The Significance of Ventral Bioluminescenee in Fishes

The positions of luminous organs and tissues are documented for all known bioluminescent families of fishes. The majority of families, species, and individuals possess photophores only or mainly on the ventral half (as opposed to the dorsal half) of the body. The position and in some cases also the photophore structure tend to direct the emitted light downwards. Review of the previously suggested functions of luminescence and a new one (proposing light organs may function in the spacing out of midwater fish in the absence of marks for fixing territories) suggests that these functions are not related to the ventral predominance of luminescence. Photophores functioning as lures for prey and as headlights do not appear to follow the general tendency for ventral positioning of bioluminescence.Study of the two previously proposed theories on ventral predominance of luminescence, the reflector theory and the "counter-shading" or disruptive illumination theory, shows that there are some cases which these theories do not explain; these two theories apply only during the day. A third theory is developed which suggests that the larger numbers of predators in upper waters would select for fishes with ventrally directed nocturnal luminescence as this would subject them to less predation. Fishes possess photophores for a number of reasons, but they are placed ventrally because this results in decreased predation. Some counter-strategies are discussed, showing how fishes have coped with certain problems created by bioluminescence.