Morphological Plasticity in Estuarine Fish

Abstract

Estuarine fish inhabit one of the most physically dynamic environments on Earth, experiencing salinity gradients from 0.5 to 35 ppt, daily tidal fluctuations in water level and current velocity, turbidity variation across orders of magnitude, and temperature ranges exceeding 20°C seasonally. This extreme environmental variability is hypothesised to drive elevated morphological plasticity in estuarine fish relative to strictly marine or freshwater congeners, yet quantitative cross-species comparisons of plasticity magnitude across estuarine gradients remain scarce. This study quantified morphological plasticity in six paired estuarine-marine species complexes using geometric morphometrics (landmark-based; 22 landmarks; n = 2,842 individuals) across salinity, turbidity, and tidal exposure gradients in four European estuaries (Tagus, Po, Rhone, Vistula). Procrustes ANOVA confirmed that species identity explained the largest proportion of shape variance (R2 = 0.48), followed by salinity zone (R2 = 0.22), turbidity class (R2 = 0.12), and their interaction (R2 = 0.08). Estuarine morphs showed consistently deeper bodies, more anterior dorsal fin insertion, and larger eyes relative to marine conspecifics across all six species pairs (all pairwise Procrustes distances p < 0.001). Reaction norm analysis confirmed that 58.4 ± 8.4% of within-species shape variance was attributable to phenotypic plasticity rather than genetic divergence, as assessed by common garden rearing experiments in four species. Condition factor (K = W/L3 x 100) was significantly higher in estuarine morphs (K = 1.84 ± 0.28) than marine morphs (K = 1.42 ± 0.22) of the same species, consistent with enhanced energy storage as a buffer against environmental unpredictability. These results quantify the morphological plasticity advantage of estuarine fish assemblages and provide a geometric morphometrics baseline for monitoring morphological change under estuarine habitat degradation and climate-driven salinity regime shifts.