The Twilight Zone
The ocean twilight zone is a layer of water that stretches around the globe, just beyond the reach of sunlight and beyond the limit of photosynthesis in the ocean. Also known as the midwater or mesopelagic, the twilight zone is cold and dark, but flashes of bioluminescence—light produced by living organisms—reveal a rich abundance and diversity of life. In fact, recent studies suggest that the biomass of fish in the twilight zone may be larger than almost anywhere else in the world’s oceans. 

So far, the twilight zone is largely unexplored. Its rich biodiversity has remained mostly beyond the reach of commercial fishing—and the international laws that govern the high seas. But some fishing fleets are poised to begin extracting the biological resources of the twilight zone, with unknown consequences for life elsewhere in the ocean and even for Earth’s climate. Watch this video to learn more about why scientists are studying the twilight zone today, and why it’s so important for the health of the oceans as a whole.

Bioluminescence
Some species of marine animals create their own light, thanks to specialized cells called photophores. Some use that light to lure prey; others use it to prevent being eaten. Others still use it as a means of communication, or to find mates in the murky waters of the twilight zone. Learn more about how bioluminescence works, and why some animals on land and sea may have evolved ways to create it.

Diel Vertical Migration
Small sea creatures, such as copepods, squid, and krill, swim up from the deep at night to feed in the cover of darkness near the surface before returning to the depths just before sunrise. This movement makes up the the largest animal migration on Earth. In fact, the U.S Navy once mistook all that life for the seafloor in sonar records. Ocean scientists think the main reason this occurs is that it lets some marine creatures eat their fill while avoiding predators that could spot them easily during the day. The migration also plays an important role in global climate: since phytoplankton help remove carbon dioxide from the atmosphere, any migrating animals that eat them help transport the carbon in their bodies away from the surface to deeper waters.

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