Spot the difference: Turbidity-driven plasticity and variation in spotted grunter (Pomadasys commersonnii) spot patterns.

Animal body markings such as spots, stripes and patterns play important roles in camouflage and visual communication. This study investigated phenotypic plasticity in the spot patterns of spotted grunter Pomadasys commersonnii, an estuarine-dependent haemulid species. First, an aquarium experiment (n = 24) examined plasticity in spot patterns under contrasting turbidity conditions. P. commersonnii reduced their spot size rapidly and reversibly in response to elevated turbidity, with spots returning close to baseline once clear less turbid conditions were restored, whereas the number and position of spots remained constant throughout the experiment, suggesting that spot patterns may be useful for short-term individual identification. Second, spot patterns of P. commersonnii (n = 139) from two estuaries with contrasting turbidities were compared. After accounting for body size, fish from the less turbid Breede Estuary had more numerous spots than those from the turbid Kei Estuary and accumulated spots more rapidly with growth, whereas spot size did not differ between estuaries. Spots from the Kei Estuary were, however, more elongated and differently orientated and individuals could be assigned to their estuary of origin with 83.5% accuracy on the bases of spot patterning alone. These findings are consistent with the sensory drive hypothesis, whereby individuals adapt their communication systems to local environmental conditions and indicates that spot size responds plastically to current turbidity while spot number reflected a longer-term, developmentally accumulated difference between estuaries. Although the precise function of these spots remains unknown, environmental variables such as turbidity evidently play an important role in the phenotypic variation of P. commersonnii.