참굴(Crassostrea gigas)의 패각운동을 활용한 바이오 모니터링 시스템 연구

Abstract

Abnormal marine environment causes fisheries damage in aquaculture farms. the occurrences and damage scale tends to increase annually. Ocean observation buoy is operating in major aquaculture farms for detecting changes in coastal environments in Korea. While these physico-chemical methods give usually may well defined answers, they do not reflect real biological harm. Organisms will react to environmental stimulus, the reaction of organism is directly related to the fisheries damage. Therefore, we tested Biological Monitoring System (BMS) using the organism's responses. The species to be tested is the Pacific oyster, Crassostrea gigas (Mollusca: Bivalvia) which accounts for the highest production of shellfish culture in Korea, and the Shell Valve Movements (SVMs) of oysters were measured by the organism's responses. The first purpose of this study is to investigate the characteristics of general SVMs of oysters and changes of SVMs according to abnormal environmental factors, which are high water temperature, low salinity water, hypoxic waters, and Harmful Algal Bloom (HAB). The second is to obtain abnormal patterns, which are different from general SVMs. The final goal is to verify the possibility of BMS through field experiments. Oysters rapidly closed their valve, and gradually started to open their valve in the laboratory and in the field. It was transiently transposed from the opened-valve condition to the closed-valve condition. Then it took 10 minutes to return to the original state. This was different from other bivalves species, and the above SVMs pattern was regarded as a typical SVMs of Pacific oyster. In other characteristics, the oyster had a resting state in which the valves were kept closed for a certain time in both laboratory and field. In laboratory, the time and duration of the resting state were different depending on the individuals. In the field, the resting state observed in the field seemed to have periodicity, and lasted for about 1 hour and was observed at a high rate during the low tide period. It seems that the external factors in the field, such as tide rhythm, seem to have affected the SVMs of oyster. It is considered that there is no problem in applying the abnormal pattern to the field because the resting state has weak periodicity and does not induce changes in SVMs. This normal SVMs of oysters captured through experiment was used as a criterion to judge the abnormal pattern. As a result of water temperature exposure experiment, the oysters were kept closed valve condition at low temperature (5℃), and the SVMs tended to be active in response to water temperature increase at 10, 20 and 30℃. Especially at 20℃ and 30℃, it was 2 ∼ 3 times and 4 times more active, respectively, than the normal SVMs of oysters in this study. The subsequent increase in water temperature induced the valve closure. This is due to the limit of water temperature tolerance, which resulted in the reduction of metabolism in oysters and thus the selection of the closed-valve condition to reduce energy metabolism. The oysters closed their valves at extreme temperature, and SVMs became active with increasing water temperature. Influence of increased temperature and acclimation temperature can not be excluded, but the increase in water temperature within the limit range has the effect of activating SVMs. Through the experiment, the increasing SVMs was defined as an abnormal pattern indicating high water temperature. In the salinity exposure experiment, there was no difference between the normal SVMs and the SVMs at 20 and 30 psu, and the closed state at 10, 0 psu. Salinity in the range of 20 ∼ 30 psu did not affect shell movement of oyster. it is considered that the oyster closed their valves in low salinity waters below 10 psu to protect the organism from the extreme environment. In the low salinity waters (15 psu) at 15℃, the oysters showed a closed-valve condition for 5 ∼ 10 hours, but showed intermittent opening action and slowed down SVMs to feeding and respiration. At this time, the SVMs velocity is decreased by about twice as compared with the normal SVMs. In the low salinity waters (15 psu) at 30℃, There were combination of SVMs at high water temperature(30℃) and low salinity waters (15 psu) at 15℃. The closed-valve condition lasted 0.2 ∼ 1 hours, the SVMs speed was 3 times slower than at the high water temperature, and 3 times faster than the 15℃ low salinity waters. SVMs combined with the characteristics of SVMs at low salinity waters and high water temperature, and the signal of the physiological crisis situation of the oyster was clearly shown. As a result, the repetitive behaviors of slow down SVMs and valve closure were determined as abnormal patterns indicating low salinity waters. Oysters showed a 5 times faster SVMs than the normal SVMs as the dissolved oxygen decreased to less than 3 O2 mg l-1. Exposure to free hypoxic waters (< 2 O2 mg l-) resulted in valve closure. This SVMs pattern was observed in both the laboratory and the field of 2016. It seemed that oysters increase the SVMs to maintain oxygen consumption when dissolved oxygen is reduced, and that the valve closure responds to maintain a reduced energy requirement once the oxygen threshold is reached. The increase of the SVMs with dissolved oxygen decrease and the subsequent valve closure were regarded as abnormal patterns indicating hypoxic environment. Especially, a series of closing action when exposed to hypoxic environment is considered to be an outstanding bio-indicator for the detection of hypoxic environment. As a results of exposure to microalgae, it showed no change of SVMs in I. galbana, which was used as a control. On the other hand, When oysters were exposed to Alexandrium affine and A. affine at the cell densities of 100 and 500 cell ml-1, the pattern of SVMs immediately exhibited with frequent spikes. The increase of SVMs with increasing cell density of Alexandrium was intensively observed at cell injection, suggesting that the oysters was sensitive to Alexandrium. In field experiment of 2017, SVMs were increased when the cell density of Alexandrium bloom in the field increased. This is similar to the abnormal pattern observed in laboratory experiments. The major characteristics of C. polykrikoides exposure experiment was that oysters had longer duration of the closed-valve condition with increasing cell density. at 100 cells ml-1 showed about 40% and 1000 cells ml-1 showed about 60% of the closed-valve condition. In addition, there was a marked reduction in gab between the valves before the valve closure and a deep and uneven SVMs. This is also an unobserved result in I. galbana, the control. This suggests that qualitative detection of C. polykrikoides is possible. The warning criteria of C. polykrikoides bloom in National Fisheries Research and Development Institute is 100 cells ml-1 or 1000 cells ml-1. it would be helpful to reduce the damage caused by C. polykrikoides. From the above results, abnormal patterns with environmental stimuli were obtained. We confirmed the possibility of BMS using SVMs of Crassostrea gigas. Particularly, the obtained results suggested that oysters may serve as excellent bio-indicators for hypoxic waters and low salinity waters. In the case of HAB, abnormal patterns in Alexandrium sp. could be used to reduce PSP damage in spring and abnormal pattern that indicates C. polykrikoides bloom seem to be a great bio-indicator. If the abnormal pattern derived from this study is used successfully, it is expected that the fisheries damage will be reduced by the early warning to abnormal marine environments.