using flow cytometry and light and electron microscopy. Three major types of hemocytes were identified in the hemolymph, including dense-granulocyte, semi-granulocyte (small and large size) and hyalinocyte. Other hemocytes, which occurred in low numbers, included granulocytes with different electron-dense/lucent granules and hemoblast-like cells. Based on flow cytometry, two subpopulations were identified. Granulocytes were larger cells, and thexa0more abundant, containing numerous granules in the cytoplasm, and hyalinocytes were the smaller and less abundant with the fewest granules. Flow cytometry revealed that the granulocytes were more active in cell phagocytosis, contained the higher lysosomal content, and showed higher esterase activity and reactive oxygen species (ROS) generation compared with hyalinocytes. Immune functions assessed byxa0the flow cytometry indicated that the granulocytes were the main hemocytes involved in the cellular defence in P.xa0viridis.

Management of marine fish farming in the sub-tropical environment: a modelling approach

Two deterministic models were applied to simulate hydrographic and water quality conditions within a sub-tropical marine fish culture site, where trash fish is used as feed. A two-dimensional, two-layer hydrodynamic model of tidal flows and salt transport calculated the water level, velocity and salinity in each grid cell of 50 m square in each layer within the culture area approximately every 30 s during a tidal cycle. Results from this flow model provided hydraulic data for input into a three-dimensional tidal water quality model, which was run to simulate water quality due to specified pollutant loadings from the marine fish culture operations. Simulated output from the models agreed reasonably well with observed field data, except for dissolved oxygen and nitrate levels in surface waters. This could be due to a conservative estimation of re-aeration rates in the model system. In applying the models to sub-tropical waters of Hong Kong, the simulation predicted the extent of pollution and area affected under varying fish stock and pollutant loadings. It was shown that impacts of fish culture activities on water quality at the test culture site were localized owing to strong advection by tidal residual flows. The higher simulated level of organic nitrogen in the bottom waters over the year at the test fish culture site was due to increased release of organic nitrogen through decay of wasted feed from the sea bed and from trash fish feed. Models of marine fish culture and environmental effects have applications in determining acceptable stocking levels, and the effects of changing feed and culture species, and for locating suitable culture sites in coastal areas.

Transplant experiments on growth and mortality of the fan mussel Pinna bicolor

Abstract Growth and mortality for different size classes (30–49 mm, 50–79 mm and 80–100 mm shell width) of the fan mussel Pinna bicolor transplanted at open sea bottom, within enclosure cages, and on suspended rafts were studied and compared over an 18-month study period. Growth, in terms of shell width, dry tissue weight and condition index of the same size class, did not show significant differences (P>0.05) between the transplanted populations at the open sea bottom and in enclosure cages. Fastest growth was noted for the small-size class during the initial 12-month period, with a net increase in maximum shell width of 50–60 mm (100–120 mm shell length). Thereafter, growth tended to slow down and all growth parameters became similar for all the three size classes. In comparison of the small-size fan mussels between the bottom (open and caged) transplants and that on the suspended rafts, a better growth result was registered in the suspended raft population possibly due to a faster induced flow and availability of food in the water column. Mortality of transplanted P. bicolor was the highest at open sea bottom, and predominantly caused by predation by fish and crabs. Highest mortality was also noted in the small-size class as the larger fan mussels were less vulnerable to predation than the small juvenile. The number of transplanted P. bicolor at open sea bottom fell to the same natural density within the first 12 months of the study and stabilized afterwards, suggesting that the natural population of P. bicolor was controlled by its predators. In rearing of fan mussels, both growth and mortality should be considered in concert, so as to select an optimal size class of transplants which would attain the maximum growth, with minimum mortality, over the culture period.

Chronic hypoxia and low salinity impair anti-predatory responses of the green-lipped mussel Perna viridis.

The effects of chronic hypoxia and low salinity on anti-predatory responses of the green-lipped mussel Perna viridis were investigated. Dissolved oxygen concentrations ranged from hypoxic to normoxic (1.5 ± 0.3 mg l(-1), 3.0 ± 0.3 mg l(-1) and 6.0 ± 0.3 mg l(-1)), and salinities were selected within the variation during the wet season in Hong Kong coastal waters (15‰, 20‰, 25‰ and 30‰). The dissolved oxygen and salinity significantly affected some anti-predatory responses of mussel, including byssus production, shell thickness and shell weight, and the adductor diameter was only significantly affected by salinity. Besides, interactive effects of dissolved oxygen and salinity on the byssus production and shell thickness were also observed. In hypoxic and low salinity conditions, P. viridis produced fewer byssal threads, thinner shell and adductor muscle, indicating that hypoxia and low salinity are severe environmental stressors for self-defence of mussel, and their interactive effects further increase the predation risk.

Combined Effects of Dissolved Oxygen and Salinity on Growth and Body Composition of Juvenile Green-Lipped Mussel Perna viridis

ABSTRACT n The green-lipped mussel Perna viridis is distributed widely in estuarine and coastal areas of the Indo-Pacific region and is regarded as a cultured mussel or by-product in aquaculture. However, in estuarine and coastal waters where salinity varies with freshwater input and rainfall during the wet season, hypoxia frequently occurs, especially when waters are highly eutrophic. The current study aimed to evaluate the effects of two key environmental factors in estuarine and coastal waters—dissolved oxygen (DO) and salinity—on growth and body composition of juvenile green-lipped mussels P. viridis. Parameters studied included shell length (SL), tissue dry weight (TDW), condition index (CI), specific growth rate (shell length, SGR L ; tissue dry weight, SGR W ), moisture content (MC), crude protein (CP), crude fat (CF), crude carbohydrate (CC), crude ash (CA), and energy content (EC). Mussels were cultured for 6 wk at 4 salinities (15, 20, 25, and 30) and 3 dissolved oxygen concentrations (1.5 ± 0.3, 3.0 ± 0.3, and 6.0 ± 0.3 mg 02/L) in a 4 × 3 factorial design. All growth parameters (SL, TDW, CI, SGR L , and SGR W ) decreased under reduced DO and salinities, but interactive effects between these 2 factors were statistically indistinguishable except for SGR W . Higher percentages of CF and CP, and lower percentages of CC were obtained at reduced salinities and DO. When changes in biochemical content (weight per individual) were compared, both CP and CC content decreased significantly as salinity or DO decreased, whereas no pattern was observed for CF. EC (calories per gram) was not significantly different among DO treatments, but varied significantly with salinity. Total energy content (calories per individual), however, increased significantly with both DO and salinity, but the interaction between salinity and DO was statistically indistinguishable.