Pacific Barreleye

Overview

The Pacific barreleye is an extraordinary deep-sea fish instantly recognized by its transparent, fluid-filled head shield. Inside this protective dome, its large, barrel-shaped eyes glow with vivid green lenses. This unique anatomy allows the fish to thrive in dim, light-scarce oceanic environments.

Appearance

Beyond its famous transparent dome, the body of the barreleye is relatively stout and covered in large scales. Its striking green eyes feature specialized lenses that capture faint downward or horizontal light, helping the fish detect bioluminescent organisms overhead. While earlier studies assumed these tubular eyes were rigidly fixed, researchers have shown they can rotate forward and upward within the protective shield.

Biology & Behavior

This species typically hovers almost motionless in the water column, using its large, broad fins to maintain stability in the dark depths. Its transparent head shield protects sensitive visual organs from potential hazards in the water column. Because traditional capture methods using nets often rupture or collapse the delicate head dome, much of our modern understanding of its living behavior comes from observations made using remotely operated vehicles.

Feeding

The barreleye feeds primarily on small drifting zooplankton, crustaceans, and gelatinous siphonophores. By pointing its rotating eyes upward, the fish spots the silhouettes of prey silhouetted against the faint ambient light filtering down from the surface. Its positioning allows it to safely pluck prey from drifting siphonophore colonies while shielding its eyes from stinging tentacles.