Elevations of somatolactin in plasma and pituitaries and increased alpha-MSH cell activity in red drum exposed to black background and decreased illumination.

The effects of different backgrounds and illumination levels on somatolactin (SL) concentrations in juvenile red drum (Sciaenops ocellatus) plasma and pituitaries were investigated using a homologous radioimmunoassay. Both plasma and pituitary SL concentrations were significantly higher in red drum exposed for 1 week to black backgrounds than in fish exposed to light backgrounds. Plasma SL levels decreased from 5.39 +/- 1.18 ng/ml in red drum adapted to a dark-blue background to 0.73 +/- 0.05 ng/ml within 1 hr of transfer to light-background tanks. Plasma SL concentrations were significantly elevated in fish 3 hr after transfer from light-background tanks (0.41-0.99 ng/ml) to black-background tanks (5.38 +/- 1.60 ng/ml) or black-background tanks without illumination (3.54 +/- 1.34 ng/ml). Maximum circulating SL levels (9.47 +/- 0.78 ng/ml) were observed 1 day after transfer of red drum to a black-background tank without illumination. Fish exposed to black backgrounds had the darkest body coloration, the greatest dispersion of melanin in their melanophore cells, and the most numerous and active alpha-melanophore-stimulating hormone (alpha-MSH) cells. These results provide evidence that SL may be involved in adaptation of red drum to different backgrounds and illuminations. However, the relative importance of SL and alpha-MSH in the regulation of this process remains to be elucidated.