Functional changes of the visual system of the damselfish Dascyllus marginatus along its bathymetric range

Shallow-water zooplanktivorous fish rely on their vision for foraging. In shallow water, feeding efficiency decreases in dim light and thus the fish cease foraging at crepuscular hours. Creatures living in the lower parts of their depth ranges are expected to be exposed to limited light levels for longer hours. However, observations of the zooplanktivore Dascyllus marginatus showed little change in foraging duration down to 40 m deep. We asked whether the visual system's functionality changes with depth along the depth range of this damselfish; we examined eye and retina anatomy for changes in visual acuity and light sensitivity and used the optomotor response to test for spatial and temporal light summation. We found only minor changes in the anatomy of the eye that are not expected to affect visual sensitivity or acuity. However, behavioural experiments showed that the deeper water fish's test performance exceeded those of fish in shallow water under lower light levels. We found that deeper water fish responded to the optomotor test at lower light levels and also had more discriminating visual acuity in low light, which can increase their potential reactive distance. The plastic adaptive ability of the visual system to low light levels may explain the fish's ability to inhabit deeper reef habitats and thus expand their depth range limits.

Research Highlights
► We asked whether and how the visual system of a coral reef fish adapts to low light.
► Regardless of lower light levels, fish at 40 m have similar foraging hours to fish at 4 m.
► Eye and retina anatomy is essentially similar along 40 m depth gradient.
► Under low light, deeper fish have higher behavioural visual acuity.
► The visual system plasticity may explain the ability to inhabit deeper habitats.