- In terms of visual perception, coral reefs are structurally complex habitats. Therefore, visual stimuli that invoke territorial behavior in fish, causing them to respond to potential intruders and competitors, may be fragmented. Amodal completion was recently shown in a fish species. Here, we presented a mirror covered by occluders, with different squared openings arranged in a vertical or a horizontal broken line or a checkerboard formation, within their natural territories in the reef. Single small openings in the occluders did not trigger an attack on the reflected image, while specific configurations of them did so, suggesting that fish recognized an intruder in the fragmented image. The results suggest the use of amodal completion by fish although other possibilities cannot be rejected.