The “fish with a see-through head” that periodically goes viral online is the barreleye, formally known as Macropinna microstoma. What looks like a brain exposed to the ocean is actually a pair of large, green-lensed tubular eyes sitting inside a fluid-filled, transparent dome that covers the top of the fish’s skull. The real brain is there too, tucked beneath those eyes, but the visual effect of a glowing green brain floating inside a glass helmet is what makes this animal one of the most genuinely alien-looking creatures on Earth. The barreleye’s anatomy is stranger and more purposeful than the memes suggest, and it belongs to a whole family of deep-sea fish with similarly bizarre eye designs.
What You Are Actually Seeing
When people share photos of the barreleye and label them “the fish with a transparent brain,” the anatomy is slightly misread. The two bright green orbs inside the dome are not exposed brain tissue. They are the fish’s eyes, specifically the large cylindrical lenses of its tubular eyes, which point upward through the transparent shield. The green pigment in the lenses is thought to filter out sunlight scattered from the ocean surface, helping the fish pick out the faint silhouettes or bioluminescent glow of prey and predators above it. The actual brain sits below and behind the eyes, inside the skull proper, and is not itself transparent.
The transparent structure covering everything is a fluid-filled shield made of soft, clear tissue. Researchers at the Monterey Bay Aquarium Research Institute (MBARI) confirmed its existence during remotely operated vehicle (ROV) dives, observing the shield intact on living fish at depth. Earlier specimens brought up in trawl nets had always lost the shield, which is fragile and collapses under pressure changes, so for decades scientists only knew the fish from mangled corpses and had no idea the dome existed at all.
How the Eyes Move Inside the Dome
The barreleye’s tubular eyes were long assumed to be fixed in an upward-pointing position, which created a puzzle: if the fish can only look straight up, how does it see food once it swims forward to eat? Its mouth is tiny and points forward, not upward. In situ ROV observations and lab work solved the paradox by showing that Macropinna can rotate its eyes from a dorsal (upward) position to a rostral (forward-facing) position, pivoting them inside the transparent dome to track prey as it moves from overhead to directly in front of the fish’s mouth.1BioOne Complete / Copeia. Macropinna microstoma and the Paradox of Its Tubular Eyes
This eye rotation means the fish likely hunts by hovering motionless in the deep water column, eyes aimed upward, scanning the dim overhead light for the silhouettes of small prey or the glow of bioluminescent organisms. Once it spots something, it tilts its body and rotates its eyes forward to keep the target in view as it swims toward it. The transparent dome makes this possible because there is no opaque bone or skin blocking the path between the eyes and the light source, regardless of the angle the eyes are pointing.
Why a Transparent Head
A transparent skull is a radical adaptation, and it did not evolve just to look impressive. The leading explanation is protection. Macropinna appears to steal food from siphonophores, colonial jellyfish-like animals that trail long tentacles armed with stinging cells. The transparent shield may serve as armor for the delicate eyes, allowing the fish to venture into a siphonophore’s tentacle curtain without risking damage to its primary sensory organs.2BioOne Complete / Copeia. Macropinna microstoma and the Paradox of Its Tubular Eyes The small crustaceans and other organisms trapped in siphonophore tentacles are thought to be a significant food source for the barreleye.
Transparency in the ocean is not unique to this fish. It is overwhelmingly a trait of open-water (pelagic) animals and is extremely rare on land or in other aquatic habitats like reefs and riverbeds.3PubMed Central. Hidden in plain sight: the ecology and physiology of organismal transparency Many pelagic creatures use transparency as camouflage in featureless open water where there is nothing to hide behind. In the barreleye’s case, though, the transparency serves a structural purpose rather than a camouflage one: the dome needs to be clear so light can reach the upward-aimed eyes, and it may double as a protective barrier.
The Barreleye Family
Macropinna microstoma is the most famous member of the family Opisthoproctidae, but it is not the only one with extraordinary eyes. The family includes roughly 20 known species, and all of them appear to possess the transparent shield that made Macropinna an internet sensation.4PLOS ONE. Preservation Obscures Pelagic Deep-Sea Fish Diversity: Doubling the Number of Sole-Bearing Opisthoproctids and Resurrection of the Genus Monacoa The dramatic footage of Macropinna mesmerized both ichthyologists and general viewers, but it also underscored how little we know about deep-ocean life: even within this single family, basic anatomy had been misunderstood for decades because trawl-net collection destroyed the very features that make these fish remarkable.
