The Tolerance Of Commercial Bivalves To Hypoxia

With the intensified human activities and the worsen eutrophication in coastal regions worldwide,both the coverage and the strength of hypoxia have risen sharply.In the background of global change,the relationships between hypoxia and other marine ecological disasters such as algal bloom and ocean acidification have become closer.Though some fishes with strong mobility can flee the hypoxic zones,most marine benthos with poor mobility often suffer mass mortalities,and as a result,the current marine ecosystems would change greatly with some serious ecological disasters such as jellyfish bloom frequently breaking out.In the culture area,the hypoxia caused by high-density mariculture and eutrophication challenge the survival of the cultured animals,bring potential threat to the industry.The studies on the tolerance of marine organisms to hypoxia have begun since 1960s.However,due to the difference in experimental method and condition,it is hard for us to compare the tolerance of different marine organisms precisely.It is also difficult to establish an accurate relationship between the tolerance of these annimals and the dissolved oxygen.Additionally,for the research on the responses of marine organism to hypoxia,some key parts are still missing.For example,some behavioral phenomenon,the key respiratory enzyme activity changes and the cellular damage remain unknown.The commercially important Manila clam Ruditapes philippinarum has strong adaptability to a wide range of salinities and water temperatures;the species has a widespread natural distribution in coastal areas of the western Pacific and has become established on the Atlantic coast of Europe.As a common and dominant species in many coastal regions,it functions as a key part of food webs and the biogeochemical cycle in these marine and brackish ecosystems.Zhikong scallop,a sessile and filter-feeding benthic animal,is widely distributed along the coast of northern China.Both the culture area and the density of Zhikong scallop have soared in the past 30 years.In particular,Zhikong scallop and Manila clam constitute the major cultured shellfish in China.However,worsening hypoxia in shallow coastal waters and estuaries increasingly challenges the survival of Manila clam and Zhikong scallop as well as the mariculture industry development.In China,the severity of summer hypoxia has been rising in coastal areas of Shandong Peninsula—an important natural habitat and mariculture region for Manila clam.Dissolved oxygen?DO?in Laizhou Bay and Rushan Bay dropped below 2.0 mg/L,and the situation was worse in the Xiaoqing River estuary,with DO concentrations of less than 0.5 mg/L being recorded.In Jiaozhou Bay,the culture of Zhikong scallops and bay scallops?Argopecten irradians?resulted in a decrease of DO levels to around 2.0 mg/L.In Sanggou Bay,another important culture area for Zhikong scallops,the risk of eutrophication and hypoxia have risen sharply due to aquaculture activities.In Japan,mass mortalities of Manila clam caused by hypoxia have occurred many times during summer,bringing tremendous economic loss to the fishery.However,studies on the effects of hypoxia on both mollusks are still limited.In the present study,we designed a novel hypoxia simulation device which featured high accuracy and good stability.We established a standard and uniform method to investigate the effects of hypoxia on survival,behavioral,physiological and key enzyme activities involved in respiration.Besides,we observed the cellular damage of Manila clam caused by hypoxia.As the results showed,Manila clam is tolerant to hypoxia as its 20-day LC₅₀ for DO was 0.57 mg/L and its LT₅₀ at 0.5 mg/L DO was 422 hours.All the clams survived the hypoxic challenge after 7 days of the experiment.In contrast,Zhikong scallop is sensitive to hypoxia as its 20-day LC₅₀ was estimated to be 1.8 mg/L and its LT₅₀ at 1.5mg/L DO was 432 hours.When DO started to drop,the survival rate of Zhikong scallop decreased immediately.Additionally,the survival of Manila clam was significantly affected by the sediment type,and the survival rate of Zhikong scallop would reduce largely in a second hypoxia event.For behavioral response,Manila clam mainly emerged from the sediment to get more oxygen,while Zhikong scallop showed possible escape responses.For physiological response,Manila clam would firstly ensure the oxygen supply and then depress its respiration to reduce its metabolic rate;Manila clam preferred the use of carbohydrate and fats instead of protein when oxygen is insufficient.Unlike Manila clam,Zhikong scallop would first depress its respiration and then keep it stable when DO dropped to 2.0-3.0 mg/L.At the DO concentration of 1.5 mg/L,Zhikong scallop would depress its heart rate?HR?at the same time.For key respiratory enzyme activity response,Manila clam exhibited significantly high activities of phosphofructokinase?PFK?and pyruvate kinase?PK?under hypoxia,which indicate the accelerated glycolysis.For Zhikong scallop,the phosphoenolpyruvate carboxylase?PEPC?and fumaric reductase?FR?were significantly activated,suggesting the dismutation of glycolysis.However,the activated lactate dehydrogenase?LDH?may be responsible for the accumulation of excessive lactate and the final death of Zhikong scallop.Under hypoxic stress,the cellular damage of Manila clam was obvious with collapsed cristae,shriveled membranes and induced cell inclusion,which would possibly reduce their resistance to other environmental stressors.