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Enter the strange world of the ocean twilight zone

The ocean twilight zone, formally known as the mesopelagic zone, exists roughly 200-1,000 meters below the surface. Home to an array of species, from the anglerfish to the vampire squid and the fairy light-like siphonophore, it’s a place where oddness reigns. In the darkness eyes are small and teeth are large; many species are transparent, many bioluminescent. And in the murk, the dominant creature, the ubiquitous bristlemouth, is smaller than a human’s little finger.

A difficult area to study and often overlooked by science, new technology is aiding its exploration, forcing researchers to re-evaluate just how much life is down there. Researchers now believe there is 10 times, maybe 100 times the volume of biomass previously thought, says Heidi Sosik, senior scientist at Woods Hole Oceanographic Institution (WHOI).

With each answer, more questions. “What we know now is how much we don’t know,” she says.

Now this quest for knowledge has become a race against time.

Some scientists fear commercial fishing operations could expand in this ecosystem, and small but abundant species could end up in fish oil, used in cosmetics and dietary supplements, or fishmeal, used in aquaculture to feed species farmed for human consumption.

“There are very basic things about the twilight zone that we just don’t know,” Sosik explains, including the lifespan of some species, and how long it takes them to mature and reproduce. Without understanding lifecycles, there is no way to know how species could be fished sustainably.

Under pressure

Technology has long been a necessary friend of those studying the twilight zone. But it has not always been advanced or nuanced enough to provide an accurate account of the region. Take the bristlemouth, for example. Dense formations of these tiny fish were once mistaken for the seabed by sonar equipment. Now researchers have the tools to estimate as many as a quadrillion (1,000 trillion) bristlemouths might live in the ocean, making it the most abundant vertebrate on earth.

“The unique challenge about working in the twilight zone is that we don’t want to disturb the animals,” says WHOI senior scientist Dana Yoerger. These creatures are sensitive to light and sound, so monitoring them means devices need to be quiet and not stir up water, and employ red lights that most animals can’t see.

Yoerger developed “Mesobot,” an autonomous robot that discretely monitors slow moving fauna. Using stereo cameras to judge a creature’s relative position (in the same way the human brain does), the robot moves with the animal at a fixed distance, allowing researchers to watch how it swims, hunts its prey, and document delicate bodily structures that would be destroyed if a physical sample were caught in a net, he explains.

Mesobot is an underwater robot designed to track and record high-resolution images of slow-moving and often delicate fauna living in the ocean twilight zone.

Trials have lasted up to 40 minutes so far, but Yoerger hopes to eventually follow a target over a 24-hour period. “Ultimately, we’d like Mesobot to think like a human scientific explorer, seeking out the most unusual animals and observe their behavior for long periods,” he says.

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In July, WHOI researchers will set sail for the first stage of a study mapping the twilight zone off the northeast US continental shelf. A team will deploy a network of sensors capable of tracking drifting research robots known as “minions,” as well as tagged specimens from upper waters entering the twilight zone, over approximately 1 million square kilometers (386,000 square miles).

Sosik explains that scientists are attempting not only to build out knowledge of the twilight zone, but how it fits in within the wider ocean. “Whales and sharks — everything we’re familiar with, the charismatic organisms of the ocean — the more we learn about them, the more it seems that they depend on interacting with the twilight zone,” she says.

That sinking feeling

Researchers have also come to the conclusion that another species might also be reliant on the twilight zone: humans.

The ecosystem is understood to have a significant role in ocean carbon sequestering. In one process, part of what is known as the biological carbon pump, phytoplankton (carbon-absorbing microalgae) growing near the surface are consumed by zooplankton and fish that swim up from the twilight zone at night in what is thought to be the largest animal migration in the world. These creatures defecate, contributing to “marine snow” — a mix of matter that also includes dying organisms and bacteria.

Marine snow is consumed by sea life including salps, gelatinous organisms living in both the surface ocean and the twilight zone and whose role may have been historically underappreciated, says Sosik. Salps are capable of clearing “huge volumes of water,” she explains, in turn excreting dense fecal pellets that sink fast into the deep ocean.

Combined, biological processes in the twilight zone sequester 2-6 billion metric tons of carbon annually, says the WHOI — the lower estimate twice the annual emissions of all the world’s automobiles.

The opportunity

The WHOI speculates commercial fishing could upset the balance of the twilight zone’s carbon pump, with knock on effects for the climate. The organization says the economic value of more precise data could be worth hundreds of billions of dollars and help society make better decisions — a strong argument for more research.

“In the past, humans have found living resources in the ocean and just went full bore over-exploiting them,” Sosik says, “and in hindsight realized that we should have been more informed and taken a more mindful approach. With the twilight zone we have that opportunity.”

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As most of the twilight zone exists outside of national jurisdiction, an international effort is underway to ensure good stewardship. The UN has backed a consortium known as JETZON (Joint Exploration of the Twilight Zone Ocean Network) to coordinate research, share technology and findings and inform policymakers. Led by the UK’s National Oceanography Centre, it brings together projects from the WHOI and entities from five other countries as well as the EU, seeking to answers questions including how much marine snow is falling through the twilight zone and how biodiversity influences the biological carbon pump.

“We have this amazing opportunity to bring together basic science and curiosity driven science, and try to generate solutions for the big challenges that humans face in interacting with our planet and our ocean ecosystems,” says Sosik.


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