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Notes From the Field Into the Twilight Zone

Notes From the Field: Into the Twilight Zone

Richard L. Pyle, Ph.D. and Sonia J. Rowley, Ph.D.


Figure1_Typical Shallow Coral Reef_Richard LWhen most people think of a coral reef, they envision vast expanses of sprawling stony corals under bright sunlight, populated by myriad bustling and colorful fishes.  Such reefs exists throughout the world’s tropical oceans and have, for the most part, been reasonably well explored and documented over the past six decades or so by marine biologists using conventional SCUBA gear. However, because of the depth limits imposed by conventional SCUBA, the vast majority of research on coral reefs has been conducted at depths of less than 50 m (165 ft) or so. Although some intrepid marine biologists probed greater depths on scuba, the complications of nitrogen narcosis and decompression illness (DCI) have made it impractical to conduct proper exploration and research within the deeper regions of the coral-reef environment.


Figure2_Submersible-Dive_Hawaii-Undersea-Research-LaboratoryStarting in the late 1960s, scientists began using research submersibles to dive in tropical locations including Hawai’i, Marshall Islands, Red Sea, and Caribbean. These pioneering efforts to explore greater depths revealed that coral-reef habitat extends far below the reach of divers using conventional SCUBA, down to at least 150 m (500 ft) or more. However, a common frustration expressed by many of these pioneering explorers was that operating from inside the confines of a submarine made it difficult to collect specimens (especially fishes), or see the small, cryptic organisms that typically inhabit coral-reef environments.  Moreover, the cost of utilizing research submersibles (upwards of $30,000 per day or more) is prohibitive for many scientists.  When it is costing that much money to use a submersible capable of descending thousands of meters deep, it’s difficult to justify spending much time at 50-150 meters (165-500 feet). As a consequence, the vast majority of research using submersibles has concentrated at depths well below the lower limits of coral-reef habitat.


Figure3_Twilight-Zone_No-Credit-NeededThus, between the physiological limitations imposed by conventional SCUBA gear, and the cost and isolating nature of research submersibles, there is a depth zone ranging from about 50-150 meters (165-500 feet) on tropical coral reefs throughout the world that has remained largely unexplored. Originally, this zone was referred to as the coral-reef “Twilight Zone”, a term that is particularly apt, given that it represents the transition zone between the brightly-lit shallow tropical reef environment, and the dark deep-sea realm.  However, that term was later used to refer to a somewhat deeper zone in the open ocean, and also has been applied to terrestrial cave environments (in all cases referring to the same thing – the transition between the well-lit regions and the perpetually dark environment). To avoid confusion, the coral-reef “Twilight Zone” was initially re-branded as the somewhat more descriptive “deep coral reefs”; until cold-water coral reefs inhabiting much greater depths were discovered, and that term was applied to those very different kinds of non-tropical coral habitats. Eventually, the term “Mesophotic Coral Ecosystem” (abbreviated as “MCE”) was coined to refer to the tropical coral-reef “Twilight Zone”, and this term seems to have now become reasonably well-established in the scientific literature. But whatever one calls it, the importance of this environment is increasingly being recognized, and there is a rapidly growing effort to start documenting the coral-reef Twilight Zone in many parts of the world.


Figure4_Richard-Pyle-examining-black-coral-with-MK4-rebreather-in-Papua-New-Guinea---1995_Bob-HalsteadThanks in large part to the efforts of Poseidon, divers are increasingly aware of electronically-controlled closed-circuit rebreathers, and how they can dramatically improve the underwater experience.  But there is a common misconception that this technology was invented for use in commercial diving applications, or by the military.  In truth, the electronically-controlled closed-circuit rebreather was invented in the late 1960’s to allow scientists to explore the coral-reef Twilight Zone.  Specifically, the world’s first electronically-controlled rebreather – the Electrolung ( – was developed by Walter Starck and John Kanwisher and used for exploring deep tropical coral reefs, as documented in an article entitled “Probing the Deep Reef’s Hidden Realm”, published in the the December 1972 issue of National Geographic magazine. Other early deep-reef exploration pioneers, such as Patrick Colin (see: extended the boundaries of conventional scuba to document the deep-reef ecosystems, using both open-circuit trimix and home-made rebreathers.  

