During the spawning migration, the overall color of the sea lamprey (Petromyzon marinus L.) becomes progressively more orange. This color development is not due to the chromatophores of the dermis, since the pigment involved is present in either the epidermis and (or) the outer mucus layer. The color, instead, is resultant upon an increase in the concentration of bilirubin, a bile pigment produced as a breakdown product of hemoglobin.