Avi KlapferA rare Cocos batfish walks along the sea floor with its limb-like fins. The author saw at least seven of these fish at a depth of about 110 feet near Cocos Island, off the Costa Rican coast.It was an extraordinary day for me. To understand why, it may be useful to explain a bit about my research. Using advanced closed-circuit mixed-gas rebreather technology, my colleagues and I have spent the past two decades traveling the tropical Pacific Ocean to to document life that occurs below the realm of scuba divers. The environment that I focus on — tropical reef habitats at depths of about 200 to 500 feet — has variously been referred to as deep coral reefs, mesophotic coral ecosystems and the coral-reef “Twilight Zone.” The last of these seems the most appropriate because this depth range under clear tropical coastal seas is a transition zone between the brightly lit shallows and the perpetually dark abyssal depths – a literal twilight zone. But the label has a dual meaning because this depth zone harbors an amazingly rich community of previously undocumented life forms and is in many ways one of the most mysterious and underexplored habitats on earth.My colleagues and I have discovered more than 100 new species of fish throughout the Pacific within this coral-reef twilight zone. Our average rate of discovery is nearly 12 new species of fish per hour of exploration time! Nowhere else on earth can one find so many new species of vertebrates so quickly. The tricky part, of course, is getting there. Breathing a helium-rich mixture in our rebreathers, we can safely access the full depth range of the coral-reef twilight zone, but we’re very limited in how much time we can spend at such depths. A typical dive to 400 feet may last as long as 30 to 45 minutes on the bottom, followed by upwards of seven to eight hours of decompression time to return safely to the surface (necessary to avoid a crippling case of the bends). Deep-sea submersibles have been very effective tools for exploring deep ocean habitat. However, they typically cost tens of thousands of dollars per day to operate and consequently are most often used to explore much deeper habitats. And it’s very difficult to capture specimens of motile fishes from submersibles. Richard L. PyleA school of Japanese bigeyes around an undersea pinnacle.But what submersibles do offer is a great deal of time on the bottom. My longstanding dream has been to combine the best of both technologies, and the reason the other day was so extraordinary was that I took another step closer to realizing that dream.For the first time in my life, I completed both a rebreather dive and a submersible dive on the same day. The rebreather dive was in the morning and wasn’t particularly deep — about 110 feet. But it was a critical first step in this project because it allowed me to test a new version of software installed on the most current generation of rebreather that I’ve been helping to develop. Although it was intended as a test dive, it was scientifically interesting as well. My dive partner was Avi Klapfer (the founder of the Group that operates the Argo, the vessel on which we’re living during this expedition), and he and I came across more than a half-dozen Cocos batfish, Ogcocephalus porrectus — a species usually found at much greater depths. Avi has been diving here at Cocos Island for many years, and he has rarely seen so many of this species in such shallow water. We’re not sure why we found so many of these fish at shallow depths, but it probably had to do with an upwelling of cooler water to this level. In any case, we will certainly be monitoring other species to see if this apparent anomaly is part of a more general trend. All in all, a very successful first dive.But the day was not over. After lunch, I was excited to climb aboard the DeepSee submersible for the first time. Unlike most such submersibles, which usually consist of a metal sphere with small portholes (a necessary design for going very deep), the pilot and two passengers aboard DeepSee sit inside a transparent acrylic sphere. The advantage of this design is that it provides for an utterly spectacular visual experience. With Avi piloting the sub, I joined Greg MacGillivray, director and founder of One World One Ocean, in the sub as we descended to the base of a large undersea pinnacle known as Mount Everest. At a depth of 280 feet, one of the first fish I saw was the rare scythe butterflyfish (Proganthodes falcifer). There are several butterflyfishes in the genus Proganthodes, one of which I discovered while rebreather diving at 420 feet in Hawaii, so this group is of particular interest to me. I was also excited to see several small rainbow basslets (Liopropoma fasciatum), the eastern Pacific member of another genus I study and of which I have several new species in the central and western Pacific to describe. We also saw vast schools of Japanese bigeyes, locally known as catalufa aleta larga” (Cookeolus japonicus) and well as countless Pacific creolefish (Paranthias colonus). As exciting as the fish-life was, it was hard to focus on science because I was so distracted by the experience of viewing the undersea world in all directions. It was an experience I will never forget and one that I hope I will soon repeat.One World One OceanThe author inside the acrylic sphere of a high-tech submersible before rejoining the Argo.Richard Pyle, an ichthyologist and database developer at Bishop Museum in Honolulu, is exploring and documenting marine biodiversity as a science adviser for the One World One Ocean initiative. In a series of posts for the Green blog, he describes his recent expedition to Cocos Island, off the Costa Rican coast.