February 22, 2011

Many deep-water marine creatures sport some sort of luminescent or light-emitting organ. These structures function to distract predators, or serve as lures to draw prey. The lure of the anglerfish, for example, can be retracted towards the mouth, bringing prey in close enough to grab. Luminescence in deep sea fish can be produced and regulated in a number of different ways. Some fish carry colonies of light-producing bacteria within their bodies, while others use a chemical reaction that gives off light. In most species these light-emitting systems are controlled by the nervous system. The fish use neurotransmitters such as adrenalin, one of the same chemicals that controls our nerve impulses, to turn their light on or off.Velvet Belly Lantern Shark (Image from Image from Claes et al. 2010. J Exp Mar Biol & Ecol, 388:28-32.)

The appropriately named velvet belly lantern shark (Etmopterus spinax) emits light from photophores, special skin organs distributed on its belly and sides. Recently this shark was shown to control its light production not with neurotransmitters, but with hormones. Hormones are chemical messengers such as estrogen, testosterone, cortisol, etc, that travel through the bloodstream to relay signals to their target organs. In the velvet belly lantern shark several hormones, including melatonin and prolactin, regulate luminescence. In mammals melatonin is known for controlling circadian rhythms, while prolactin is involved in stimulating milk production following birth. These hormones are also present in sharks, but carry out very different functions.

Looking more closely at the lantern shark luminescent system though, researchers realized that hormones alone couldn’t account for the complicated light regulation they displayed. Additional work by Claes et al has now shown that lantern shark luminescence IS under neuronal control, and also under hormonal control. A neurotransmitter called GABA, widely used in the human nervous system, appears to turn off the luminescence that is turned on by melatonin and prolactin. So velvet belly lantern sharks use a complex interplay between at least two entirely different control systems (there is preliminary evidence for a third system as well) to regulate light production.

The article is: Claes JM, Krönström J, Holmgren S, Mallefet J. 2011. GABA inhibition of luminescence from lantern shark (Etmopterus spinax) photophores, Comparative Biochemistry and Physiology, Part C 153:231-236.

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