Brackish Beginnings: Rethinking the Role of Salinity in Shaping Mosquito Proboscis Morphology and Disease Risk

Abstract Climate change driven saltwater intrusion is expanding coastal brackish water habitats, promoting the proliferation of salinity-tolerant mosquitoes such as Culex sitiens , a vector of lymphatic filariasis and Japanese encephalitis. This study investigated whether environmental salinity affects mosquito morphology, specifically proboscis length, a trait of ecological significance related to feeding efficiency and vector competence. Late fourth-instar larvae and pupae of Cx. sitiens were collected from habitats with varying salinity levels in the Muttukadu Backwater, Tamil Nadu, India, and reared under controlled conditions using habitat specific water. Adult female body and proboscis lengths were measured, and water quality parameters were analysed to characterize environmental variation. Statistical analyses (Welch’s ANOVA, regression, and ANCOVA) revealed a significant positive relationship between salinity and body length (R 2 = 0.94, p=0.0003) as well as with proboscis length (R 2 = 0.90, p=0.001). Additionally, ANCOVA indicated that the proboscis elongation remained significant after adjusting for body length (F□, □□□ = 32.36, p < 0.001, partial η 2 = 0.257). This confirmed that the salinity exerts an independent effect on this morphological trait. These findings provide the first field-based evidence that the environmental salinity drives proboscis elongation in Cx. sitiens , indicating an adaptive response under saline stress. This may have implications for disease transmission in climate-affected coastal regions.