Gallbladder (GB) bile of most cyclostomes, elasmobranchs, and teleosts contains appreciable amounts of biliverdin (BV) and bilirubin (BR) conjugates with lesser amounts of unconjugated BR in certain species. Certain elasmobranch and teleost species have been reported to have primarily BV or BR in bile. The appearance of the enzyme BV reductase, which converts BV to BR in mammals, evolved quite early in the evolution of vertebrate species; however, exceptions exist in certain fishes, amphibians, reptiles, and mammals whose bile contains primarily BV. Nearly all analytical studies on bile pigment composition in fishes to date have utilized only GB bile, which may or may not always be representative of the pigments excreted in freshly collected hepatic duct bile. The concentration of BV and BR in GB bile of fishes increases markedly during prolonged fasts. From the limited data currently available, there appears to be no systematic development from primitive to advanced forms in the appearance of certain bile pigments in fishes. While bile of most aquatic species contains appreciable amounts of both BV and BR, it is interesting that the bile of most terrestrial avian and reptilian forms contains primarily BV.
The serum of fishes, except for certain bony species such as eels (Anguilleformes) and cottids (Scorpaeniformes), is a light yellow color due to the presence of BR and is similar to that observed in higher vertebrates. Serum from certain eels and cottids is bluish green in color due to the presence of a variety of chromoproteins that contain BV firmly bound as the prosthetic group.