Reduction in thermal stress of marine copepods after physiological acclimation

Abstract

We studied the phenotypic response to temperature of the marine copepod Paracartia grani at the organismal and cellular levels. First, the acute (2 days) survival, feeding and reproductive performances at 6-35°C were determined. Survival was very high up to ca. 30°C and then dropped, whereas feeding and fecundity peaked at 23-27°C. An acclimation response developed after longer exposures (7 days), resulting in a decline of the biological rate processes. As a consequence, Q₁₀ coefficients dropped from 2.6 to 1.6, and from 2.7 to 1.7 for ingestion and egg production, respectively. Due to the similarity in feeding and egg production thermal responses, gross-growth efficiencies did not vary with temperature. Respiration rates were less sensitive (lower Q₁₀) and showed an opposite pattern, probably influenced by starvation during the incubations. The acclimation response observed in the organismal rate processes was accompanied by changes in body stoichiometry and in the antioxidant defense and cell-repair mechanisms. Predictions of direct effects of temperature on copepod performance should consider the reduction of Q₁₀ coefficients due to the acclimation response. Copepod population dynamic models often use high Q₁₀ values and may overestimate thermal effects.

Keywords: Paracartia grani; Q10; oxidative stress; temperature; thermal performance curves.

Fig. 1

Thermal response of P. grani exposed for 2 days to temperatures between 5 and 35°C. (a) Survival in starvation, (b) survival in saturated food conditions, (c) egg production rate and (d) fecal pellet production rate. Egg and fecal pellet data were obtained only during the second 24-h period in the saturated food treatment. Thermal performance curves in (c) and (d) were fitted to a double-exponential model (see text). Error bars are SE.

Fig. 2

Survival (a), egg production rate (b) and fecal pellet production rate (c) of P. grani exposed for 7 days to temperatures between 10 and 28°C under saturated food conditions. “Short term” refers to the first 2 days of exposure, whereas “medium term” refers to the whole 7-day exposure period (see text for further details). Thermal performance curves in (b) and (c) were fitted to a double-exponential model (see text). Error bars are SE.

Fig. 3

Feeding (a) and egg production (b) rates of P. grani after short-term and medium-term exposures at 16, 19, 22 and 25°C. Lines correspond to exponential fits. Error bars are SE.

Fig. 4

Respiration rate of P. grani after short- and medium-term exposures at 16, 19, 22 and 25°C. (a) First trial, concurrent with feeding and egg production experiments; (b) additional experiment with a different copepod cohort. Lines correspond to exponential fits. Error bars are SE.

Fig. 5

Arrhenius plots of carbon-specific ingestion rates (a), carbon-specific egg production rates (b), and carbon gross-growth efficiencies (c) of P. grani after short- and medium-term exposures at 16, 19, 22 and 25°C. Lines correspond to linear regression fits.

Fig. 6

Arrhenius plots of P. grani respiratory carbon losses after short- and medium-term exposures at 16, 19, 22 and 25°C. (a) Incubations concurrent to experiments showed in Fig. 5; (b) additional respiration experiment (see text for further details). Lines correspond to linear regression fits.

Fig. 7

Body stoichiometric ratios of P. grani after short- and medium-term exposures at 16, 19, 22 and 25°C. (a) C:N, (b) C:P, and (c) N:P. Notice that these samples were taken at the start of the physiological incubations, and hence the actual exposure was 1 day shorter than for rate processes. Error bars are SE. Lines correspond to linear regression fits. n.s.: not significant.

Fig. 8

Enzymatic biomarkers and LPO levels of P. grani after short- and medium-term exposures at 16, 19, 22 and 25°C. Notice that these samples were taken at the start of the physiological incubations, and hence the actual exposure was 1 day shorter than for rate processes. Error bars are SE.