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Tiny sharks glow in the dark ... for camouflage

The smalleye pygmy shark uses light-emitting organs in its belly to actually hide itself from predators, a new study suggests.

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The smalleye pygmy shark is one of the world's tiniest sharks, reaching no longer than 8.7 inches.

Julien Claes

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Tiny sharks about the size of a human hand have a superpower of sorts: their bellies glow, according to new research that also showed these smalleye pygmy sharks use the glow to hide from predators lurking below.

Scientists had proposed the smalleye pygmy shark (Squaliolus aliae) sported light-emitting organs called photophores for use in camouflage, but that was never really tested, said study researcher Julien Claes of the Université catholique de Louvain in Belgium. "It wasn't even known if these organs were really functional, able to produce light," Claes added.

The small shark, which reaches a maximum length of just 8.7 inches (22 centimeters), lives well below the water surface in the Indian and western Pacific Oceans. The new research, detailed this week in The Journal of Experimental Biology, suggests their glowing bellies (a type of bioluminescence) would replace the downwelling light from the sun, or the moon and stars, that is otherwise absorbed by their bodies.

For the study, Claes and his colleague Jérôme Mallefet, along with Hsuan-Ching Ho from the National Dong Hwa University, Taiwan, captured 27 adult, smalleye pygmy sharks off the coast of Taiwan and brought them to the National Museum of Marine Biology and Aquarium. In the lab, the scientists took skin samples from the sharks and tested how they responded to various chemicals known to trigger biological processes such as light production. Sure enough, melatonin caused the shark's skin to glow; neurotransmitters known to regulate light production in deep-sea bony fish had no effect on the pygmy's skin. When the team added the hormone prolactin to the samples, the glow faded. [Bioluminescence Quiz: Do You Know About Glow?]

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