It takes innovation to survive in a dark world. For the barreleye fish, that means directing its tubular eyes through the clear shield covering its head to identify the silhouettes of jellies and other prey drifting though the deep sea.
For the past 70 years, scientists have thought the barreleye’s globby green peepers could only stare up through the top of its head, limiting its field of vision to a narrow tunnel. Not so, say Monterey Bay Aquarium Research Institute scientists Bruce Robison and Kim Reisenbichler in a new paper, according to a Feb. 23 MBARI press release: The barreleye can also rotate its weird eyes forward to focus on what it’s eating.
The scientists analyzed videos collected by MBARI’s remotely operated vehicles in the deep waters off Central California’s coast. “At depths of 600 to 800 meters below the surface,” MBARI reports, “the ROV cameras typically showed these fish hanging motionless in the water, their eyes glowing a vivid green in the ROV’s bright lights.” A transparent, fluid-filled shield surrounds the fish’s eyes and gives it a clear view through the top of its head.
Robison and Reisenbichler hypothesize that the barreleye hangs motionless in the water most of the time, its body horizontal and its eyes peering upward. When it sees prey, it rotates its eyes forward and swims toward it vertically.
“It is only with the advent of modern underwater robots that scientists have been able to observe such animals in their native environment,” MBARI reports, “and thus to fully understand how these physical adaptations help them survive.”