Leiognathids are shallow-water, Indo-West Pacific fishes that have a circumesophageal, bacterial light organ. Visual observations of living fishes revealed a mottled ventral luminescence pattern, which was analyzed both behaviorally and morphologically. In behavioral experiments, these fishes responded to increases in intensity of downwelling light with increases in the intensity of ventral luminescence. However, while the absolute luminescence levels tracked the ambient light levels, they did not increase in direct proportion to those of increasing downwelling light; luminescence levels were closer to the intensity of downwelling light at low light levels.
The tissues that intervene between the internal light organ and the external environment are responsible for the observed mottled pattern of the ventral luminescence. Furthermore, these tissues, which have been incorporated into the light organ system, are involved in the control of the intensity, spectral quality and angular distribution of the fish's luminescence. The spectral peak of the bacterial luminescence from whole fish (500 nm) was shifted about 10 nm towards the green relative to the spectral peak of cultured light organ symbionts (485–490 nm). The luminescence had the greatest intensity of outward expression at an angle of 20–25° from the ventral midline and was undetectable dorsally. The ventral illumination behavior of leiognathids, with their associated morphology, is compared and contrasted with the counterillumination systems that have been described in a number of mesopelagic fishes, shrimps and squids.
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