Metabolic Activities And Bacterial Contamination According To Shell Sizes Of Two Commercial Shellfish In South Korea

As filter-feeding organisms, bivalves often play a significant ecological role in many shallow fjords and coastal water because the remove of phytoplankton and other suspended particles from the water. The interaction between shellfish and ambient environment has always been considered as a research focus of culture ecology and attached with more and more attentions. To get a clear understanding of interactions between cultured shellfish and their ambient environment, especially of the potential function of bivalves in coastal water purification, the knowledge of the influences of physiological and metabolic activities of shellfish on their ambient environment and the influences of environmental factors on physiological and metabolic activities is necessary.The aim of the present study was to evaluate the clearance and oxygen consumption rate of Mytilus galloprovincialis and Scapharca broughtonii, which are two common commercial shellfish in South Korea, with different shell sizes to various environmental factors. Results of this study may provide essential data to assess the cleaning potential of these bivalves. Depending on local eutrophication situation, this information could be of value for water quality control throughout south coast of South Korea. And the results also can provide essential data to estimate the carrying capacity for culture of S. broughtonii.Simultaneously, as filter feeding organisms, bivalve molluscs are an important source of foodborne diseases. These invertebrates can accumulate high levels of microbial pathogens within their internal tissues, so they are implicated more than other marine animals in seafood-borne illness. Therefore, the aim of the present study was also to have a better understanding of the contamination status of M. galloprovincialis and S. broughtonii with various shell length, and the effects of refrigerated storage on these two shellfishes, as well as the necessity of applications of refrigerated storage.Although the physiological responses and bacterial characteristics of many bivalves have been studied, but few studies discussed the effects of body size (commonly means shell sizes for bivalve) on physiological response and bacterial contamination. In present study, the influence of body size was focused on.The physiological indexes of shellfish are affected by many environmental factors, and further the controlling role of bivalves which live in estuarine and coastal environments are influenced by many environmental factors such as water temperature, salinity, algae concentrations, nutrient loads, dissolved oxygen and environmental pollutants. As poikilothermic animal which is often distributed in estuarine, shallow fjord and costal waters, water temperature and salinity which vary seasonally can cause significant changes in physiological activities that further influence the ability of the organisms to ingest the phytoplankton. Precise knowledge about the physiological response, especially the filtration and respiration, to different temperature and salinity is essential to evaluate the role of bivalves on water quality control.To comprehensively understand the relationship of shellfish and their environment and evaluate the seafood safety, the knowledge of the bays in which they are cultured is essential.Gwangyang Bay (34°55ˊ N, 127°50ˊ E), where the M. galloprovincialis was cultured, is a typical semi-enclosed water bay, connecting with the sea with a narrow mouth, with a surface area of 230 km2. Some seasonal streams empty into the bay with varying water and sediment loads. Some rivers have become canals of industrial and domestic waste discharge with the advance of economic activity and increase of population in the region. The changes of nutrient regime and phytoplankton structure are attributed to increased human activity in this region, changed water circulation and increased aquaculture. The maximum water depth is 35 m, and the average water depth is 10 m. The current exist in the bay are significantly affected by tidal current. The tidal current speed during the flood phase ranges from 10 cm s-1 (at the bottom of the bay) to 60 cm s-1 (at the bay mouth), and the range of tidal current speed during the ebb was from 10 cm s-1 (at the bottom of the bay) to 80 cm s-1 (at the bay mouth). In Myodo of Gwangyang bay, the monthly average water temperature ranged from 1°C in Jan to 26.0°C in Jul and Aug. The monthly average rainfall ranged from 3 mm in Dec to 361 mm to Aug. The monthly average wind speed ranged from 1 to 3 m s-1, and the monthly maximum wind speed ranged from 4 m s-1 in Oct to 10 m s-1. In Suncheon of Gwangyang bay, the monthly average water temperature ranged from 1.0°C in Jan to 26.0°C in Aug. The monthly average rainfall ranged from 1.5 mm in Nov to 551.8 to Sep. The monthly average wind speed ranged from 0.8 to 1.7 m s-1, and the monthly maximum wind speed ranged from 1.7 m s-1 in Jul to 5.4 m s-1 in Mar. Mediterranean mussels M. galloprovincialis is dominantly cultivated bivalves in Gwangyang Bay. In Gwangyang Bay, the M. galloprovincialis was farmed on cultivation raft which is different from traditional bottom sawing culture.Nearby Gwangyang Bay there are four towns (Gwangyang, Suncheon, Yeosu and Hadong-gum) which have a total population of 260,392 inhabitants, about 727,411 economic animals and about 198,270 m2 hatchery areas, having potential negative impacts on the bay.Gamag Bay (34°40ˊN , 127°40ˊ E), where the S. broughtonii was cultured, is a large embayment on the south coast of Korea peninsula. The Gamag Bay is an important mariculture