Barreleye Fish: The Deep-Sea Telescope — Passage and Quiz

In the dark depths of the ocean, the barreleye fish (Macropinna microstoma) swims with one of nature's most bizarre adaptations—a transparent, fluid-filled head with glowing green eyes that can rotate like binoculars. Found 2,000–2,600 feet below the surface, this strange creature has evolved to hunt with precision in near-total darkness.

The barreleye’s most striking feature is its see-through dome head, which protects its upward-facing eyes while allowing it to spot faint silhouettes of prey above. Scientists discovered that its eyes can swivel forward when feeding, giving it a "telescopic" view of small fish and jellyfish. The green pigments in its eyes may filter out sunlight that barely penetrates the depths, helping it focus on bioluminescent signals.

To avoid predators, the barreleye uses its flat, silvery scales to blend into the dim light from above—a camouflage tactic called "counter-illumination." Its large, fin-like pectoral muscles allow it to hover motionless for hours, conserving energy in the food-scarce deep sea.

Fun Fact: When first discovered in 1939, scientists thought the barreleye’s eyes were fixed looking upward. Only in 2009 did deep-sea robots reveal their rotating ability!