Effects of temperature and body size on the physiological energetics of the stalked sea squirt Styela clava

•The effect of temperature on physiological processes of Styela clava was measured.•Physiological rates were higher in the larger individuals regardless of temperature.•The SFG was positive with relatively constant net growth efficiency at 5–15 °C.•Increased metabolic costs resulted in lowered SFG and growth efficiency at 20–25 °C.•The absence of compensatory adjustment to warm condition causes energetic disruption.

An assessment of the patterns of thermal acclimation of physiological processes of the stalked sea squirt Styela clava is needed to further understand growth patterns associated with large seasonal fluctuations in ambient temperature. The effects of different exposure temperatures on physiological processes of S. clava were measured at 5, 10, 15, 20, and 25 °C. The impact of temperature on energy balance at a whole organism level was then assessed by scope for growth (SFG) and net growth efficiency (K2) measures. Physiological rates of S. clava were well correlated with its dry tissue weight (DW). Positive allometry with DW indicated that all physiological rates investigated were generally higher in the larger individuals regardless of temperature. Some allometric equations (e.g., feces production rate vs. DW and respiration rate vs. DW) exhibited identical estimates of exponents, enabling accurate comparisons of the rates across temperatures. In contrast, disparities across temperatures of the values for the weight exponent were detected in other allometric equations, revealing that thermal effects on these rates have different degrees for large and small individuals. The SFG value in S. clava of different sizes was similar at lower temperatures (5–10 °C), peaking at 15 °C. The SFG was positive with a relatively constant K2 in this temperature range. This positive energy balance reflects the reduction in metabolic costs at lower temperatures. In contrast, increased metabolic costs at higher temperatures (20–25 °C) resulted in lowered SFG and K2 values, indicating that the feeding rate did not increase as greatly in compensation for the increased metabolic costs at higher temperatures. The lowered absorption at 25 °C (compared with 20 °C) resulted in negative SFG and K2 values. Finally, our results confirm that the absence of compensatory adjustment to warmer conditions leads to energetic disruption at a whole organism level at such temperatures.