area in South Korea which has a total water area of 1,020 km2. The maximum water depth is 40 m, and the average water depth is 9 m. The current exist in the bay are significantly affected by tidal current. The tidal current speed during the flood phase ranges from 10 cm s-1 (at the bottom of the bay) to 40 cm s-1 (at the bay mouth), and the range of tidal current speed during the ebb was from 10 cm s-1 (at the bottom of the bay) to 40 cm s-1 (at the bay mouth). The monthly average water temperature ranged from 4°C in Jan to 26.1°C in Aug. The monthly average rainfall ranged from 0.1 mm in Nov to 330.5 mm to Jun. The monthly average wind speed ranged from 3 to 5 m s-1, and the monthly maximum wind speed ranged from 11 m s-1 in Oct to 26 m s-1 in Jul. Ark shell S. broughtonii is dominantly cultivated bivalves in Gamag Bay. In Gamag Bay, this production is made according to traditional bottom sawing culture.Nearby Gamag Bay there are five towns (Yeosu, Dolsan, Hwayang, Hwaieong, and Nam-myeon) which have a total population of 249,229 inhabitants, about 18,468 economic animals, and about 312,352 m2 hatchery areas, having potential negative impacts on the bay.In order to provide essential data to evaluate the effect of filter-feeding bivalves on water quality of Gwangyang Bay in South Korea, the clearance and respiration rates of M. galloprovincialis with different shell length (S: 21.0±1.2 mm; M: 50.6±3.3 mm; L: 31.1±2 mm) were estimated under different temperature and salinity conditions. The results showed that, in all of the three groups, the highest clearance rates appeared at 20°C, and the highest one was 4.53±0.22 Lg-1h-1 in the group with the smallest shell length. The clearance rates of every group had the highest values at salinity 30, and the highest one is 4.94±0.24 Lg-1h-1 in the smallest shell length group. The highest respiration rates were 1.405±0.070 mgg-1h-1 at 25°C and 1.353±0.068 mgg-1h-1 at salinity 40 in the group with the smallest shell length. And the shell size was negatively correlated with clearance and oxygen consumption rates.To investigate the effects of temperature and salinity on clearance and oxygen consumption rate of S. broughtonii spat with different growth stages, this study was conducted under 6 different water temperatures and 5 different salinity conditions. The individuals were divided into three groups (S: 11.8±2.1mm; M: 21.8±1.1 mm; L: 30.1±3.0 mm) of different shell sizes and clearance rates were estimated by Neutral red. The results showed that in all of the three groups, the highest clearance rates appeared at 20°C, and the highest one was 1.89±0.12 Lg-1h-1 in the group with the smallest shell length. The clearance rates in every group increased with the rise of salinity in the range from 10 to 40, and the highest one is 2.62±0.21 Lg-1h-1 in the smallest shell length group. The trends of oxygen consumption rates were similar to clearance rates. The highest rates were 0.189±0.003 mgg-1h-1 at 20°C and 0.308±0.018 mgg-1h-1 at salinity of 40 in the group with the smallest shell length. And the results showed that the clearance and oxygen consumption rates were negatively correlated with shell size. With the aim of having a better understanding of the contamination status of M. galloprovincialis and S. broughtonii with various shell lengths, as well as the necessity of applications of refrigerated storage, in this study we determined: the initial level of coliform, fecal coliform and total plate count (TPC) in M. galloprovincialis and S. broughtonii with various shell length; the level of coliform, fecal coliform and total aerobic bacteria in these two shellfish under refrigerated and non-refrigerated storage conditions.The initial level of coliform, fecal coliform and TPC in M. galloprovincialis were 960±90 MNP 100g-1, 230±40 MNP 100g-1 and 8,930±870 CFU g-1, which were significant high than those in S. broughtonii (450±60 MNP 100g-1, 90±30 MNP 100g-1 and 1,570±380 CFU g-1). It should attribute to the higher contamination level of aquatic environment where M. galloprovincialis was cultured. The initial levels of coliform, fecal coliform and TPC in M. galloprovincialis were negatively correlated with shell length (S: 18.7±4.0mm; M: 42.6±5.4 mm; L: 61.4±7.4 mm). The initial levels of coliform, fecal coliform and TPC in S. broughtonii were also negatively correlated with shell length (S: 30.0±2.0mm; M: 39.1±0.7 mm; L: 45.9±1.1 mm).The bacteriological characteristics of these shellfish were determined after 48-h refrigerated (4±1°C) and non-refrigerated (28±1°C) storages. Under refrigerated conditions, the inhibitory rate of bacterial in M. galloprovincialis was significantly higher than that in S. broughtonii indicating that refrigerated storage was more effective for M. galloprovincialis than S. broughtonii.After 48 h refrigerated storage, the level of coliform, fecal coliform and TPC in M. galloprovincialis were 2,000±120 MNP 100g-1, 430±60 MNP 100g-1 and 10,780±1,240 CFU g-1, which were significantly higher than those in S. broughtonii (850±70 MPN 100g-1, 130±30 MNP 100g-1 and 3,360±410 CFU g-1). Refrigerated storage is an effective control measure to slow the rate of bacteria growth in live seafood, and thus can maintain the initial microbial status of the shellfish, but cannot improve it.After 48h refrigerated storage, the inhibitory rates in both M. galloprovincialis and S. broughtonii were negatively correlated with shell length. The inhibitory rates of bacterial were significantly different between M. galloprovincialis (39.4±4.5% for coliform, 54.7±4.8 for fecal coliform and 78.1±8.1 for TPC) and S. broughtonii (29.2±3.5% for coliform, 40.9±4.7 for fecal coliform and 72.3±8.1 for TPC).