The deep oceans are full of extraordinary creatures, adapted to the frigid environment — but few are as striking as the barreleye fish. After all, a fish with a transparent head and two vivid green eyes that can swivel sounds like the stuff of Sci-Fi. But it’s not one fish species with a transparent head: there’s at least three types of them, and they’re all weirder than you think.
Spooky fish
Barreleye fish are technically called Opisthoproctidae, sometimes referred to as spookfish. They’re not exactly a uniform group. In between the stout, deep-bodied Macropinna and the slender and elongated Dolichopteryx, you have several different types of spookfish, each with their own particularities. Yet, they all have one thing in common: large, telescopic eyes that protrude from a large transparent head they are enclosed within.
Macropinna microstoma is probably the most famous fish from this family. It’s instantly recognizable with its transparent, fluid-filled head. This species, which inhabits depths between 600 to 800 meters (2,000 to 2,600 feet), is equipped with tubular eyes that are capped with vivid green lenses.
The green lenses act as sunglasses. The green tint (actually created by a yellow pigment) allows barreleyes to differentiate between the blue sunlight trickling down from above and the glow of bioluminescence that many deep-sea creatures use to disguise their silhouettes. This adaptation gives the barreleye a visual edge, enabling it to detect the faint, telltale glimmers of prey that other animals would overlook as they try to blend into the backdrop of the open water.
But that’s not the only surprise regarding their eyes.
A New Perspective
For years, marine biologists mistakenly believed that the barreleye’s eyes were fixed in place, granting it only upward vision. Such an adaptation is logical for a creature living in the dark, looking for silhouettes against the faint surface light. However, MBARI researchers Bruce Robison and Kim Reisenbichler revealed a surprising behavior in 2008. Macropinna microstoma can rotate their eyes within their transparent heads. So, the fish can move its gaze from straight upward to directly forward, aiding it in targeting prey with remarkable accuracy.
“It always puzzled me that their eyes aimed upward, but the field of view did not include their mouths,” Robison said when making the discovery. Imagine trying to eat stuff that’s floating in front of you while your eyes force you to look upwards to the ceiling — it doesn’t make too much sense.
The mystery was unraveled when researchers caught a live specimen. Despite their biology which suggests the eyes are rigid, the fish can actually move their eyes. “Suddenly the lightbulb lit and I thought ‘A-ha, that’s what’s going on!’,” he says. “They can rotate their eyes.”
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But why the transparent heads?
It may all boil down to the eyes again.
Tubular eyes maximize the amount of light that can be captured, allowing these fish to detect faint silhouettes and movements. By concentrating light into a small field of view, tubular eyes provide a high level of sensitivity, which is crucial for spotting prey in near-darkness. The transparent head allows the eyes to collect even more incident light. But there’s another part to it: protection.
When researchers studied barreleyes, they found siphonophores’ tentacles, as well as animals that siphonophores feed on.
Siphonophores are complex, colonial marine organisms belonging to the phylum Cnidaria, which includes jellyfish and corals. Some siphonophores, like the Portuguese man o’ war, have long, venomous tentacles that can deliver painful stings. Based on the findings, the researchers say it could mean that barreleyes swim up to siphonophores and steal the prey from their tentacles. If this is the case, the transparent head would act as a helmet that protects their eyes from stings.
Curious Creatures
Despite being documented in the early 20th century, it wasn’t until 2004 that scientists managed to observe Macropinna microstoma alive in its natural habitat. The breakthrough came courtesy of researchers from the Monterey Bay Aquarium Research Institute (MBARI), who utilized specialized remotely operated vehicles (ROVs) to dive into the inky depths.
Although researchers have been looking for them for decades, they’ve only spotted them a handful off times. “We do spend a lot of time exploring down there, so I can say with some confidence that they’re quite rare,” Robinson mentioned.
It’s not just Macropinna — other types of barreleyes are just as mysterious.
Opisthoproctus barreleyes also possess tubular eyes, but they lack the fully transparent head seen in Macropinna. Instead, their eyes point upward to detect faint light sources, making them highly efficient at spotting prey in near-total darkness.
Opisthoproctus species tend to be more elongated and slender compared to their Macropinna cousins. They are also masters of stealth, often floating motionless in the water column, waiting for tiny crustaceans or other small organisms to drift within range. This sit-and-wait hunting strategy conserves energy — a vital adaptation in the nutrient-poor environment of the deep ocean.
Dolichopteryx is perhaps even more unusual. This fish takes tubular eye evolution one step further: in addition to its upward-facing eyes, Dolichopteryx has evolved a second set of reflective mirrors located behind the main lenses. These mirrors direct light to its retina from below, allowing it to spot prey and potential predators coming from any direction.
By using mirrors instead, these fish gain a wider field of view, critical for detecting the faintest bioluminescent signals in their dark, deep-sea habitat. The mirrors are composed of tiny, precisely arranged crystals, a marvelous and unusual example of natural engineering.
There’s much we don’t yet know
Despite decades of research, much of the deep ocean remains unexplored, and the barreleyefish are a perfect example of the strange, wondrous life forms that await discovery. Every time scientists venture into the mesopelagic and bathypelagic zones, they uncover new species that challenge our understanding of life on Earth.
Advancements in remotely operated vehicles (ROVs) and deep-sea cameras have opened up new possibilities for studying these elusive creatures in their natural habitats. As we push the boundaries of exploration, the barreleyefish and their relatives serve as a reminder of nature’s capacity for astonishing adaptations. Each new discovery adds to our knowledge of the complex and diverse ecosystems that thrive far below the ocean’s surface.