Seeing a fish float through water so dark and deep that sunlight has never touched it has a subtly humble quality. That is geography, not poetry. Most marine biologists dedicate their entire careers to gathering the kind of footage that scientists using remotely operated vehicles and submersibles captured in waters off Puerto Rico at depths exceeding 6,000 meters. There was more than just data returned. It was a window into a world that continues to astonish despite all of our ocean exploration.
Under the direction of SUNY Geneseo researchers and with funding from NOAA Ocean Exploration, the study examined video from thirty deep-sea dives that ranged in depth from 250 to more than 6,300 meters. Throughout the tropical Atlantic and Caribbean Sea, the participating vehicles—NOAA’s ROV Deep Discoverer, ROV Global Explorer, and Woods Hole’s submersible Alvin—recorded 1,137 observations of at least 94 fish species. Reaching this depth and returning biological footage that can be used is a true first for NOAA and advances our knowledge of how ocean life organizes itself under extreme cold and pressure.
The footage shows something striking, but not particularly shocking. Fish diversity decreases with increasing depth. The number of fish decreases. The diversity of behaviors and forms thins out into something more specialized and austere. Cusk eels, long-bodied members of the Ophidiidae family, were the most commonly seen fish at abyssal and hadal depths, the lowest zones, close to the Puerto Rico Trench. In the video, they can be seen almost continuously, moving through areas where very little else is able to survive. They are silent and methodical.
It’s worth stopping to consider that particular detail. Not only were the cusk eels there, but they were also in charge. This largely unstudied group has found a solution in environments where conditions are as hostile as biology permits. The prevalence of cusk eels at such depths indicates that they should receive far more research attention than they have in the past, though scientists are still figuring out exactly what this means.
Body shape is one of the findings’ more fascinating themes. Fish found at deeper depths typically had an elongated, eel-like appearance. According to the researchers, those in shallower zones appeared more “normal.” This pattern’s explanation is still up for debate. One theory is that longer bodies enable deep-sea fish, which are endurance swimmers in a setting where meals are infrequent and far apart, to travel the great distances required to locate food. According to a different theory, a longer body increases the number of receptors in the lateral line, the sensory organ that fish use to sense movement and vibrations in the water. This would be crucial in almost complete darkness. Both factors might be true. It’s also possible that something completely different is happening that hasn’t been discovered yet.
This study’s methodological interest stems from its use of in situ observation instead of trawl nets. Nets can’t show you how a fish behaves, such as whether it hovers above the seafloor, how it reacts to a predator, or what it actually does with its hours. However, physical collection has its place in marine science. That calculus was altered by the video footage. Fish that are normally categorized as open-water swimmers were captured on camera interacting with the seafloor and, in a few instances, barely escaping being ambushed by bottom-dwelling anemones. In a net sample, those moments would not have been visible.
The video also showed trash, which is socially significant but less exciting from a scientific standpoint. Throughout the deep-sea recordings, human debris frequently showed up in footage from depths that most people believe are unaffected by human activity. In the midst of an otherwise remarkable scientific accomplishment, it is difficult to ignore that particular detail.
Part of the research was done as a course-based undergraduate experience at SUNY Geneseo, where students assisted with data analysis, result writing, and hypothesis generation. This framing is important because it indicates that a new generation of scientists took the time to carefully examine an area of the ocean that is still largely unknown in many significant ways.
