Thermal effect on energetic physiology in the ark shell Scapharca subcrenata

Abstract

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.