Museum specimens of Macropinna had been known since 1939, but the delicate dome covering the head collapses when a fish is hauled up in a net. Every preserved barreleye therefore appeared to be a fish with two upward-staring green tubes and no shield, and the eyes were assumed to point permanently at the surface.
In 2004 MBARI researchers observed the fish with remotely operated vehicles and later kept individuals alive briefly on deck. The dome was intact, and the eyes could rotate from vertical to forward-facing, letting the barreleye track prey overhead and then look ahead at it as the body pitched up.
The green pigment in the eyes is thought to filter downwelling sunlight, which would help distinguish the faint bioluminescence of prey from light coming from above. The barreleye is often described as stealing food from siphonophores, but that remains an inference from anatomy and habitat rather than a filmed act.