Deep beneath the Pacific Ocean, where sunlight barely reaches, scientists encountered a fish that looks almost impossible to believe. Its head is completely transparent, revealing two green eyes seemingly suspended inside a glasslike dome.
The unusual creature is the barreleye fish (Macropinna microstoma), a deep-sea animal whose extraordinary appearance has fascinated researchers. During a recent expedition, scientists captured remarkably clear 4K footage of the rarely seen fish, offering a closer look at an animal whose unusual anatomy is closely connected to how it finds food and survives.
The encounter took place Oct. 22 during an expedition by researchers from the Monterey Bay Aquarium Research Institute (MBARI). While exploring Monterey Canyon aboard the research vessel David Packard, a member of the team spotted the fish on the camera of its remotely operated vehicle, Doc Ricketts.
The research site was approximately 90 kilometers (55 miles) off the California coast.
But what makes this fish particularly fascinating isn't simply its transparent appearance. Its unusual head conceals a remarkable visual system that helps it hunt in the darkness of the deep ocean.
A Transparent Head That Helps It Survive in the Deep Sea
The barreleye fish has one of the most unusual head structures found among deep-sea animals.
Its head is covered by a gelatinous, translucent dome, giving it the appearance of wearing a clear diving helmet. Inside that structure are its eyes, positioned within the head rather than on the outside like those of most familiar fish.
The fish lives in the ocean's mesopelagic zone, commonly known as the twilight zone. With limited sunlight reaching these depths, it relies on specialized adaptations to find food and survive. Its unusual eyes allow it to scan the water ahead and above for potential prey, including subtle shadows moving overhead.
The transparent dome also serves a practical purpose when the fish feeds. Some of its prey are found among the stinging tentacles of siphonophores, marine animals related to jellyfish. The barreleye can collect small crustaceans trapped among these tentacles while its gelatinous covering helps shield its eyes from the stinging structures.
Its hunting strategy involves more than its unusual head. Large fins allow the fish to hover in place while searching for food, and its eyes can rotate to keep prey within sight.
What looks like a bizarre transparent helmet is actually part of a specialized system that helps the barreleye find food and survive in the deep ocean.
Even Experienced Scientists Were Amazed by the Encounter
Seeing a barreleye in its natural environment is a special opportunity, even for researchers who have spent years exploring the ocean.
Steven Haddock, the expedition's senior scientist, told Discover Wildlife that he had encountered the fish a few times before. However, his previous sighting had been nine years earlier, before the research team had 4K cameras on its submersible.
Describing the animal, Haddock said:
“It is such an amazing creature, which looks like it was created by an imaginative child. If you compare historical drawings of trawled specimens, the transparent dome surrounding their eyes has been stripped away. I didn’t even think they were illustrating the same species. So to see it alive with the dome of its ‘cockpit’ intact is a real privilege.”
His observation highlights an important challenge in studying deep-sea animals. When barreleye fish specimens were brought to the surface in trawling operations, their delicate transparent domes had been stripped away. Historical illustrations therefore did not always show the fish's complete appearance.
The new footage provides researchers with a clearer view of the animal as it actually lives underwater.
The fish's unusual appearance also caught the attention of YouTube viewers. As reported by the New York Post, one viewer described the mixed reaction that the transparent bodies of deep-sea creatures can inspire:
“The transparency of deep-sea critters always amazes me, intrigues me to look closely and examine it, but also kind of icks me at the same time.”
Another viewer summed up the strange appeal of the ocean's creatures in just a few words:
“beautiful and horrifying, just like the ocean.”
What the Footage Could Reveal About Deep-Sea Vision
The discovery is also valuable because it offers researchers an opportunity to study how animals see and survive in the ocean's middle depths.
Haddock explained that the barreleye is an example of the remarkable ways marine animals have adapted to life in the midwater environment.
As he told Discover Wildlife:
“Macropinna reminds us of the surprising ways that animals have adapted to thrive in the ocean’s midwater. So many of them survive by looking for subtle optical cues, like a shadow passing overhead, and this sets off evolutionary competition, pitting bioluminescent camouflage against increasingly sophisticated visual capabilities.”
The 4K footage could help scientists further understand how vision works in the darkness of the mesopelagic zone. It also provides a closer look at the adaptations that allow deep-sea animals to detect prey and navigate an environment with very little sunlight.
The barreleye was filmed during the eight-day Diving Deep for Delicate Drifters expedition. MBARI scientists and their collaborators were studying how gelatinous animals, including jellies, siphonophores and ctenophores, have adapted to deep-sea conditions.
For a fish that looks as though it belongs in a science-fiction film, its transparent head has a surprisingly practical purpose. It helps the barreleye watch for prey above it while providing protection as it feeds among the ocean's stinging tentacles.
In the deep sea, where finding food and avoiding danger can be difficult, even an extraordinary-looking feature can serve as an ingenious survival adaptation.
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