Tropical vacationers know well that the coral reefs of the Caribbean are home to hundreds upon hundreds of unique and gorgeous fish species. But what you can see with a snorkel and mask, or even full scuba gear, doesn’t begin to cover the breadth of life present in these marine biology wonderlands. To properly explore, you’d need to helm a deep-diving submersible equipped with state-of-the-art tools—exactly the sort of vehicle that research zoologist Carole Baldwin from the National Museum of Natural History commands with her team off the island of Curaçao.
The spacious five-person sub was conceived and commissioned by local businessman and scuba junkie Adriaan “Dutch” Schrier on the occasion of his 60th birthday, as a means of more safely accessing deep waters in his later years. With its bulbous transparent cockpit and cutesy orange-and-white paint job, the whimsical craft would do Jacques Cousteau—or Steve Zissou—proud. Capable of descending to depths of up to 1,000 feet, its potential utility as a scientific tool was immediately evident to its owner.
Since 2011, Schrier has granted submersible access to Carole Baldwin and her fellow Smithsonian researchers, members all of the Deep Reef Observation Project (DROP). Christened the Curasub, the vessel has made regular dives along a species-rich reef slope off the island’s coast. What Baldwin and the others have observed has upset fundamental notions of reef biology and its distribution.
In a just-published Nature paper titled “Below the Mesophotic,” Baldwin and her coauthors lay out major revisions to the zonal classification scheme scientists have long used to describe reef ecosystems. Whereas the prevailing notion among marine biologists was that fishes from the mesophotic (“medium light”) zone abruptly gave way to deep-sea fauna at a depth of around 500 feet, a vast dataset Baldwin compiled with the Curasub strongly suggests otherwise.
Having collected 4,500 distinct depth observations of more than 70 fish species along the Curaçao reef slope, the DROP team plotted these data points graphically and subjected them to cluster analysis, determining the extent of various groupings of fish species and linking them with discrete ranges of depth. What the scientists had observed below the mesophotic zone was not the expected deep-sea fishes, but rather an unacknowledged community all its own.
“When we looked at our data,” Baldwin says, “we came out with not only this mesophotic cluster, but this whole other cluster about 400 to 1,000 feet below the surface.” Baldwin and her co-authors coined a name for this region: the rariphotic (“scarce light”) zone.
“The mesophotic does not transition into the deep sea,” Baldwin says—the evidence is conclusive. “There is a previously unrecognized zone that bridges the gap between those two. That’s what this new rariphotic zone is.”
Many of the species in the rariphotic were unfamiliar, and required on-the-fly labeling and classification by the science team. On the whole, the fishes here were much more closely related to shallow reef fishes than to the creatures of the deep sea. “We weren’t seeing anglerfishes and viperfishes and other deep-sea things,” Baldwin says.
Within the rariphotic, the team pinned down two distinct but related sub-clusters of species, leading to an “upper rariphotic” and “lower rariphotic” dichotomy neatly mirroring that of the upper and lower mesophotic. This breakdown might be subject to revision, however, as the full extent of the rariphotic becomes known. Right now, the team is certain that the newly named zone penetrates to at least 1,000 feet in depth—but it’s quite possible rariphotic species don’t give way to deep-sea fishes until a point significantly beyond that.
For completion’s sake, the team also invented a name for the previously anonymous region extending from the surface to a depth of 130 feet: the altiphotic (“high light”) zone.
While exciting on its own, Baldwin’s overhaul of the zonal classification system is also likely to pave the way for more accurate follow-up science—in particular, research concerning the effects of temperature on the migratory movements of fishes.
The DROP team has been monitoring temperature in the waters off Curaçao since nearly the project’s inception. Eleven oceanographic thermometers spaced between 50 and 800 feet in depth have continually provided temperature data on the reef slope for years. Now, with all the new species and depth data laid out in the rariphotic study, Baldwin’s cohort has a wonderful opportunity to apply its temperature data in a way that was previously impossible.
“We’re beginning to overlay the temperature data on these depth profiles of fish species,” Baldwin says, “to try to come up with each species’ preferred temperature regime. What we want to know is whether fish species are moving deeper in response to warming surface waters.”
It’s been well established in recent literature that fishes are capable of changing latitudes in response to rising temperatures, but Baldwin wants to know if fishes near the equator—for whom escaping excess heat in this way would be less feasible—are changing depths instead.
“If you live in the tropics,” Baldwin says, “you might have to go quite a distance north or south to get to cooler waters, but you only have to go a few meters deeper.” DROP has already uncovered species of goby that long ago branched off from their shallow-reef ancestors to descend to cooler waters, so Baldwin thinks it likely that modern fishes are making the same move in response to climate change. The classification of the rariphotic zone will make such migration patterns much easier to detect and understand.
Equally exhilarating for Baldwin is the frequent discovery of new species in the region, which continues apace to this day and is only likely to ramp up as the team takes Curasub on field trips further and further from the Curaçao mainland. Where once it was confined to a vanishingly small operational swath of sea (about a tenth of a square mile in area) along the Curaçao reef slope, the team now has access to a large ship on which they can haul Curasub to destinations of their choosing. Given the amazing diversity of life DROP has uncovered even in its narrow slice of Curaçao’s reef, these trips are sure to turn up plenty of fresh findings down the road.
(One recently discovered Curaçao species that Baldwin is keen to christen is a striking rariphotic seabass in the genus Baldwinella, which ichthyologists Phil Heemstra and Bill Anderson titled in her honor back in 2012. “I’ll probably name that one for a very big donor,” Baldwin says with a chuckle. “It’s a really beautiful fish.”)
Baldwin has been exploring the seas for quite a while now, but her passion for marine biology is unflagging. Accounting for depth, she says, “More than 95 percent of the livable space on the planet is in the oceans. And we’ve explored less than five percent of that. To me, that’s just unbelievable.”
“Articles like this one in Nature help people understand just how little we do know about the ocean,” Baldwin says. “And when we describe new species, that lets people know that there’s still a heck of a lot to learn.”