Physiological and Transcriptional Responses to Acute and Chronic Thermal Stress in the Ark Shell Scapharca subcrenata

Ark shells (Scapharca subcrenata) grown on the tidal flats are often exposed to high temperature stresses in summer. In order to better understand their adaption to extreme or natural high temperature, we first determined the 96-h upper lethal temperature of ark shell and then investigated their physiological and transcriptional responses to acute or chronic thermal stress at the 96-h upper median lethal temperature (32°C). A significantly higher cumulative mortality (52% in 96 h) was observed in the acute heating treatment (AHT) group than that (22% in 7 days) in the chronic heating treatment (CHT) group. The apoptosis and necrosis rates of hemocytes were increased significantly in a time-dependent manner under both thermal stress strategies. Activities of antioxidant enzymes [superoxide dismutase (SOD) and catalase (CAT)] increased dramatically in a short time followed by a quick decline and reached to a lower level within 12 h in the AHT group, but maintain relatively high levels over a long period in the CHT group. The contents of malondialdehyde (MDA) were increased significantly firstly and restored to the original later in both acute and chronic thermal stress. Moreover, expression of the genes related to heat shock proteins (HSPs; HSP90, HSP70, HSP20, and sHSP), apoptosis [TNF receptor-associated factor 6 (TRAF6), glucose regulated protein 78 kD (GRP78), and caspase-3 (Casp-3)] and antioxidant responses [glutathione S-transferase (GST) and multidrug resistance protein (MRP)] could be induced and up-regulated significantly by thermal stress, however, expression of regucalcin (RGN), metallothionein (MT), and peroxiredoxin (PRX) was down-regulated dramatically under the two heating treatments. These results suggested that anti-apoptotic system, antioxidant defense system and HSPs could play important roles in thermal tolerance of ark shells, and the heat-resistant ark shell strains could be selected continuously by properly chronic thermal stress.

Ameliorative effects of ark clams (Scapharca subcrenata and Tegillarca granosa) on endothelial dysfunction induced by a high-fat diet

Endothelial dysfunction is directly involved in consequence of various metabolic syndromes such as diabetes and hypertension. In this study, we investigated the preventive effects of two ark clams [ark shell (AS, Scapharca subcrenata) and granular ark (GA, Tegillarca granosa)] on endothelial dysfunction induced by a high-fat diet. Wistar rats were divided into four groups as follows: control (normal diet), HF (high-fat diet), AS (high-fat diet + 5% AS powder), and GA (high-fat diet + 5% GA powder) for 12 weeks. AS and GA diets enhanced vascular reactivity of the rat thoracic aorta and significantly increased expression levels of vascular relaxation-related proteins (p-Akt-ser473 and p-eNOS-ser1177). Ark clam supplement reduced endothelin-1 expression level, as compared to the HF group. Additionally, AS and GA showed a trend of improving insulin sensitivity compared to HF. Our results suggest that AS and GA enhance vascular reactivity and ameliorated endothelial dysfunction induced by a high-fat diet.

Thermal effect on energetic physiology in the ark shell Scapharca subcrenata

Background: Clams inhabiting temperate coastal zones are affected not only by seasonal thermal variation, but also by changes in the prevailing thermal regime of their habitats. Understanding the physiological processes required for adjusting the energy balance of the ark shell Scapharca subcrenata to varying thermal conditions is pivotal for predicting its growth and further phenology, and ultimately promoting successful aquaculture activity. Thermal effects on the physiological processes and the combined energetic physiology at the organism level of S. subcrenata were assessed over a temperature range corresponding to field conditions (3–28 °C). Results: Physiological rates of S. subcrenata were well correlated with its dry tissue weight, formalizing allometric relationships. Extremely low weight exponent values for filtration rate and metabolic rate were detected at lower (3–8 °C) compared to higher (8–28 °C) temperatures. In addition to marked reductions at 3 °C, weight exponents were identical and intercept estimates increased progressively with rising temperature over the temperature range (8–28 °C). Identical weight exponents and increasing intercept estimates for both feces production and excretion rates across the experimental temperatures indicated that energy losses by egestion and excretion increased gradually with rising temperature. Scope for growth and net growth efficiency showed relatively constant and positive values at 8–23 °C, suggesting an optimal temperature range for production, but dropped drastically to negative values at 3 and 28 °C, indicating thermal (both cold and heat) stress. The Q10 values revealed that the metabolic and filtration rates are more sensitive at 23–28 and 3–8 °C, respectively. Conclusions: Allometric size-scaling of physiological rates in S. subcrenata highlights species-specific responses to changes in temperature. The observed weight exponents and intercept estimates for filtration and metabolic rates reveal the variation of the thermal effects according to size as well as an incapability of acclimation to varying temperatures. Reversed thermal sensitivities in both components confirm that energy acquisition by feeding does not offset the metabolic energy cost outside the optimal thermal range. Our empirical analysis allowed further understanding of the seasonal energy dynamism and biological cycle of S. subcrenata in temperate habitats subject to highly variable thermal regimes.