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Blue glow on a chocolate shark
Photo: Stevens Mallefet / Duchatelet / CC BY 4.0
It is almost always completely dark in the depths of the oceans.
But sometimes something also glows where the sun's rays are no longer sufficient.
Bioluminescence is the name of the phenomenon when living things emit visible light by means of chemical reactions.
It is known, among other things, from various deep-sea fish, but also from jellyfish, shrimp and squid.
Researchers off New Zealand have now found out that three types of shark have also mastered the lighting trick, of which this was previously unknown.
In the specialist magazine "Frontiers in Marine Science" a group around Jérôme Mallefet from the Catholic University in Leuven, Belgium reported observations on the Chatham Rise off the east coast of New Zealand in January 2020. They noticed that both the chocolate shark and the black-bellied and the southern lantern shark can emit light.
It has a bluish shimmer and can be seen mainly in the animals' abdominal region.
"It is becoming increasingly obvious that the production of light in the depths has to play an important role in structuring the largest ecosystem on our planet," the researchers write in their report.
Particularly interesting: With a body length of up to 1.8 meters, the chocolate shark is also the largest vertebrate known to date that can shine.
The five and a half meter long and extremely rare basking mouth shark probably also uses bioluminescence.
However, researchers assume that the glow here is caused by captured plankton and that the shark only reflects the light.
The three species of shark we are talking about now live in the shadowy realm of the so-called Mesopelagic Zone, at a depth of 200 to 1000 meters.
The interesting question is what actually brings the animals their ability to shine here.
A light belly may help them that they cannot be seen so easily from below because the contrast between the backlight and the light water surface is no longer so great.
But maybe it's also about being able to search the ocean floor better in search of food.
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