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

Organisms living in highly variable environments are expected to exhibit a strong phenotypic variability within populations due to plastic responses to environmental fluctuations. Yet, this phenomenon is not commonly seen in nature, since compensatory mechanisms can promote phenotypic stability. Here, we hypothesized a potential compensatory effect that could result from the interaction between behavioural and morphological trait plasticity. These traits are highly responsive to environmental factors, and compelling research has demonstrated that behavioural plasticity can impact individual life history traits. In microcosm experiments, we analysed plasticity in body size, larval and pupal behaviour and carry-over effect across developmental stages in the sea rock-pool mosquito Aedes mariae in response to water salinity changes. Analyzing morphological traits, we found that larvae raised under increasing salinity were smaller than those raised under constant conditions. Additionally, a smaller body size at the pupal stage was observed, which indicated the presence of carry-over effects from the larval to pupal stages. However, no differences were observed in adult sizes between the two salinity conditions. When examining behavioural traits, we observed that higher salinity promoted plastic changes in larval activity behaviour and pupal diving behaviour. In particular, pupae exposed to increased salinity spent 20.6% less time underwater and performed fewer abdominal contractions than those under constant conditions. Since pupal energy expenditure is proportional to the time spent underwater, we suggest that the plastic pupal behaviour promoted compensatory energy allocation to growth, thereby reducing the carry-over effects from the pupal to the adult stage. This study emphasizes that plasticity at multiple traits during development can impact carry-over effects, ultimately leading to the convergence of adult phenotypes.