Barreleye

The barreleye, also known as the spookfish, is a deep-sea fish best known for its transparent, fluid-filled dome-shaped head, through which its barrel-shaped, tubular eyes can be seen. Macropinna microstoma is the most well-documented species, found in the North Pacific. For many years scientists misunderstood the orientation of the eyes because specimens brought to the surface were typically damaged, until deep-sea video footage revealed that the eyes rotate within the transparent head to look either upward through the dome or forward. Barreleye inhabit the deep mesopelagic zone and represent one of the most visually unusual adaptations among fish for detecting faint light and prey silhouettes in the deep ocean.

Barreleye fish have one of the most unusual heads of any vertebrate, making them highly distinctive when observed intact.

• Transparent, dome-shaped shield covering the top of the head
• Large, green, tubular eyes visible through the transparent dome, capable of rotating to look upward or forward
• Small, unremarkable mouth relative to head size
• Flattened, dark body with large, wing-like pectoral fins
• Overall dark brown to black body coloration contrasting with the pale, translucent head

The transparent dome and rotating tubular eyes are unique among deep-sea fish and immediately separate barreleye from other mesopelagic groups such as hatchetfish or lanternfish, which lack any transparent head structure.

Barreleye are found in the mesopelagic zone of the North Pacific, typically at depths between about 400 and 800 meters, within a region often called the twilight zone where only faint sunlight penetrates. They are believed to spend much of their time relatively motionless, hovering in open water while using their upward-facing eyes to detect the faint silhouettes of prey or predators above them against the dim downwelling light. Barreleye live entirely within the water column, with no association with the seafloor, and their transparent-headed anatomy is thought to be an adaptation to the very specific, low-light conditions of their narrow depth range.

Barreleye are believed to hover nearly motionless in the water column for extended periods, an energy-conserving strategy suited to the food-scarce mesopelagic zone. Their rotating, upward-facing tubular eyes allow them to watch for the faint silhouettes of prey or the stinging tentacles of siphonophores above without needing to reposition their whole body, and researchers believe barreleye may steal prey caught in the tentacles of siphonophores. The transparent dome covering the eyes is thought to protect them from stinging cells while still allowing a wide field of upward vision. Very little is known about barreleye reproduction and general behavior due to the difficulty of observing the species alive in its deep natural habitat.