Bioluminescence, simply put, is the ability of an organism to produce its own light. Deep sea fish commonly do this by having specialized organ structures for housing glowing bacteria. However, in rare instances, fish and other creatures have intrinsic luminescence. Such organisms have tissues that produce luciferin and the enzyme substrate luciferase, which degrades it such that light is produced. While bioluminescence is common in fish, it is rare in elasmobranchs (rays, skates, and sharks) and rarer still that the lantern shark, Etmopterus spinax, produces intrinsic luminescence that is controlled by visual input initiating endocrine signalling! In fact, it is currently the only known fish to use hormone regulation of luminescence.
Elasmobranch skin contains malleable melanophores that have been documented in the lantern shark’s shallow water cousins to help them adjust coloration over time to camouflage into their surroundings. It seems that the lantern shark has co-opted these pre-existing structures to emit light in the dark depths of the ocean. These melanophores widen and narrow like the aperture of a camera to change how much light is emitted or how much pigment is visible in the skin. This light is emitted by photocytes via the luciferin-luciferase reaction.
This system of light emittance and control, though fascinating, is not particularly unique. Many fish have neural control over these melanophores connected to eyes or eyespots that help them adjust the brightness of their glow to match ambient downwelling light. When looking up, a fish can easily spot the dark shadow of a predator blocking the light coming down through the water. However, a fish that is able to glow the same color and intensity of light as the light around it is invisible. It is true that the lantern shark likely evolved to bioluminesce for similar counter-downwelling camouflage purposes, the control of light is actually endocrine regulated. Lab experiments showed that prolactin and melatonin applied to the skin of specimens caused light emittance and melanophore dilation. Interestingly, light production of photocytes was faster than melanophore dilation, which is likely a way that the shark ensures maximal light emittance by the reaction before controlling it with melanophore contraction or dilation.
Interestingly, I read that the lantern shark habitat ranges from 200-2,490 meters, where downwelling light ranges from the sunlight (maximum light penetration) to the midnight zone (no light penetration). It is likely that the evolution of the bioluminescent control mechanism was influenced by its time in the strongly selective midnight zone, where even a small bit of light can be a death sentence. However, some species also use bioluminescent patches to identify species, such as was discussed by Dr. Matt Davis at a recent biology series talk on dragonfish. Dragonfish, he posited, have ventral illumination for counter-downwelling and specific patterns of lateral illumination spots to communicate species. So, while it is fascinating that the lantern shark mediates luminescence by endocrine signalling, I am not sure all attention should be on counter-downwelling. Considering different purposes of bioluminescence and related physiological structures, we may come to understand why the lantern shark evolved such a unique way to glow.
Velvet Belly Lantern Shark
Illuminating lantern shark
(if this isn't cool, then nothing is)
References
Amundson, B. (2014, June 30). Missing link found on sharks. Retrieved November 24, 2017, from http://sciencenordic.com/missing-link-found-sharks
Carpenter, K. E., & Sampang-Reyes, A. G. (n.d.). Etmopterus spinax summary page. Retrieved November 24, 2017, from http://www.fishbase.org/summary/687
Claes, J. M., & Mallefet, J. (2010). The lantern sharks light switch: turning shallow water crypsis into midwater camouflage. Biology Letters, 6(5), 685-687. doi:10.1098/rsbl.2010.0167
Claes, J. M., Nilsson, D., Straube, N., Collin, S. P., & Mallefet, J. (2014). Iso-luminance counterillumination drove bioluminescent shark radiation. Scientific Reports, 4(1). doi:10.1038/srep04328
Davis, M. P., Holcroft, N. I., Wiley, E. O., Sparks, J. S., & Smith, W. L. (2014). Species-specific bioluminescence facilitates speciation in the deep sea. Marine Biology, 161(5), 1139-1148. doi:10.1007/s00227-014-2406-x
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