Luminous fishes: Endocrine and neuronal regulation of bioluminescence

Bioluminescence is the ability of organisms to emit their own light which can be self-produced in light organs by photogenic cells (photocytes) or symbiotically by bearing cultures of luminous bacteria. Luminous animals are mainly comprised in the ocean, nocturnal coastal shallow-water species or, as the majority of species, inhabiting deep-sea where dim light conditions are optimal for communication by luminous signals. Due to difficult access to species, fisheries of bioluminescent fishes worldwide are rare, mostly for traditional cuisine and fishing bait. Fishes are the only bioluminescent vertebrates and thus their light organs present an astonishing level of complexity for light modulation to adequate light signals for specific ecological roles. Fishes control the output of light intensity using different mechanisms which can be categorized as being under control of hormonal or neuronal pathways. Usually, neural control allows fast and direct response to the photocytes while endocrinal systems allow to control the light produced by bacteria, via manipulation of individual melanophores. In some species, mechanical movement of melanophore-filled tissues by muscular action is under neuronal or hormonal control. Sexual dimorphism of light organs, driven by hormonal regulation during ontogeny, takes a crucial role in communication and reproductive success of these species.