10 Things You Didn’t Know About Life in the ‘Twilight Zone” of the Deep Sea

Diving deep below the ocean’s surface – 200 meters and beyond – offers an illustrative life of extremes. Temperatures near the ocean floor often hover near freezing, with nearly no light. The pressure can feel like you have an elephant (or two) standing on your head.

Still, this mysterious miracle of a watery world is alive with life. And at risk. While it is widely documented that biodiversity is extraordinary down to the deepest depths of our oceans, an upcoming treaty vote by member countries of The International Seabed Authority would allow deep-sea mining that scientists warn could be devastating to marine life, the environment and our fragile climate. Here are ten amazing reasons why everyone should care.

1. Scientists are just beginning to study many of the species that live at the deepest depths of our oceans, from spectacular deep-sea fauna like sea anemones to unique worms and octopods. And all agree there are countless species yet to be discovered.

2. One researcher says nearly every one of their deep-sea expeditions collects new animal species that are not familiar to science. And many are rare species not thought to be living anywhere else in the deep sea. For example, beaked whales and yeti crabs were only discovered in the last 20 years.

3. Rising and falling from the deep seafloor is a wonderland of dark shadows, hidden craters and creviced high peaks. While its ecosystems are dominated by corals, sponges, and other filter feeders, it’s also home to larger animal inhabitants the likes of tuna and sharks, even dolphins and sea turtles.

4. Enormous undersea volcanic mountains called deep seamounts rise hundreds of thousands of miles from the seafloor. They support not only turtles, crabs and other crustaceans like shrimp, but they’re also feeding areas for many types of fish.

5. The highest mountain on Earth is a seamount. Mauna Kea is a dormant volcano that plunges 18,000 feet beneath the surface of the Pacific Ocean, measuring more than 30,000 feet from its base on the seafloor to its peak more than 2.6 miles high on the island of Hawaii.

6. A real-life “twilight zone” exists 200-1000 meters below the area of ocean illuminated by surface sunlight (that’s about 650-3300 feet). It’s cold and the light is dim, but flashes of light are produced by living organisms there through an extraordinary process called bioluminescence.

7. The twilight zone (or mid-water) is teeming with life. According to some recent studies, there is as much as ten-times greater life thriving in this environment than previously thought. They suggest that the biomass of fish in the twilight zone may actually represent more than in the rest of the ocean combined!

8. The largest animal migration on Earth appears to take place underwater and includes animals in size from microscopic to among the largest on the planet, according to the Woods Hole Oceanographic Institution. Some organisms spend their lives in the shadowy depths of the deep sea, but others travel hundreds or even thousands of miles each day. A host of tiny animals known as zooplankton as well as larger crustaceans, fish, squid, and many kinds of gelatinous animals like jellyfish regularly move to and from the surface, feeding on surface waters at night, then returning “home” before becoming a predator’s delicacy themselves when sunlight arrives.

9. “Visions of fantasy or science fiction” are how many deep ocean scenes and animals are often described. This is particularly true in the twilight zone, where some of its inhabitants look eerily prehistoric. While most fish that live in this part of the deep sea are only a few inches long, they are still considered a powerful force in the ocean. One example is the bristlemouth—a small mid-zone fish with relatively large jaws full of spiny teeth. It’s reportedly the most abundant vertebrate on Earth, possibly numbering in the quadrillions.

10. Animal movement helps transport enormous amounts of carbon from surface waters into the deep ocean, helping to regulate global climate.

Protecting deep-sea species of all sizes and shapes is not just critical to the planet, and to the balance of nature below the surface, it is also about the abundance of information scientists don’t yet have available. In fact, NOAA (The National Oceanic & Atmospheric Administration) says that while the ocean takes up about 71 percent of Earth's space, 95 percent of that ocean is still completely unexplored.

From what we do know, there are likely to be irreplaceable genetic resources there that could play a critical role in today’s and tomorrow’s modern medicine and industrial processes.

Joining other international advocates and scientists, a recent report from scientists at Fauna & Flora International (FFI) strongly recommend a moratorium on deep seabed mining because it is likely to cause “significant disruption to the ocean’s life-support systems, its carbon capture and a loss of biodiversity.” The report adds: “We are now beginning to appreciate the extent to which life in the deep sea also affects the health of the planetary systems on which we all depend.”