Jason Farmer for RedOrbit.com
There are a number of sharks that have specialized light-emitting organs on their undersides that allow them to glow in the darkest depths of the ocean. In fact, according to researchers, at least 10% of all known shark species are luminous, and this ability is one of the most mysterious areas of shark biology.
A new study of a tiny predator known as the Smalleye pygmy shark (Squalious aliae), led by Dr Julien Claes from the Catholic University of Louvain in Belgium, suggests this ability may have evolved from a shallow water ancestor and may yield a greater understanding of the specific molecular mechanisms sharks use to achieve this bioluminescence.
Pygmy sharks only grow to 22 centimeters in length and swim many hundreds of feet below the surface in the Indian and western Pacific oceans, so they can be very difficult to locate and study. When the world´s smallest sharks are found, researchers observe a bluish glow on their underbellies.
Dr Claes also wanted to compare this ability across shark species.
Previous research by Claes showed that in another group of sharks, appropriately named lantern sharks, two hormones found in many animals (melatonin and prolactin) are responsible for creating their glow. However, researchers were uncertain that the pygmy shark and it´s close relatives relied on the same hormones.
So, off the coast of Taiwan Claes and his colleagues collected close to thirty pygmy sharks. The researchers removed patches of the fish’s skin and soaked them in a variety of chemicals known to trigger luminescence in other shark species. When Claes tried melatonin, the skin of the pygmy shark would glow, just as it does in lantern sharks. The melatonin causes hundreds of thousands of very small pores covering the sharks’ skin to open, exposing cells that glow brightly by chemically producing their own light.
Research indicates the tiny sharks use this ability as a kind of glowing camouflage for hunting and protection. Without it, any predator or prey could easily spot the sharks’ bodies silhouetted against the bright sky above the surface of the water.
It is also known that lantern sharks use the hormone prolactin to create bright blue bursts of light to communicate in the dark water.
However, research reveals that in pygmy sharks prolactin turns off the animals’ light-producing organs.
Dr Claes thinks that luminescence in earlier species of sharks functioned in a similar way to that of pygmy sharks. When the ancestors of modern lantern sharks emerged, they repurposed prolactin into a cue to light up for communication.
“The pygmy shark, in that sense, is the missing link in the evolution of shark luminescence,” Claes told BBC News.
“It is likely that the control of shark [luminescence] evolved from the [camouflage] of shallow water sharks,” he added.
Sharks that swim in shallow water are more susceptible to threats from above. They can use their ability to change the colors of their skin to blend with their surrounding as a defense. By producing different hormones, they trigger darker and lighter patches of skin. The research suggests this is the basis for the evolution of the deep-water shark´s ability to control their luminescence.
“This result is very important, since it gives clues to the evolutionary pathway taken by a soft-tissue phenomenon which leaves no or extremely few fossil tracks,” Dr Claes told BBC’s Ella Davies.
The findings are published in the Journal of Experimental Biology.