Adaptation to water salinity changes: phenotypic plasticity meets and moulds carry-over effects in sea rock-pool mosquitoes

Environmental conditions experienced during early-life can shape trait expression after metamorphosis. Direct carry-over effects occur when the value of a trait expressed at an early stage directly determines its expression at later stages, maintaining phenotypic continuity across metamorphosis. However, environmental factors generally affect multiple traits whose interaction might shape developmental trajectories. Here, we tested the hypothesis that trait interactions can shape direct carry-over effects by examining the interplay between behavioural and morphological plasticity in the sea rock-pool mosquito Aedes mariae under varying salinity conditions. We found plastic changes in body size and behaviour. Higher salinity reduced larval body size and increased resting behaviour at the water surface while browsing activity decreased. Under constant conditions, larval body size was positively correlated with pupal size, indicating a direct carry-over effect. However, this relationship was disrupted as salinity increased due to larval size-dependent behavioural plasticity which decouples pupal morphology from larval size. Our results show that environmental conditions modulate trait integration and modify direct carry-over effects. These findings highlight the importance of considering multiple traits when studying developmental plasticity and contribute to the ongoing debate on the extent to which one life stage is coupled to the others across metamorphic boundary.