**Background:** The visual systems of fish are adapted to their light environments over evolutionary time, but can also show plasticity within an individual's lifetime in response to regular changes (e.g., ontogenetic, seasonal). However, less is known about responses to irregular, short-term changes such as algal blooms or light pollution. This study investigated whether the convict surgeonfish (Acanthurus triostegus), a diurnal reef fish, exhibits phenotypic plasticity in its retina at both molecular (opsin gene expression) and cellular (retinal cell densities) levels when exposed to altered light conditions, and whether such plasticity occurs across life stages (juveniles and adults).
**Methods:** Juvenile and adult A. triostegus were collected from Moorea (French Polynesia) and Lizard Island (Australia), respectively. Fish were exposed to five light treatments for juveniles (12L12D outdoor control, 12L12D indoor control, 12L12AL simulating light pollution, 24L constant bright light, 24D constant dim light) and three for adults (12L12D indoor control, 24D, and a rescue treatment where fish were returned to control after 6 days of 24D). Opsin gene expression was quantified using real-time qRT-PCR normalized to two housekeeping genes (actb and elf1a) and also as proportional expression. Retinal cell densities (rods, cones, inner nuclear layer [INL] cells, ganglion cells [GC]) were assessed histologically in juveniles. Statistical analyses used nonparametric tests (Mann-Whitney, Kruskal-Wallis with Dunn's post-hoc).
**Key Results:** Juveniles showed the most pronounced changes under constant dim light (24D): significant increases in absolute expression of cone opsin genes sws2a (p<0.05), rh2a (p<0.01), rh2c (p<0.001), and lws (p<0.01) at day 6, and decreased cone densities in both dorsal (p<0.01) and ventral (p<0.0001) retina, as well as decreased INL and GC densities in dorsal retina (p<0.05). Under constant bright light (24L), juveniles increased absolute expression of sws2a (p<0.001) and lws (p<0.0001) and proportional expression of sws2a, rh2a, and lws (p<0.01). Under 12L12AL, sws2a increased (p<0.05) and sws2b decreased (p<0.01) in absolute expression. Adults under 24D showed trends toward increased absolute expression of sws2a, rh2c, and lws, and significant increases in proportional expression of sws2a (p<0.05) and rh2c (p<0.05). The rescue experiment demonstrated that shifts in opsin expression reverted to control levels within 24 hours, with rh2c proportional expression significantly different between 24D and rescue (p<0.05). No changes in rod densities or rh1 expression were observed. Survival was 100% in all treatments.
**Clinical Implications:** This study provides evidence that reef fish can rapidly (within days) and reversibly adjust their visual system at both molecular and cellular levels in response to extreme light changes, particularly dim light. The plasticity is more pronounced in juveniles than adults, suggesting developmental stage influences adaptive capacity. These findings have ecological relevance for understanding how fish may cope with natural (e.g., overcast days, seasonal changes) and anthropogenic (e.g., light pollution) alterations to their light environment. The rapid reversibility of changes is promising for recovery after environmental restoration. However, the study was conducted in controlled aquarium settings, and responses may differ in natural reef conditions. Further research is needed to explore mechanisms (e.g., thyroid hormone signaling, transcription factors) and long-term effects.