Barreleye

A deep-sea fish in the family Opisthoproctidae, known for upward-facing tubular eyes and, in some species, a transparent head.

Tubular eye: An elongated eye with a narrow field of view and a large lens, adapted for detecting faint light. Barreleyes use tubular eyes to spot silhouettes and faint bioluminescence above them.
Opisthoproctidae: A family of deep-sea marine fishes commonly called barreleyes or spookfishes. Barreleye names commonly designate members of this family.
Macropinna microstoma: A barreleye species with a transparent, fluid-filled head and eyes that can rotate forward. Its transparent dome made the family's unusual head anatomy widely known.
Spookfish: A common name for deep-sea fishes in the family Opisthoproctidae. The name overlaps with barreleye and helps distinguish common naming from a single species.
Parabolic mirror: A curved reflecting surface that focuses light toward a point or line. In Macropinna microstoma, reflective structures help redirect light within the eyes.
Argentiniformes: An order of mostly marine ray-finned fishes that includes barreleyes and smelts. This order places barreleyes among a broader group of deep-water and coastal fishes.
Opisthoproctus soleatus: A deep-sea barreleye fish with a flattened body and reflective underside. It illustrates that barreleyes include forms unlike the transparent-headed Macropinna.
Lanternfish: A small deep-sea fish in the family Myctophidae, often bearing light-producing organs. Lanternfish produce visible light themselves, unlike barreleyes' reliance on detecting light.
Bioluminescence: The production and emission of light by living organisms through chemical reactions. Faint flashes from deep-sea organisms can reveal prey against the dark water.
Ray-finned fish: A diverse class of fishes whose fins are supported by bony or cartilaginous rays. Barreleyes belong to this dominant group of living fishes.

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Mesopelagic fishBroader topic: It illustrates specialized visual adaptations among deep-water fishes.