Growth And Physiological Response Of The New Strain Of Razor Clam(Sinonvacula Constricta) To High Temperature And Low Salinity Stress

The razor clam（S.constricta）is widely distributed in intertidal and estuarine areas off the coast of the western Pacific Ocean and is one of the four traditional marine aquaculture shellfish in China.Razor clams are poikilothermic animal and osmoconformer,external temperature and salinity changes can directly affect their growth,reproduction,development,metabolism,and feeding and even survival,therefore,temperature and salinity are environmental factors that should not be ignored in the razor clam culture industry.In the southern China outdoor mud pond culture area,direct sunlight in summer tends to overheat the water,in addition,strong rainfall weather in the summer will also lead to a sudden drop in the salinity of the culture water.Therefore,in order to cultivate new strain of razor clams with excellent resistance to high temperature and low salinity,and to better understand the growth and physiological response of the new strain of razor clams under environmental stress,the new razor clam strains selected for this study were firstly analyzed for growth under different feeds and culture densities to determine the suitable culture conditions,and then high temperature and low salinity exposure experiments were set up to analyze the growth and physiological response of the new razor clam strains in response to high temperature and low salinity stress,the main results are as follows:1.Growth performance analysis of the new strain of razor clam under different diets and culture densitiesIn this study,we investigated the suitable feeds and culture density of conventional razor clam juveniles,“Shen-zhe No.1”razor clam juveniles and new strains of razor clam juveniles.Four different culture densities（1000,2000,3000 and 4000 inds/m²）were set up with three different species/strains of razor clam juveniles for each culture density.The results showed that the new strain of razor clam had the best growth performance at 1000inds/m²（P<0.05）.And the new razor clam strain had better growth performance than the conventional razor clam at the same culture density.Isochrysis galbana was used as bait for single algae feeding experiments,while I.galbana,Chaetoceros muelleri and C.muelleri were used as a mix of algae for feeding.Four feeding gradients(50×10⁴,100×10⁴,200×10⁴ and 400×10⁴ cells·ind^-1)were set.The results showed that the best growth performance was observed when the new razor clam strain juveniles were fed with the optimal feeding amount of 200×10⁴ cells·ind^-1 single algae or mixed algae（P<0.05）.In addition,the shell length and wet weight of razor clams increased faster in the same time,indicating that the mixed algae feeding razor clams is more effective than the single algae.The wet weight growth rate of the new razor clam strain was better than that of the conventional razor clam at a feeding amount of 200×10⁴ cells·ind^-1（P<0.05）.This study initially revealed the most suitable breeding density and feeding conditions for the new razor clam strain.2.Growth and physiological response of the new strain of razor clam to high temperature stressIn order to investigate the growth and physiological response of the new strain of razor clams under high temperature environment,the high temperature experiment was conducted for 30 days at 30°C in the control group and 34 and 38°C in the high temperature treatment group,and the results showed that the survival rate of the new strain of razor clams was significantly reduced due to high temperature stress,and the survival rate was 0%（P<0.05）at day 30 in the 38°C treatment group.In the 34°C treatment group,the survival rate of the new razor clam strain at day 30 was the highest,37.50%,while the survival rate of the conventional razor clam was only 20.00%.The growth rate of shell length and wet weight of the new strain of razor clams decreased under high temperature conditions（P<0.05）.Under the same temperature conditions,the shell length and wet weight growth rate of conventional razor clams were the lowest and the growth was slow,while the wet weight growth rate of the new strain of razor clams was the highest（P<0.05）.The digestive enzyme activities were found to be highest in the new razor clam strain at 30°C for pepsin andα-Amylase activities,and the increase in temperature led to a decrease in enzyme activities（P<0.05）.The results showed that the survival and growth rate of the new strain of razor clams were reduced due to high temperature stress,and the digestive enzyme function was impaired,but the overall heat tolerance performance was better than that of the conventional razor clams.This study initially revealed the high temperature tolerance characteristics of the new strain of razor clams,which can provide a reference basis for the promotion of culture and the development of high temperature prevention and control management measures.3.Physiological response of the new strain of razor clam to low salinity stressTo investigate the low salinity tolerance of the new strain of razor clams,a 7-day low salinity stress experiment was conducted with low salinity groups 1,3,5 and 10 and a control group of 20 psu.The results showed that the survival rate of the new strains of razor clam was 100%in groups of 10,5 and 3 psu.The survival rate decreased in the environment with salinity of 1 psu（P<0.05）.Low salinity stress resulted in a reduction in the adaptation of haemolymph osmolality in razor clam（P<0.05）.The activity of Na⁺/K⁺-ATPase in the gill of razor clam in the low salinity treatment group was significantly higher than that in the control group（P<0.05）.Taurine,alanine,arginine and ornithine were the main free amino acids in haemolymph of razor clam.The proportion of taurine,arginine and ornithine in haemolymph increased while that of alanine decreased under low salinity stress（P<0.05）.The results indicated that free amino acids in haemolymph played an important role in the regulation of osmolality of razor clam.Histological analysis showed that severe hyperplasia,haemocyte infiltration,and cellular swelling in the haemolymph cavity were observed in the gill treated with low salinity,indicating that low salinity stress caused adaptive changes in the gill tissue.The activity of immune-related enzymes in the haemolymph was measured.The activities of total superoxide dismutase（T-SOD）,lysozyme（LZM）,acid phosphatase（ACP）and alkaline phosphatase（AKP）in haemolymph were measured.Compared with the control group,the activity of T-SOD in haemolymph of razor clam decreased significantly under salinity stress of 1 psu（P<0.05）,LZM activity of razor clam increased in salinities of 10and 5 psu,but decreased in salinities of 1 psu（P<0.05）,salinities of 5 and 3 psu resulted in increased ACP activity（P<0.05）.16S r RNA gene sequencing method was used to analyze haemolymph microbial community of razor clam in different salinities.The results showed that:low salinity stress caused significant changes in the relative abundance of Proteobacteria,Acidobacteria and Cyanobacteria（P<0.05）;low salinity stress significantly changed the relative abundance of Marinobacterium,Marinomonas and Pseudomonas（P<0.05）.LEf Se analysis showed that different microbial populations existed in haemolymph of razor clam in different salinities.These results further revealed the physiological response and potential adaptation strategies of the new strains of razor clam to low salinity stress,and provide a reference for the promotion of the new strain of razor clam for culture.