Effects of Sudden Drop in Salinity on Osmotic Pressure Regulation and Antioxidant Defense Mechanism of Scapharca subcrenata

Abstract

Salinity is an important ecological factor that impacts the growth and survival of aquatic organisms. The salinity of seawater in coastal and estuarine areas is often subject to dynamic changes because of seasonal rainfall and continental runoff. Thus, the current study investigated the effects of sudden changes in salinity on the survival rate and osmotic pressure regulation mechanisms of bottom-sowing seedlings of the economically important ark shell, Scapharca subcrenata. By simulating the sudden changes that occur in seawater salinity after rainstorms, the results showed that the osmotic pressure of the hemolymph and Na⁺, K⁺, Ca²⁺, and Cl^- concentrations first decreased and then increased. When the salinity decreased from 30 to 14‰, hemoglobin, soluble total protein, taurine, and total free amino acid gradually increased; maximum levels of hemoglobin, soluble total protein, and taurine occurred once the salinity increased to 22‰ at 96 h. After 96 h, the total free amino acid content increased until 144 h. The reactive oxygen species (ROS) content and total antioxidant capacity (T-AOC) peaked at 96 h, whereas the expression levels of Mn-superoxide dismutase (MnSOD) and catalase (CAT) increased earlier, indicating that, with continuous ROS generation, antioxidant defense mechanisms were activated to avoid oxidative damage. Expression levels of cathepsin C (CTSC), cathepsin D (CTSD), heat shock protein 20 (HSP20), and heat shock protein 70 (HSP70) were significantly higher than in the control group at 48 h (salinity level 14‰); the expression levels of HSP20, heat shock protein 90 (HSP90), MnSOD, and glutathione peroxidase (GP _x ) remained high, indicating that they were still required for osmotic pressure regulation to maintain the dynamic balance between the generation and removal of ROS as the salinity level increased. These results not only add to our basic understanding of the aquatic ecology of S. subcrenata, but also provide a theoretical ground for improving the survival rate of bottom-sowing, propagation, and release of S. subcrenata seedlings.

Keywords: Scapharca subcrenata; antioxidant defense; cathepsin; heat shock proteins; osmotic pressure; reactive oxygen species; salinity.

FIGURE 1

Diagram of the salinity change mode used in the experiment. First, salinity decreased gradually from 30 to 14 psu at a rate of 8 psu every 24 h (first stage), and then was maintained at low levels for 24 h (second stage). After 72-h stress, salinity increased gradually to 30 psu at a rate of 8 psu 24 h^–1 (third stage) and was then maintained at 30 psu for 24 h (fourth stage).

FIGURE 2

Effects of salinity changes on the survival rate of ark shell (Scapharca subcrenata). Values are expressed as mean ± SE (n = 4). Statistical analysis was performed by one-way analysis of variance (ANOVA) followed by Tukey’s test, using SPSS version 18.0. Means with the different lower case letters are significantly different at P < 0.05 level.

FIGURE 3

Effects of salinity changes on the osmotic pressure of hemolymph in ark shell (Scapharca subcrenata). Values are expressed as mean ± SE (n = 4). Statistical analysis was performed by one-way analysis of variance (ANOVA) followed by Tukey’s test, using SPSS version 18.0. Means with different lower case letters are significantly different at P < 0.05 level. Asterisks indicate significant differences between salinity change treatments and control treatment for the same time points, P < 0.05.

FIGURE 4

Effects of salinity changes on the Na⁺, K⁺, Ca²⁺, and Cl^– concentration of hemolymph in ark shell (Scapharca subcrenata). Values are expressed as mean ± SE (n = 4). Statistical analysis was performed by one-way analysis of variance (ANOVA) followed by Tukey’s test, using SPSS version 18.0. Means with the different lower case letters are significantly different at P < 0.05 level. Asterisks indicate significant differences between salinity change treatments and control treatment for the same time points, P < 0.05.

FIGURE 5

Effects of salinity changes on the activity of Na⁺/K⁺-ATPase, content of reactive oxygen species, and total antioxidant capacity in ark shell (Scapharca subcrenata). Values are expressed as mean ± SE (n = 4). Statistical analysis was performed by one-way analysis of variance (ANOVA) followed by Tukey’s test, using SPSS version 18.0. Means with different lower case letters are significantly different at P < 0.05 level. Asterisks indicate significant differences between salinity change treatments and control treatment for the same time points, P < 0.05.

FIGURE 6

Relative mRNA expression levels of antioxidant enzyme and heat shock proteins’ genes in the gills of ark shell (Scapharca subcrenata). Values are expressed as mean ± SE (n = 4). Statistical analysis was performed by one-way analysis of variance (ANOVA) followed by Tukey’s test, using SPSS version 18.0. Means with different lower case letters are significantly different at P < 0.05 level. Asterisks indicate significant differences between salinity change treatments and control treatment for the same time points, P < 0.05.

FIGURE 7

Schematic overview of the osmotic pressure and immunoregulation mechanism of Scapharca subcrenata in response to a sudden drop in salinity.