The diversity of eye designs across the family is striking. Some opisthoproctids have relatively simple tubular eyes pointing upward, relying on a lens to focus downwelling light onto a main retina. Others have evolved far more elaborate structures, including lateral outgrowths called diverticula that allow a single eye to simultaneously focus light from above (using the main lens) and from the sides or below (using either reflection or refraction within the diverticulum).5Frontiers in Ecology and Evolution. Diversity and evolution of optically complex eyes in a family of deep-sea fish: Ocular diverticula in barreleye spookfish (Opisthoproctidae) In practical terms, these fish have eyes that work like bifocals, splitting their visual field between two different optical systems in a single organ.
The Spookfish and Its Mirror Eye
One of the most remarkable relatives of Macropinna is Rhynchohyalus natalensis, commonly called the brownsnout spookfish. This species takes the dual-focus eye concept to an extreme. Each of its tubular eyes has a large lateral diverticulum containing a curved mirror that focuses light from below and to the side, while the main tubular portion of the eye focuses light from above using a conventional lens. Modelling suggests the mirror produces a bright, well-focused image, making it the only known vertebrate to use a mirror rather than a lens as its primary image-forming optic.6PubMed Central. Reflecting optics in the diverticular eye of a deep-sea barreleye fish (Rhynchohyalus natalensis)
Why a mirror? At the depths where these fish live, in the mesopelagic zone roughly 200 to 1,000 meters down, light comes from two very different sources. A faint wash of sunlight filters down from above, providing enough illumination to silhouette objects overhead. Meanwhile, bioluminescent flashes come from every direction as other animals produce their own light. A mirror-based optic can gather and focus the dim, scattered bioluminescent light from the sides more efficiently than a lens alone, giving the spookfish a near-360-degree awareness of its dimly lit world.
Life in the Mesopelagic Twilight Zone
The barreleye and its relatives occupy the mesopelagic zone, the layer of ocean between roughly 200 and 1,000 meters deep where sunlight fades to near-total darkness. This is one of the largest habitable zones on Earth by volume, and it teems with life: vast communities of small fish, shrimp, squid, and jellyfish form what oceanographers call the deep scattering layer (DSL), a band of organisms so dense it shows up on sonar. The upper and lower boundaries of this layer shift with light levels and dissolved oxygen concentration, moving shallower at night and deeper during the day as animals undertake massive daily vertical migrations to feed near the surface under cover of darkness.7Deep Sea Research Part I: Oceanographic Research Papers. Dissolved oxygen as a constraint on daytime deep scattering layer depth in the southern California current ecosystem
Macropinna typically occupies the lower reaches of this zone in Monterey Bay and across the subarctic and temperate North Pacific.8BioOne Complete / Copeia. Macropinna microstoma and the Paradox of Its Tubular Eyes It hangs almost motionless in the water column, its flat, broad body and large fins giving it remarkable stability, like a helicopter hovering in place. That stillness is probably essential: at these depths, food is sparse and energy is precious, so a hunting strategy that involves waiting and watching for the occasional morsel passing overhead makes more metabolic sense than active pursuit.
Climate Change and Mesopelagic Fish
The twilight zone is not as insulated from surface-world changes as its depth might suggest. Declining oxygen levels in intermediate waters off the coast of California have already been linked to sharp drops in mesopelagic fish abundance. Analysis of ichthyoplankton records from 1951 to 2008 found that mesopelagic fish populations were reduced by an average of about 63% during periods when intermediate-water oxygen concentrations were low, compared with periods when oxygen was higher. The oxygen levels themselves fluctuated by roughly 20% between the worst and best decades in that time series.9Marine Ecology Progress Series. Impact of declining intermediate-water oxygen on deepwater fishes in the California Current
The proposed mechanism is straightforward: when oxygen drops, the hypoxic boundary (the depth below which oxygen is too low for many animals) rises closer to the surface. That compresses mesopelagic fish into a narrower band of livable water, which in turn makes them more vulnerable to predators that hunt by sight from above. Climate-driven ocean deoxygenation is expected to worsen this compression over time, with the upper boundary of the deep scattering layer shoaling faster than the lower boundary, effectively squeezing the habitable layer thinner and thinner.10Deep Sea Research Part I: Oceanographic Research Papers. Dissolved oxygen as a constraint on daytime deep scattering layer depth in the southern California current ecosystem For specialized species like the barreleye, whose entire suite of adaptations is tuned to a narrow depth band and a specific light environment, habitat compression could be a serious threat, though direct data on barreleye population trends are essentially nonexistent because the fish are so rarely encountered.
