Phenotypic plasticity shapes carry-over effects in sea rock-pool mosquitoes.

Environmental conditions 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 affect multiple traits whose interaction might shape developmental trajectories. Here, we tested whether trait interactions can shape direct carry-over effects, examining the interplay between behavioural and morphological plasticity in the mosquito Aedes mariae under varying salinity conditions. We found that higher salinity caused a reduction in larval body size and an increase in resting behaviour at the water surface, at the expense of browsing activity. Furthermore, we found that larval body size was positively correlated with pupal size under constant conditions, indicating a direct carry-over effect. However, this relationship was disrupted as salinity increased, due to different behavioural response according to larval body size, 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 debate on the extent to which one life stage is coupled to the others across the metamorphic boundary.