Life-history and thermal tolerance traits display different thermal plasticities and relationships with temperature in the marine polychaete Ophryotrocha labronica La Greca and Bacci (Dorvilleidae)

In order to gain insights into links between fitness and physiological function, the phenotypic plasticity of life-history traits and thermal tolerances has been investigated in a laboratory strain of the polychaete Ophryotrocha labronica acclimated to a broad range of temperatures. Reproductive pairs were kept in isolation for two months at one of six temperatures (10, 15, 20, 25, 30 and 35 °C), and survival and a number of life-history traits recorded. Tolerance to heat and cold was assessed in individuals that survived the acclimation period by measuring upper and lower thermal limits. Life-history and thermal tolerance traits showed different relationships with temperature: generally non-monotonic for life-history and monotonic for thermal limits. The thermal history of the strain used in the study seemed to be partly responsible for this pattern, affecting the thermal responses of most life-history traits, but not those of the upper and lower thermal limits. Survival correlated positively with tolerance to cold but not with heat tolerance. This result supports the idea that tolerance to heat is under stronger selection in O. labronica, resulting in a lower level of plasticity for this trait. Some life-history features positively co-varied with tolerance to heat (e.g. body size), whilst in other cases reproductive traits showed apparent trade-offs with thermal limits. In all cases correlations amongst traits were apparently driven by their relationship with temperature. Even if we found no evolutionary trade-offs among the thermal responses of life-history traits and thermal limits, the strategies evolved by organisms to cope with thermal variability result from the mutual interaction of these traits, which plays a key role in defining organisms' responses to temperature changes and fluctuations.