Most of us think of the ocean as what we see on the sunny surface. But beneath the shimmering waves, there's a deeper layer called the twilight zone.

Referred to by scientists as the mesopelagic, this dimension is considered a "dark hole" in our understanding of ecosystems and one of the most understudied regions in the world.

The twilight zone can be found 200 to 1,000 meters (about 650 to 3,300 feet) below the ocean surface, at the point where the sun's rays can no longer reach, according to the Woods Hole Oceanographic Institute (WHOI) in Massachusetts. Because it's so deep and there's no sunlight, it's cold and dark.

But that doesn't mean this deep layer is still and quiet. Instead, it's full of life including fish, crustaceans, jellyfish, squids and worms. Occasionally, there are bursts of bioluminescence, when living creatures give off their natural glows.

Researchers estimate there may be up to 1 million undiscovered species in the zone. Oceanographers who want to study this life don't have much natural light to observe them. But if they use too much artificial light, they risk frightening them. So researchers are still trying to find the right balance.

Creatures in the zone

bristle worms The twilight zone of the ocean is full of organisms such as bristle worms. (Photo: Silke Baron [CC BY 2.0]/Flickr)

Studies have suggested that the biomass or weight of fish in the twilight zone might be as much as 10 times greater than they had originally thought, which is more than in the rest of the entire ocean. It could, in fact, make up more than 90% of all fish in the sea, according to the Blue Marine Foundation.

Recently, researchers with the 6-year, $35 million Ocean Twilight Zone (OTZ) initiative sent their 5-meter-long (16-feet) "Deep-See" sled exploring into the twilight zone, Science reports. The sled is packed with cameras and audio sensors and can take samples from this "neglected" ocean layer.

"We kept seeing organisms all the way down," says Andone Lavery, a physicist with WHOI, which is leading the project. "That was really surprising."

Not only are there so many of these fish, they have unusual appearances and behaviors.

"Mesopelagic fishes are small, quirky looking and many of them undertake a daily commute, migrating vertically at night to feed in shallow waters above 200 m in the safety of darkness and then retreating to the depths by day," Blue Marine Foundation writes.

The fishing question

commercial fishing ships Some countries like Norway and Japan allow fishing in the twilight zone. (Photo: Iulianna Est/Shutterstock)

Because there are so many fish in the twilight zone, the fishing industry is naturally interested in this dark and mysterious layer.

Some of the organisms that make the trek to the surface are being harvested by industrial fishing operations in countries such as Japan and Norway, according to WHOI. Large numbers of tiny crustaceans such as krill and copepods are harvested and processed for use in pet foods, livestock feeds and human nutritional supplements.

These open-water fisheries far from land are mostly without regulations. Researchers and environmentalists are concerned about the consequences of removing so many organisms from this little-understood layer.

The U.S., reports the Blue Marine Foundation, has banned commercial fisheries from removing mesopelagic fish in the Pacific because of concerns over potential negative impacts on the ecosystem. The United Nations is negotiating a new international agreement to improve management and conservation of marine biodiversity.

The role of mesopelagic fish

Fish in the twilight zone are key for the environment.

Researchers know fish play an important role in the ocean's food web by carrying large amounts of carbon from water near the surface into deeper areas of the ocean. This helps prevent it from escaping into the air as greenhouse gases.

In addition, they are an important source of prey for marine mammals so when fisheries remove large quantities of twilight-zone fish, it can upset ocean biodiversity.

So the fishing and research communities are balancing the need to protect the ecosystem with the benefits of finding new sources of food to deal with world hunger issues.

A perspective article in the journal Frontiers in Marine Science looked at the various sides of the fishing argument in the twilight zone.

They quote Andrew Mallison, director general of IFFO, the fish meal and fish oil producers and consumer's organization, who said:

"The industry is certainly in need of more raw material – demand exceeds supply and demand is forecasted to continue growing as global aquaculture (and feed) increases. However, these deeper water fish will be more costly to harvest, and there would have to be a good set of science based harvest control rules to satisfy any environmental or ecosystem impact concerns. If the science indicates a potential sustainable fishery with a reasonable yield, there are several IFFO member companies who could look at the economics of fishing effort and return."

Mary Jo DiLonardo writes about everything from health to parenting — and anything that helps explain why her dog does what he does.

Why we must protect the ocean's 'twilight zone'
Deep in the ocean is a 'twilight zone' teeming with 1 million undiscovered species, yet fishing there isn't regulated.