Phenotypic Plasticity Drives Seasonal Thermal Tolerance in a Baltic Copepod

Seasonal changes in environmental conditions require substantial physiological responses for population persistence. Phenotypic plasticity is a common mechanism to tolerate these changes, but for organisms with short generation times rapid adaptation may also be a contributing factor. Here, we aimed to disentangle the impacts of adaptation from phenotypic plasticity on thermal tolerance of the calanoid copepod Acartia hudsonica collected throughout spring and summer of a single year. We used a common garden (11 {degrees}C and 18 {degrees}C) design to determine the relative impacts of plasticity versus adaptation. Acartia hudsonica were collected from five time points across the season and thermal tolerance was determined using critical thermal maximum (CTmax) followed by additional measurements after one generation of common garden. As sea surface temperature increased through the season, field collected individuals showed corresponding increases in thermal tolerance but decreases in body size. Despite different thermal tolerances of wild collections, common garden animals did not differ in CTmax within thermal treatments. Instead, there was evidence of phenotypic plasticity where higher temperatures were tolerated by the 18 {degrees}C versus the 11 {degrees}C treatment animals across all collections. Acclimation also had significant effects on body size, with higher temperatures resulting in smaller individuals, consistent with the temperature size rule. Therefore, the differences in thermal tolerance and body size observed in field collected A. hudsonica were likely driven by plasticity rather than adaptation. However, the observed decrease in body size suggests that nutrient availability and ecosystem functioning could be impacted if temperatures consistently increase with no change in copepod abundance. This is the first record of A. hudsonica in the Baltic Sea known to the authors. O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=113 SRC="FIGDIR/small/551281v1_ufig1.gif" ALT="Figure 1"> View larger version (33K): org.highwire.dtl.DTLVardef@814a70org.highwire.dtl.DTLVardef@3e17f0org.highwire.dtl.DTLVardef@95cb3corg.highwire.dtl.DTLVardef@51eb1c_HPS_FORMAT_FIGEXP M_FIG Graphical Abstract C_FIG HighlightsPhenotypic Plasticity Drives Seasonal Thermal Tolerance in a Baltic Copepod Alexandra Hahn, Reid S. Brennan O_LIAcartia hudsonica shows strong seasonality in thermal tolerance. C_LIO_LIThe observed seasonal differences in CTmax are driven by phenotypic plasticity not adaptation. C_LIO_LIBody size in A. hudsonica is negatively correlated to environmental and developmental temperature. C_LIO_LIThis is the first record of A. hudsonica in the Baltic Sea known to the authors. C_LI