The background adaptation of the flatfish, Paralichthys olivaceus

The background adaptation of the flatfish, Paralichthys olivaceus, was studied by sliding beneath the fish a long strip of plastic sheet with various patterns to serve as a background. The body shade and pattern differed depending upon the pattern used. In general, the fish showed maximal pallor on a white background and darkened when lying on a black background. With a pattern of large checks, several black patches became conspicuous. The fish showed the same tendency in terms of response even when the left eye was covered. However, the fish was prevented from manifesting these changes when the right eye was covered. The results suggest that the left eye of the fish does not see the immediate background. This conclusion is supported by an anatomical study.