Figure5_Richard-Pyle-collecting-fishes-with-a-Mk-VP-rebreather-in-Vanuatu_John-LBut it wasn’t until the late 1980’s, when modern technical diving practices began to emerge, that exploration of the coral-reef Twilight Zone began in earnest. After several years of using open-circuit trimix to collect coral-reef specimens on deep coral reefs in Hawai’i and the Cook Islands, one of us (Pyle) contacted Bill Stone about using his then prototype Cis-Lunar rebreathers. Starting in 1994 with the Cis-Lunar Mk-IV (see image above of Richard Pyle examining Black Coral with Poseidon MK4 Rebreather in Papua New Guinea), our efforts to explore deep coral-reef habitat throughout the tropical Pacific has continued to expand and improve over the years. In 1997 we upgraded to the Cis-Lunar Mk-VP rebreather (see image of Richard Pyle collecting fishes with MK-VP rebreather in Vanuatu), which served us very well for many years.

Figure6_Richard-Pyle-using-a-prototype-Poseidon-Technical-rebreather-at-100m-in-Indonesia_John-LIn 2005 we began development on what became the Poseidon Discovery (Mk-6), and by 2011 we started test-diving prototypes of the Poseidon SE7EN rebreather. After 20 years of using four generations of Poseidon/Cis-Lunar closed-circuit rebreathers to explore the coral-reef Twilight Zone, in many ways it feels like we are only just getting started!


As exciting as it has been to have helped in the design, development and testing of these revolutionary rebreathers, our real focus is on the exploration of biodiversity on deep coral reefs. In that regard, we have been extremely successful over the years; particularly in terms of discovering new species of fishes. With improving equipment and techniques, we discover new fish species at an average rate of about twelve new species per hour of exploration time!

Figure7_Cirrhilabrus-earlei_Richard-LThese fishes aren’t just interesting because they are new to science; but they are also revealing patterns of geographic distribution (biogeography) that are somewhat surprising and unexpected (see image to left of Cirrhilabrus earlei by Richard L. Pyle). For example, on shallow coral reefs, there is greatest diversity in the so-called “Coral Triangle” (a region including Indonesia, Papua New Guinea, the Philippines and adjacent regions).  As one moves eastward across the Pacific, the shallow reef-fish diversity begins to drop, such that in the tropical southeastern Pacific (e.g., French Polynesia), there is barely one-fifth as many species as there are in the Coral Triangle. This pattern does not seem to exist on the Pacific deep coral reefs, where the diversity is more or less consistent from the west to the east. Also, there seems to be higher rates of endemism – that is, species found in only restricted geographic ranges – among the deep-reef fishes in the Pacific compared with the shallow-reef fishes. These and other exciting research questions are being addressed by our team in collaboration with Dr. Brian Bowen and his graduate student Joshua Copus of the Hawai’i Institute of Marine Biology.


-particularly-gorgonian-corals_Richard-LAnd fishes are only the tip of the iceberg:  for every new fish we discover, there may be dozens of undiscovered marine invertebrate species. In fact, these mesophotic coral reefs are typically characterised by a huge wealth of invertebrates, specifically gorgonian (sea fan) corals. These large, slow growing archives of the ocean traverse great depths with some groups known to inhabit from 5 - 2000 m depth! Providing secondary space to many other reef animals (e.g. the charismatic pigmy seahorse), sea fan corals act as nursery habitats specifically for many fish species, in addition to providing a protective place for certain fish species to lay their eggs! As we are increasingly able to penetrate the mysteries of these little studied reefs with the invaluable aid of the Poseidon rebreathers, the sheer diversity of these gorgonian corals is overwhelming. Yet, research on such species is still in its infancy, with many undoubtedly new to science and armed with a chemical battery of defense which provides an endless resource of potential new pharmaceuticals. Like the fishes, the biogeographic distribution of these corals show a dramatic decline eastwards across the Pacific on the shallow reefs, yet maintained on deeper reefs until you get to Hawai’i! Much is yet to be discovered of the distribution and diversity of these enigmatic corals, currently classified as having up to three times more species than hard (scleractinian) corals. It is clear that the exploration of these reefs and their inhabitants has only just begun.     


These are exciting times for undersea exploration!  With each new generation of diving technology, our research becomes logistically easier, less expensive, and – most importantly – safer! In particular, the Poseidon SE7EN rebreathers represent a major step forward in our ability to document biodiversity in the coral-reef Twilight Zone. We will continue to report our “notes from the field” over the coming months and years, and we hope you will find this work as exciting, fun, and interesting as we do!


Next Installment: Adventures in Anilao


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