Differential eco-physiological performances of two pseudocryptic species of the Eurytemora affinis complex (Copepoda, Calanoida) in the St. Lawrence estuarine transition zone: a reciprocal transplant experiment

Abstract

The goal of this study was to evaluate and compare the short-term performances and the physiological plasticity of two cryptic species Eurytemora carolleeae and Eurytemora affinis (North-Atlantic clade) by simulating rapid advection from freshwater to brackish water conditions and reciprocally. To do so, two reciprocal transplant experiments without acclimation and under non-limited food condition were performed in the St. Lawrence estuarine transition zone during summer 2011. Results revealed that both species diverged in their short-term acclimation response when facing acute salinity changes that they might encounter when advected through the highly dynamic estuarine transition zone. We show that E. carolleeae could use the brackish environment without loss of performance and energy, while E. affinis needed to reallocate energy from other processes (i.e., reproduction) and required food intake to maintain itself in the freshwater environment. In addition, the transplant experiment highlighted that only 40% of the E. affinis showed short term capacity to acclimate to freshwater conditions, indicating that in situ advection by currents from brackish water to fresh water could be dramatic even for a short time period. Furthermore, the survivors of E. affinis in fresh water might not be able to reproduce, which limits establishment of a sustainable population of E. affinis (North-Atlantic clade) in the tidal freshwater part of the estuarine transition zone. Finally, we highlighted for the first time that both species of this pseudocryptic complex could use lipid remodelling to overcome temperature effects on membrane structure, but further studies are needed to determine the influence of membrane lipid remodelling on salinity tolerance.