Why the Barreleye Is So Hard to Study
Almost everything we know about living Macropinna comes from a handful of ROV encounters. The fish is fragile, solitary, and lives at depths that are impractical to visit repeatedly. Trawl nets, the traditional tool for sampling deep-sea fish, destroy the transparent dome and distort the body so severely that earlier researchers described the fish as having a flat, featureless head. For more than 70 years after its initial description in 1939, nobody knew the dome existed. The PLOS ONE study on opisthoproctid diversity made the point explicitly: preservation methods obscure the very features that define these animals, and the true diversity of barreleye fish has been significantly underestimated as a result.11PLOS ONE. Preservation Obscures Pelagic Deep-Sea Fish Diversity: Doubling the Number of Sole-Bearing Opisthoproctids and Resurrection of the Genus Monacoa
This problem is not unique to barreleyes. Many deep-sea organisms have gelatinous or transparent tissues that collapse or become opaque when brought to the surface. ROVs and submersibles have revolutionized our understanding of deep-sea life by observing animals in their natural habitat, but ROV time is expensive and the ocean is vast. We are still in the early stages of cataloguing what lives in the mesopelagic zone, let alone understanding how those animals behave, reproduce, or interact with one another.
Other Fish With Transparent Heads and Bodies
The barreleye is not the only fish whose internal anatomy is visible from the outside. A very different group of tiny freshwater fish, the genus Danionella, has attracted growing attention from neuroscientists precisely because their skulls remain optically transparent into adulthood. Most vertebrates’ skulls become opaque as bone mineralizes during development, but Danionella species keep their bones thin and translucent enough that researchers can image brain activity through the intact skull without surgery.
Danionella cerebrum, for example, has one of the smallest known adult vertebrate brains, and researchers have demonstrated that these fish can perform spatial navigation tasks despite their tiny brain volume, making them a promising model for studying how vertebrate neural circuits produce complex behavior.12Current Biology. Visual spatial navigation and its neural correlates in adult Danionella cerebrum A related species, Danionella dracula, has been used for label-free imaging of brain development from larval through adult stages, taking advantage of optical access that would be impossible in almost any other vertebrate.13PubMed Central. Label-Free Multiphoton Imaging Reveals Volumetric Shifts Across Development in Sensory-Related Brain Regions of a Miniature Transparent Vertebrate
The transparency of Danionella and the transparency of Macropinna evolved for completely unrelated reasons in completely unrelated lineages. In Danionella, small body size and reduced skull ossification are likely byproducts of miniaturization. In the barreleye, the dome is a specialized structure that enables a unique visual hunting strategy. But both cases illustrate the same broader principle: when an animal’s internal structures become visible, scientists (and the internet) pay attention. For the barreleye, that visibility is a hunting tool millions of years in the making. For Danionella, it is an accident of anatomy that turned a tiny, unremarkable-looking fish into one of neuroscience’s most exciting new model organisms.
The Two Small Dots That Fool Everyone
One final anatomical confusion worth clearing up: those two dark spots on the front of the barreleye’s face, just above its tiny mouth, are not its eyes. They are its nares, the fish equivalent of nostrils, used for detecting chemical traces in the water. Nearly every photo or video description of Macropinna labels those nares as eyes because they sit exactly where you would expect eyes to be on a normal fish. The actual eyes are the large, bright green cylinders inside the dome, pointing upward. Once you know this, the fish looks less like a cartoon character and more like what it is: a precision-built predator with optics so specialized that the entire front half of its head was remodeled to accommodate them.