When flatfishes are cultured at high-density in an artificial facility, the bright background color and the
carpeted bottom substratum (sand or gravel) help significantly to enhance growth and prevent blindside hypermelanosis. However, how the bright background and carpeted gravel enhance growth and
prevent the hypermelanosis is unknown. Thus, we performed three experiments using the olive flounder, Paralichthys olivaceus. In the first, the acupressure ofgravel to blind skin and the burrowing behavior
of flounder were assessed to determine which eventually suppressed hypermelanosis. In the second
experiment, we assessed whether the color of the carpeted gravel affected flounder hypermelanosis and
growth. In the final experiment, we assessed whether hypermelanosis offlounder reared on a gravel bottom was continuously inhibited after removal of the gravel substratum, and whether tank color affected
the hypermelanosis and fish growth. The results showed that the burrowing of flounder into gravel was
more important than the stimulus of rough gravel on blind-skin to prevent blind-side hypermelanosis.
The color ofthe gravel and tanks did not affect hypermelanosis. Removal ofthe gravel substratum resulted
in enhancement of blind-side hypermelanosis. However, fish growth was enhanced by a bright-colored
tank but not a bright-colored bottom substratum. In conclusion, these results confirm that the absence
of shelter in which flounder can burrow or hide induces blind-side hypermelanosis, and that flounder
consistently burrow beneath the substratum to prevent malpigmentation. Additionally, our results show
that a bright-colored tank resulted in increased growth, indicating that flounder should be reared in a
bright-background to enhance growth