Scientists Catch a Rare Glimpse of a Fish with a See-Through Head

Researchers using a remotely operated vehicle recorded about 20 minutes of 4K footage of a barreleye fish, Macropinna microstoma, nearly 2,000 feet below the surface in California’s Monterey Canyon. The encounter was unusually rare: it was the first time the Monterey Bay Aquarium Research Institute team had seen one since 2021, and only the 11th sighting across roughly 7,000 dives. Barreleyes inhabit the ocean’s twilight zone, where sunlight is too weak to support photosynthesis, and their elusive deep-water habitat makes close observation difficult.

The fish’s translucent forehead reveals its unusual anatomy: its eyes sit inside its head and can move to scan in different directions, especially upward for prey silhouettes. The transparent tissue also shields the eyes from stinging siphonophores, organisms whose food the barreleye is known to steal. The footage offers researchers a rare chance to study how vision functions in dim midwater conditions and how the species evolved. Its adaptations reflect a broader evolutionary contest in the ocean’s depths, where animals detect faint visual signals such as passing shadows while prey and predators develop camouflage, including bioluminescence, and increasingly specialized vision.