Transitory colour-change mechanism in a fresh-water teleost, Clarias batrachus (L).

The rate of colour change in the fish Clarias batrachus (L.) has been studied as a response from black to white background and vice versa. The 'Derived Ostwald Index (D.O.I.)' method proposed by Healey (1967) has been used to measure the melanophore responses of the fish. Observations show that the fish is quite sensitive to background responses. The rate of colour change is fast. The colour change is quite rapid in both, the paling and the darkening experiments in the beginning as compared to the later stages which are slow and gradual. The results indicate that the process of colour change is mainly under nervous control in the initial stages while in the later stages the chromatic response seems to be regulated mainly by hormone(s). The fish is better adapted on a white background (maximum paling 1.7 grades of D.O.I. scale in 10 hours) than on a black background (maximum darkening 6.35 grades of D.O.I. scale in 20 hours). It is suggested that chromatic control mechanisms in the fish include the sympathetic pigment-aggregating fibres constituting the nervous control and at least a pituitary principle, the melanophore-aggregating hormone (MAH) constituting the hormonal control.