Burrowing and Feeding Responses of Different Populations of Sinonovacula constricta to High-Salt Culture Environment

The razor clam (Sinonovacula constricta, Class Bivalvia) is a kind of burial filter-feeding shellfish. Salinity fluctuation is an important source of pressure for water habitats. High salinity in some coastal areas of Shandong and Jiangsu impact the survival and germplasm conservation of razor clam. To study the ecological behavior response of S. constricta to high salt culture environment, two populations of razor clams were used, including "Shenzhe No.1" population (SZSC) and a natural population (ZRSC). The semi-lethal salinity level of each population was determined. The effects of control group (20) and high salinity (24, 28, 32) on burrowing and feeding behavior of razor clams were studied. The differences in burrowing indices and feeding physiology between the two populations were compared. In the burrowing behavior experiment, two groups were set; razor clams from the temporary pond were put into each salinity group to start the experiment, while the other group of razor clams were stressed under each salinity condition for 24 h and then put into each salinity group to start the experiment. The results showed that the 120 h LC50 of SZSC was 34.04, while the 120 h LC50 of ZRSC was 32.04. The burrowing behavior of razor clams could be divided into four periods: The preparation period of shell closure, the period of axe foot movement, mud digging period, and the end period of mud diving. In the non-stressed group, the burrowing time of 50% (BT50) of SZSC was significantly higher than that of ZRSC (P<0.05). The BT50 of SZSC at 24 salinity was the minimum, which was significantly lower than that of BT50 at 28 and 32 salinity. The distribution of burrowing depth of SZSC was highly concentrated: 50% of the individuals were between 7.29 and 7.55 cm. The burrowing rate was 88.33% at 32 salinity, which was significantly higher than that of ZRSC (P<0.05). In the stressed group, the BT50 of SZSC was significantly lower than that of ZRSC, while the burrowing rate was significantly higher than that of ZRSC (P<0.05). With the increase in salinity, the burrowing rate of ZRSC decreased significantly (P<0.05). The average burrowing depth of ZRSC was 7.45 cm at 32 salinity, which was significantly higher than that of SZSC (P<0.05). By comparing the experimental results of the two populations, whether in SZSC or ZRSC, the BT50 of razor clams in the stressed group was higher than that in non-stressed group at each salinity. However, there was no significant difference in BT50 between the stressed and non-stressed groups at 20 and 28 salinity among SZSC (P>0.05). The BT50 of SZSC was closer to that of the control group at 24 and 28 salinity, and the vitality of SZSC was significantly better than that of ZRSC after 24 h of salinity stress. Under high salinity, the distribution of SZSC in mud was more concentrated than that in ZRSC, and the burrowing depth was shallower. In terms of feeding physiology, the feeding rate of SZSC was significantly higher than that of ZRSC under high salinity (P<0.05). The feeding rate of SZSC reached the maximum 89.54 mL/(g·h) at 24 salinity, which was significantly higher than that of other salinity groups (P<0.05). In summary, the ecological behaviors of both populations were affected by high salinity. The higher the salinity, the stronger the stress response, with the SZSC having a higher salinity tolerance than ZRSC. In this study, the tolerance of two populations of razor clams to high salinity was evaluated at the level of ecological behavior, and the vertical distribution and feeding ability of S. constricta in sediment in a high-salt environment was revealed. The results provide a theoretical reference for the further breeding of novel high-salt-tolerant strains of S. constricta.