While we adapt to shorter, darker days with less sun and more snow, an ultra-rare sea creature well adapted to darkness was recently sighted again.
In the deep sea—some 600 to 800 feet underwater, where sunlight dwindles to near pitch-black—there dwells a fish with unusual features well suited to that extreme environment. Several species of the Opisthoproctidae fish family are aptly dubbed “barreleyes,” because of their elongated, tube-shaped eyes that allow them to make their living in the ocean’s “twilight zone.”
The barreleye Macropinna microstoma is not a large fish, measuring only 15 centimeters (6 inches) long; nor are its black scales and large black fins that unusual. The strangeness of this fish is in its extraordinary headgear.
The two small cavities just above its mouth (where its eyes should be) are not actually eyes, but “nares,” or fish nostrils, connected to its olfactory organs. The two large, glowing green orbs inside the fish’s transparent, fluid-filled forehead are its actual eyes, looking directly upward through its forehead like a pilot looks out the window of a fighter plane. Researchers believe the glowing green color filters out sunlight that would otherwise make spotting prey difficult—barreleyes feed on bioluminescent jellyfish and small crustaceans caught in their tentacles.
Before a remote operated vehicle (ROV) sighted this fish in 2009, this transparent forehead feature was excluded in barreleye illustrations, as previous specimens caught were all damaged and lacked this part. As of 2021, barreleyes have only been observed by ROVs nine times. Scientists from MBARI also managed to net one and bring it to the surface, uncovering the mystery of how barreleyes, with their upward-facing eyes, manage to hunt and feed without looking forward.
Researchers previously thought its eyes were fixed in the upward position, but the netted catch confirmed what the 2009 ROV sighting pointed to: when the fish spots prey and swims in for the kill, its tube eyes tilt forward to focus on its food.
In 2009, MBARI researchers Bruce Robison and Kim Reisenbichler hypothesized the deep-sea dweller’s modus operandi: Most of the time, barreleyes use their large fins as stabilizers in the water (similar to ROVs) as they scan upward for prey—such as bioluminescent jellies that can grow to over 10 meters (33 feet) long called siphonophore, which drag a “net” consisting of thousands of long, stinging tentacles. Maneuvering carefully, barreleyes pick off copepods and other small animals captured by these net jellies.
The latest barreleye sighting was on Dec. 1, 2021, when aquarist Tommy Knowles and a team aboard MBARI research vessel Rachel Carson deployed ROV Ventana to collect jellies for the aquarium’s upcoming “Into the Deep” spring exhibition. Of the sighting, the researchers wrote on MBARI’s YouTube page: “The team stopped to marvel at Macropinna before it swam away.”
Although we have much to learn about this strange swimmer of the deep, advancements in remote underwater robotics allow us to uncover some of their mysteries and understand barreleyes’ adaptations for surviving in darkness—as we must do for a few more months.
