Off the coast of California near Monterey, researchers filmed one of the most unusual creatures in the deep sea. The Barreleye fish is the kind of creature that captures the imagination and evokes wonder about what remains undiscovered in the deep ocean. Known by the scientific name Macropinna microstoma, this fascinating fish resembles something straight out of a sci-fi movie. With a transparent head and tubular eyes, the Barreleye fish navigates the dark depths of the ocean with an appearance and biology that is both stunning and biologically fascinating.

First described in 1939, the fish astonished scientists who were stunned by its unique optical anatomy. The barreleye is found in the Pacific Ocean, with sightings ranging from the coasts of California, particularly around Monterey Canyon, to the mid-Pacific near Hawaii. Most commonly, it resides between 400 to 2,500 feet below the surface—a region known as the mesopelagic or “twilight” zone. At these depths, little light penetrates, making the area a seemingly inhospitable place for most life forms. But the Barreleye fish thrives here, adapting to its surroundings in the most bizarre ways.

Its most distinctive features, and the ones that give it its name, are its transparent head and barrel-shaped eyes that are usually directed upwards. These eyes are extremely sensitive to light, which is a scarce commodity where it lives. Interestingly, the eyes are encased in a dome-shaped, transparent head. This clear cranium allows the fish to capture as much light as possible, increasing its visual field. The upward-facing eyes allow the Barreleye fish to see silhouettes of prey or predators against the faint light filtering down from the surface. It’s like a built-in periscope for scanning the world above, allowing it to spot the bioluminescent glow of jellies or small fish that meander above it.

In 2009, researchers at the Monterey Bay Aquarium Research Institute elucidated a longstanding mystery surrounding the barreleye. For many years, marine biologists were under the impression that these specialized eyes were immobile and only provided the fish with a limited, tunnel-vision perspective, focused solely on the waters above its transparent head. Such a constraint would significantly limit the fish’s situational awareness, making it highly dependent on what occurs directly above it for both prey detection and predator evasion.

However, a groundbreaking paper by researchers Bruce Robison and Kim Reisenbichler overturned this conventional wisdom. Their findings reveal that the eyes of the barreleye fish are not static but can actually rotate within a transparent protective shield that envelops the fish’s head. This adaptation enables the fish not only to look upwards to identify potential prey but also to focus forward, thereby expanding its field of vision and facilitating more effective foraging.

Robison and Reisenbichler conducted their research using footage obtained from the Monterey Bay Aquarium Research Institute’s remotely operated vehicles (ROVs) to investigate the barreleye fish in the deep-sea regions adjacent to Central California. Situated at depths ranging from 600 to 800 meters (approximately 2,000 to 2,600 feet) beneath the ocean’s surface, the ROVs’ cameras typically captured images of these fish in a state of immobility, floating like zeppelins with their eyes radiating an intense green hue due to the illumination provided by the ROV’s powerful lighting system. The remotely captured video data also divulged a hitherto undocumented anatomical feature—namely, that the eyes of the barreleye fish are encased in a transparent, fluid-filled protective shield that encompasses the upper region of the fish’s cranial structure.

The implications of this discovery extend beyond mere academic curiosity. Understanding the unique visual system of the barreleye fish provides crucial insights into the mechanisms of evolutionary adaptation. It showcases how even seemingly minor anatomical modifications can result in significant survival advantages in the highly competitive and challenging marine environment. Moreover, it challenges our existing perceptions and encourages scientists to revisit and reevaluate other long-standing assumptions in marine biology.

The Barreleye isn’t just a pair of eyes swimming around in the ocean, though. It has a suite of other adaptations to its challenging environment. For instance, it has large, flat fins that let it remain almost motionless in the water, conserving energy in an environment where every calorie counts. Also, it’s got a small mouth. This mouth is adapted to consume tiny organisms, like zooplankton, that are abundant in the deep ocean. So while the Barreleye may not be the apex predator down there, it has carved out its own unique niche.

The fish also has what are called “lateral line canals” that are filled with fluid and are sensitive to changes in pressure. This allows the Barreleye to detect movement in the water, effectively giving it a “sixth sense” to sense prey or predators around it. Imagine you’re trying to navigate a pitch-black room—every little bit of extra information helps.

Despite its enigmatic nature, we know relatively little about its breeding habits, lifespan, or social interactions. Part of the reason is the difficulty in studying a creature that lives in such an extreme environment. Researchers have managed to capture only a few specimens, and observations in their natural habitat are relatively scarce.

While the Barreleye fish might look like a figment of a wild imagination, it’s very much a real creature, adeptly adapted to its harsh surroundings. It represents the myriad ways life can evolve to meet the unique challenges of extreme environments. As we continue to explore the depths of our oceans, who knows what other fantastical creatures we might find? But for now, the Barreleye fish remains one of the most compelling arguments for the strange and wondrous biodiversity hidden in the ocean’s twilight zone—a testament to the endless creativity of evolution.