Adaptations for reflection of bioluminescent light in the gas bladder of Leiognathus equulus (Perciformes: Leiognathidae).

The gas bladder of leiognathid fishes functions not only in buoyancy but also in reflection of bioluminescent light from the circumesophageal light organ. Purine distribution, quality (guanine/hypoxanthine ratio), and concentration, as the basis for reflectivity, were assayed enzymatically for different portions of the gas bladder lining of the common leiognathid, Leiognathus equulus (Forskal). For highly reflective areas, the percentage of tissue wet mass and dry mass represented by purine was also determined. The results indicate that total purine content in the reflective areas of the leiognathid bladder was significantly higher than values determined for other similar, shallow water fishes; instead, purine content in these reflective areas was similar to that known for very deep-dwelling fishes, in which heavy purine deposition is correlated with high pressures and high oxygen concentrations in the bladder. In addition, the results show that differential purine distribution within the bladder correlates strikingly with the path of bioluminescent light. The dorsal bladder lining, the primary site of incident luminescence, had extremely high purine concentrations (averaging 2.80 mg/cm2), whereas the secondary reflective surfaces, the lateral (1.81 mg/cm2) and ventral (1.22 mg/cm2), portions, although high in purine content, had concentrations significantly lower than the dorsum. Areas through which light is transmitted, the light organ-bladder interface (0.09 mg/cm2) and the posterior region (0.19 mg/cm2), were greatly reduced in purine content. The enhancement of purine in the reflective portions of the bladder and the correlation of the differential distribution of purines with the path of light indicate that the L. equulus gas bladder is exquisitely adapted to function as a reflector of bioluminescent light.