We don't often encounter species that produce their own light here on land. Fireflies do it. Some millipedes and fungi do it. That's about it.
But in the murky depths of the ocean, it's a whole different glowing story.
About 1,000 to 1,500 feet beneath the ocean surface, in a region known as the deep scattering layer, there can be so much bioluminescence that the sea looks like it is twinkling with blue stars.
"As you get deeper and deeper in the water column, it becomes less and less penetrable to sunlight," said Matt Davis, an ichthyologist at St. Cloud State University in Minnesota. "The only light many of these animals see is made from other organisms."
The almost magical ability of creatures to chemically create their own light serves a lot of purposes in the deep sea. Fish use bioluminescence to communicate with one another, to hide their silhouettes from low-lurking predators, as a way to lure prey and even as a defense mechanism.
The shining tubeshoulder fish got its name because it squirts a blob of bioluminescent goo out of its shoulder when it is being attacked, using the flash of light to confuse its predators as it escapes into the darkness.
Bioluminescence is so useful to underwater creatures that it turns out it evolved independently at least 27 times in marine fish alone, according to a new study in PLOS One.
"When we see something that's repeatedly evolving over and over again, that's a good clue that it is of biological importance," said Davis, who led the work.
Previous studies on bioluminescence have found that the ability for an organism to create its own light evolved 40 times across the tree of life. The new study, which focused entirely on ray-finned fish, suggests that this evolutionary adaptation occurred many more times than that.
Not all fish use it in the same way. The authors' genetic analysis suggests that intrinsic bioluminescence — when a fish creates and emits light without any help from bacterial symbiosis — evolved eight times. Bacterially mediated bioluminescence evolved 17 times.
The genetic analysis also allowed them to see that the evolution of this light-making phenomena began as early as 150 million years ago and continues.
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