Rare Deep Sea Fish With Transparent Head Captured in Stunning New Footage

Marine biologists operating out of the Monterey Bay Aquarium Research Institute (MBARI) have successfully captured high-resolution footage of one of the ocean’s most elusive inhabitants, the barreleye fish (Macropinna microstoma). The rare sighting occurred during a research mission aboard the vessel David Packard, marking only the 11th time the institute has encountered the species in nearly four decades of deep-sea exploration.

The encounter took place during the “Diving Deep for Delicate Drifters” expedition, a project dedicated to documenting the complex lives of gelatinous marine organisms. While the team was preparing to retrieve their remotely operated vehicle (ROV), Doc Ricketts, research specialist Shannon Johnson spotted the distinct silhouette of the barreleye on the live video feed. This serendipitous moment halted the retrieval process, allowing scientists to observe the creature in its natural habitat for approximately 20 minutes.

A Rare Glimpse Into the Twilight Zone

The barreleye fish is a master of the ocean’s “twilight zone,” an environment characterized by limited light and extreme pressure. Its most striking anatomical feature is a transparent, fluid-filled dome atop its head, which protects its sensitive, tubular eyes. These specialized eyes can rotate independently, allowing the fish to scan the water column above for the silhouettes of potential prey.

“Seeing a barreleye fish is a rare treat,” noted MBARI senior scientist Steven Haddock. “Crew from across the ship joined the science team in the control room, mesmerized by this remarkable animal.”

The research team utilized the ROV’s advanced 4K camera technology to document the specimen, while ROV pilot Cameron Law navigated the vehicle to capture three-dimensional stereo imagery. This data is invaluable, as the species is infrequently observed despite MBARI having completed roughly 7,000 ROV dives and over 41,000 hours of underwater recording. The last documented sighting by the team occurred in 2021, and as per standard research protocols, the animal was left undisturbed in the wild.

Evolutionary Engineering in the Deep Sea

The transparent dome is more than a mere curiosity; it serves as a protective barrier for the fish’s eyes, particularly when it engages in its unique feeding strategy. Barreleye fish are known to hover near deep-sea siphonophores, effectively “stealing” prey that becomes entangled in the stinging tentacles of these colonial organisms. The shield prevents the barreleye from being harmed during these high-stakes maneuvers.

Furthermore, the eyes contain unique green pigments that are believed to filter out ambient sunlight, enhancing the contrast of prey silhouettes against the dim backdrop of the surface. This adaptation is similar to that of the strawberry squid (Histioteuthis heteropsis), showcasing a convergent evolutionary path among deep-sea species.

Historically, specimens of this species collected by trawling nets were often brought to the surface with their fragile, gelatinous domes ruptured or missing, leading to confusion about their anatomy in early biological studies. By observing the fish in its native, high-pressure environment, researchers are gaining a much clearer understanding of how it survives in the midwater column.

"Macropinna reminds us of the surprising ways that animals have adapted to thrive in the ocean’s midwater," Haddock added. For the MBARI team, the footage stands as a vital record, providing new insights into the behavioral ecology of one of the ocean’s most mysterious residents.