The safe answer to how a lantern shark turns its luminescence on and off is: “Any way it wants.” Now researchers have looked into the belly of the beast and found that three hormones act as on-off switches for these glow-in-the-dark sharks. It is the first discovery of hormones controlling bioluminescence in animals, the scientists report in the Nov. 15 Journal of Experimental Biology.
Belgian researchers identified melatonin, prolactin and alpha-MSH, three hormones known to control sharkskin coloration, as key players in setting sharks aglow.
In all animals investigated up to this point, luminescence is triggered by nerve cells. Finding a parallel pathway to bioluminescence — one that’s controlled by hormones, not nerves — strongly supports the notion that light-emitting powers have evolved multiple times in animals, comments marine scientist Jim Gelsleichter of the University of North Florida in Jacksonville, who was not involved in the research.
The light-emitting cells in some sharks aren’t connected to prominent nerve cells, and the slow onset of their glow hinted that something other than nerves were involved. Exposing patches of skin from lantern sharks to hormones and to nerve signaling molecules confirmed that hormones turn on the sharks’ bluish glow.
Melatonin, which in humans is an important hormone for sleep regulation, induced a slow, long-lasting glow in the skin patches that persisted for several hours, researchers show. This light probably serves to camouflage these velvet belly lantern sharks, Etmopterus spinax, counter-illuminating them from below as they descend to darker depths of the sea, says Julien Claes, coauthor of the study with Jér´me Mallefet of the Catholic University of Louvain in Belgium.
Prolactin, which plays a major role in reproductive physiology in people, spurred a quicker shine that lasted up to an hour the scientists report. The prolactin-induced glow might be a means of communication with other sharks and potential mates, the scientists speculate. A third hormone, alpha-MSH, turns the shark’s lights off. Several common nerve signaling molecules had no effect, the researchers found.
In bony fishes, nerves control luminescence and skin coloration — an “on” switch that is speedy and precise, allowing very fine-tuned control, notes Gelsleichter. A flounder, for example, that’s moved from a light to a dark background quickly changes color to match the backdrop. “If you put it on a checkerboard, it would probably turn checkerboard, there’s such fine nervous control and it’s very quick,” he says.
But in sharks and the closely related rays, hormones control skin coloration. Like luminescence, this color change is “slower and not as finely regulated,” says Gelsleichter. “If you take a stingray from a light background and put him against a dark background — it will take him a little longer. He’ll almost get it right.”
Even if melatonin doesn’t allow super fine-tuning, it’s actually a very good choice for regulating light for the sharks. Known as the “dark hormone” for its role in sleep and in seasonal shifts in animal physiology and behavior, melatonin is released by the pineal gland, which receives information about the amount of light in the external environment. Many animals secrete more melatonin when the long nights of winter arrive. Similarly, whenever a shark descends it will encounter darker waters, so a hormone that already is tuned into the dark is an ideal one to co-opt for turning on light, notes Seppo Saarela of the University of Oulu in Finland.
While Claes says he is reluctant to generalize, he suspects that other bioluminescent sharks also have hormone switches in their light-producing organs, the photophores. About one in eight shark species does some kind of glowing, says Claes, and he intends to investigate other species. “It’s amazing — this work just shows that bioluminescence is a very complex phenomenon. We are still really at the beginning of this story.”