The observations on the fascinating aspect of animal behaviour dealing with physiological colour changes in the fish as a result of adaptation to a lighter and a darker background has been taken up to understand the mechanisms relating to this aspect in a nocturnal African catfish, Clarias geriepinus. The rate of colour change were studied in Clarias geriepinus (Burchell, 1822). The studies in Clarias geriepinus include visual background related transitory colour changes, the nonvisual chromatic responses in the blinded fish on exposure to light and darkness. The data obtained with regard to the rate of colour changes over two contrasting backgrounds i.e., a lighter (white) and a darker (black) background do point that when the fish are subjected to background reversal (i.e., a black adapted fish is transferred to a white background and vice-versa), they change their colour initially at a faster speed for 10 min. which is followed by a slow and gradual colour change at a later stage i.e., the colour change accomplished is biphasic. Apparently, the nervous system of fish has evolved to allow faster chromatic adaptation than can be achieved via the hormonal system. The rate of colour changes as a result of adaptation to different backgrounds i.e., white and black do point for a biphasic control, where they appear to be initiated by neural control and completed by simultaneous and synergestic hormonal control in the fish.
