Natural habitats contain dynamic elements, such as varying local illumination: can such features mitigate the salience of organism movement? Dynamic illumination is particularly prevalent in coral reefs, where reticulate patterns known as ‘water caustics’ play chaotically in the shallows. In behavioural experiments with a wild-caught reef fish, the Picasso triggerfish (Rhinecanthus aculeatus), we demonstrate that the presence of dynamic water caustics negatively affects the detection of moving prey items, as measured by attack latency. Manipulating two features of water caustic form (sharpness and scale) implies that the masking effect should be most effective in shallow water. Due to the direct impact upon foraging efficiency, we expect the presence of water caustics to influence decisions about habitat choice and foraging by wild prey and predators.