“It’s estimated that 95 percent of the livable space on our planet is in the ocean,” said Carole Baldwin, curator of fishes at the Smithsonian’s National Museum of Natural History, lead author of the study and director of the Smithsonian’s Deep Reef Observation Project (DROP). “Yet only a fraction of that space has been explored. That’s understandable for areas that are thousands of miles offshore and miles deep. But tropical deep reefs are just below popular, highly studied shallow reefs—essentially our own back yards. And tropical deep reefs are not barren landscapes on the deep ocean floor: they are highly diverse ecosystems that warrant further study. We hope that by naming the deep-reef rariphotic zone, we’ll draw attention to the need to continue to explore deep reefs.”

The authors defined the rariphotic based on depth observations of about 4,500 fishes representing 71 species during approximately 80 submersible dives to as deep as 309 meters. Most of the fishes in the rariphotic zone not only look similar to shallow reef fishes (photos) but are related to them rather than to true deep-ocean fishes, which belong to quite different branches of the evolutionary tree. This research showed that assemblages of the kinds of reef-fishes that inhabit shallow water in fact have double the depth range they were previously thought to have.

Since 2011, when DROP began, more than 40 researchers, most from the National Museum of Natural History and the Smithsonian Tropical Research Institute (STRI), have intensively studied deep-reef fishes and invertebrates off Curaçao. They named six new genera and about 30 new species as they explored a 0.2 square kilometer (0.08 square mile) area of reef, much of which is too deep for enough light to penetrate to support the algal symbionts on which reef-building corals rely.