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Yellow and orange sponges contrast well with the deep blue of the “twilight zone” (depth: 70m or 230ft)
 

Since the beginning of 2017, Olivier Konieczny and I have been diving on the outer side of the coral reef at the Indian Ocean French archipelago of Mayotte, between 70 and 120 meters (230–390 feet) deep. These dives were motivated by a big question: What happens under well-known reef areas, in what is called the mesophotic zone? Our goal is to observe what researchers cannot see and to collect information they don’t have access to.

The biotopes of coral reefs, between the surface and 30–35 meters (100–115 feet), are almost all known. They have long been the subject of scientific studies and monitoring. Due to their ease of acess and how shallow they live, the data, counts and measurements provide an accurate picture of their current evolution around the world. The bathyal depths (300–3,500 meters or 985–9,850 feet) and abyssal bottoms are being studied, both industrially (polymetallic nodules containing rare metals) and in the search for new organisms.
 

A rebreather deep diver at the foot of the second wall (depth: 77m or 253ft)

Between the Surface and the Abyss: Poorly Known Areas

However, between the two, there are still areas that have not been studied as much—mysterious places called the mesophotic zone and the twilight zone. The greater the depth, the less light there is, until it disappears completely. However, before total darkness, there is a zone of darkness, called mesophotic: meso means “medium” and photic denotes “in relation to light.”

As such, the mesophotic zone corresponds to the water column being poorly lit by sunlight. In the tropics, it is located between 50 and 150 meters (165–490 feet). Even deeper is the twilight zone, where the light gradually disappears until the darkness becomes almost total. And this environment, due to a lack of sufficient exploration, suffers from a significant lack of knowledge and data, which are essential to its understanding.
 

A panoramic shot composed of 10 photos of a cavity at the base of the second wall (depth: about 80m or 260ft)
 

Under the twilight zone: A panorama composed of 11 photos covering an angle of almost 180 degrees (depth: 80m or 260ft)
 

A panorama at the foot of the second wall, not far from the Bouéni Pass (depth: 85m or 279ft)

Rarely Accessible to Researchers

The main reason for this lack of knowledge remains the complexity of access. At these depths, the constraints placed on divers are enormous. We are no longer talking about recreational diving but technical diving. The equipment used is expensive and cumbersome, and the procedures are long and burdensome.

Specialized training and experience are essential to consider diving in the twilight zone. The use of a mixture of several gases adapted to the different depths, as well as the control of a closed-circuit rebreather, are imperative. The human investment, both temporal and material, is very heavy, as is the commitment. Diving in an environment very hostile to human physiology, imposing pressures of more than 10 bar on your body, and long hours of decompression hold back most divers.

Among all these constraints, there are also the constraints of photography in the deep. In the twilight zone, underwater photography no longer has much in common with that when doing conventional recreational diving, since the equipment—housing, dome, port, and strobes—must withstand much higher pressures.
 

Less than 20m (65ft) to reach the target depth of 120m (390ft): We are at the edge of the third drop-off, which plunges almost vertically to more than 200m (650ft) deep

A Life Raft for Coral Reefs?

In the face of climate change, we urgently need to have a global understanding of reefs, including those at depth. “Superficial coral reefs [up 30–40 meters or 100–130 feet],” expains Professor Pichon, a coral specialist, “are increasingly being affected by various types of impacts such as climate change, leading to, for example, increasingly severe and frequent bleaching phenomena. It has also been suggested that these mesophobic coral stands could be used as ‘nurseries’ to replenish or restore devastated surface reefs. There are, of course, many aspects of the biology of mesophobic coral environments that need to be elucidated to assess the validity of these hypotheses, and these are the research topics we are currently addressing.”

Therefore, there is an urgent need to ascertain a “state of play” of these mesophotic zones. In addition to counting species, observations of the biodiversity of this environment as a whole can enrich the understanding of the different interactions that can occur between deep and near-surface areas.
 

A colored mosaic in the mesophotic zone: Although only a tiny fraction of the sunlight reaches these depths, organisms living in this environment adopt a variety of colors worthy of the most beautiful rainbow (depth: 78m or 256ft)
 

Despite the lack of light, some corals are able to thrive in the twilight zone (depth: 78m or 256ft)

Oases of Life Hidden in the Darkness

In Mayotte, deep coral reefs are present on the outer slope of the lagoon. This is where we focus our dives, between 70 meters (230 feet) and a little over 120 meters (390 feet), near the southern passes, the boat pass and the Bouéni Pass. During our explorations, we have been able to identify several specific areas where important biodiversity is concentrated—real “islands of life” well hidden in the shadows of the depths.

For example, we have discovered prominent massifs, located on a sandy beach whose base is nearly 100 meters (330 feet) deep. At the bottom of the second wall, at a depth of 80 meters (260 feet), we were also able to explore small cavities, created thousands of years ago by wave action, when the sea level was much lower. Today, they constitute a refuge for many species, despite the very low light levels that prevail there.
 

The cavities are lined with organisms, some like lace. Sponges of different colors form a beautiful mosaic
 

Unusual decor at 93m (305ft): Whip corals of various colors
 

 A stingray cruises past as Olivier looks on (depth: 75m or 246ft)

Rare and Sometimes Unknown Species

We are still at the very beginning of our explorations, but the number of specimens we have been able to identify so far is significant. In addition, within these “hotspots” of deep biodiversity, we have already found some rare, if not yet identified, subjects, including fish, crustaceans and shellfish. Two examples are a Galathea squat lobster on a soft coral and a tiny symbiotic shrimp, both of which seem to be unknown to specialists.

Marine biologist Thierry Mulochau, Director of the BiORécif research group and leader of the MesoMay program, says: “We’re just getting started, but several fish species have already been listed as new to Mayotte. Some crustacean species may be new to science, but this requires sampling to describe them and validate these new species.”
 

This squat lobster has not yet been identified by specialists. What is certain is that it is rarely photographed
 

New to science? A tiny shrimp perfectly camouflaged on its soft coral host (depth: 85m or 279ft)

Familiar Species at Surprising Depths

Our photos have also allowed us to provide additional information to science and to advance knowledge on species that are generally well known. For example, while the literature suggests that the ribbon moray eel (Rhinomuraena quaesita) lives at a maximum depth of 60 meters (197 feet), I have photographed one at a depth of 92 meters (302 feet). The delicate ghost pipe fish (Solenostomus leptosoma) is apparently only found at a depth of 40 meters (130 feet), whereas we encountered it at 100 meters (330 feet). Similarly, the semicircle angelfish (Pomacanthus semicirculatus), which should also not go deeper than 40 meters (130 feet), we observed at more than 120 meters (390 feet).

These few examples, we are certain, are only the beginning of many discoveries to come. Many others, we hope, will improve our knowledge of mesophobic areas and their interaction with shallower areas.
 

At 95m (310ft), my eye was attrached to what turned out to be a delicate red ghost pipefish, probably a juvenile since this specimen was no more than 3cm (1.2") long. Once again, we proved that the mesophotic zone is very poorly understood: This fish is supposed to dwell no deeper than 35m (115ft)!
 

We regularly come across spider crabs in the twilight zone, at 70 to 100m (230–330ft). This specimen—which looks a lot like a spider crab from the Indo-Pacific—was photographed in a cave at 70m. Is it a new species? Only DNA analysis can tell
 

Shrimp gather in almost total obscurity at the bottom of a cave at 72m (236ft)
 

A blue angelfish couple swims peacefully at 80m (260ft)—despite being described as living at a maximum depth of 40m (130ft). We have observed others at more than 120m (390ft)

Shooting in the Deep

Deep sea photography is very different to recreational diving photography. The commitment involved in deep dives—the long decompression times, the complexity of the equipment—are real constraints on the photographer. A simple image shot in 30 meters (100 feet) of water is no longer a straightforward matter at 100 meters (330 feet).

The use of a closed circuit rebreather requires great vigilance at all times, and this only intensifies the deeper you go. At the same time, the bulky bailout bottles that must be carried—often three or four—can make it tricky to get into position for the shot.
 

A three-inch spiny seahorse (Hippocampus hystrix) swimming a few inches from the wall at 75m (246ft)
 

The camera housing also plays a very critical role, much more so than in recreational diving. Deep diving puts a great deal of stress on the housing and the slightest defect in materials can be disastrous. With my past housings, I’ve had buttons ceasing to function at 80 meters, and even ports imploding at 100 meters. Fortunately, my current Seacam housing has proven to be 100-percent reliable.

Above all else, the biggest handicap is the pressure of time. At great depth, the minutes spent at the bottom cost you hours of decompression. Time is extremely valuable. You don’t have the time to wait for subjects to come to you. When you’ve found your subject, you won’t have much time with it, so you have to work fast. But finding subjects, especially for macro photography, is the biggest challenge, and it can sometimes take several dives to successfully capture what you’re after. As underwater explorer and photographer Laurent Ballesta put it: “In deep diving, patience is not a virtue but a luxury.”
 

A Javanese moray eel receives a good cleaning session (depth: 65m or 213ft)

Participatory Science: A Necessity?

The data we are sharing with scientists through our photos are extremely valuable because of their rarity and reliability. “[Deep divers] photographs of organisms and especially their macro photographs allow us to identify animals at least at the level of orders and families, and sometimes genera or even species,” says Prof. Bernard A. Thomassin from the French National Center for Scientific Research (CNRS). “However, to refine these identifications, it is often necessary to have a sample of these species available, and this is where collection is required. But collection must, above all, respect the environment and the density of the population in place.”

There is little doubt that participatory science is a viable alternative solution in the acquisition of knowledge. Without committing a large budget, it’s possible to mobilize, around a common objective, a large number of participants from different backgrounds and multiple areas of expertise. A perfect example is the International Association of Nitrox and Technical Divers (IANTD), which enables us to access little-known or unknown areas.

Ultimately, it is the passion for the world under the sea that we share with the men and women of science that fuels our blossoming collaboration. For us divers, the scientific side of our dives and our exciting discoveries reinforce our thirst for exploration—and provide even greater meaning to what we’re doing.
 

A trimix rebreather deep diver takes a sample of a live coral colony in the twilight zone at 80m (260ft). Once this sample is brought to the surface, a small fragment is placed in alcohol for DNA analysis
 

About Gabriel Barathieu: Gaby first discovered the underwater world in the French overseas territory of Réunion at the age of 16. A decade later, after deciding to make the island home, he rediscovered scuba diving and became fully immersed in underwater photography. He soon because a rebreather diver and then gained Normoxic Trimix certification, allowing him to dive to 70 meters (230 feet). Since 2017, he has been deep diving in the twilight zone off the French island of Mayotte. In 2019, he and his friend Olivier Konieczny created Deep Blue Exploration, an association whose main objective is the exploration and study of the deep reefs of Mayotte. www.underwater-landscape.com