Barreleye

What it is

The barreleye is a small, deep-bodied fish that rarely exceeds ten centimetres in length. Its body is flattened from side to side and covered in large, thin scales. The most striking feature is its pair of tubular eyes, which point upwards rather than forwards. These eyes are enclosed in a transparent shield on the head, allowing the fish to look up while keeping its profile low. The snout is translucent, and the back is dark, helping it blend into the deep water. A forked tail and small dorsal fin aid in its movement through the dark ocean depths.

Where it lives

This species inhabits tropical and temperate waters across the globe. It is present in the Atlantic, Pacific, and Indian Oceans. In the eastern Atlantic, it is often found between western Ireland and Mauritania, as well as from Sierra Leone to Angola. The fish lives in the mesopelagic zone, typically at depths between 500 and 700 metres. It prefers water temperatures around eight degrees Celsius, which usually limits its vertical range to below 400 metres where sunlight fades significantly.

How it lives

The barreleye hunts in near-total darkness by looking up for the silhouettes of prey against faint surface light. Its primary food source is bioluminescent siphonophores. The fish’s large lenses and specialized retinas help it detect these glowing organisms despite their natural camouflage. It likely lives alone, avoiding detection by larger predators. Beaked whales and some seabirds have been known to eat them. The fish uses internal light production to mask its own silhouette from below, a strategy called counter-illumination.

Breeding and life cycle

Like other members of its order, the barreleye reproduces by laying eggs. The young larvae look very similar to the adults in body shape and proportion. However, the larvae are more heavily pigmented, especially at the base of the tail and between the pelvic fins and the anus. As they grow, this pigmentation changes, and the distinctive transparent features of the adult head develop. The early stages of life remain largely hidden in the deep ocean environment.

Fish and people

Humans rarely encounter this fish due to its deep-water habitat. It is not targeted by commercial fisheries and appears only occasionally as bycatch. Scientists study it primarily for its unusual biological adaptations, such as its tubular eyes and light-producing organs. Museum records indicate it is distributed widely, but it holds no significant economic or cultural value for human populations. Its existence is mostly known through deep-sea research expeditions and occasional strandings of its predators.

Status and threats

The International Union for Conservation of Nature lists the barreleye as Least Concern. Because it lives in the deep ocean, it is difficult to monitor population trends accurately. However, it is recorded frequently across a vast geographic range. There are no major threats identified, such as overfishing or habitat destruction, that currently endanger the species. Its remote habitat provides a natural buffer against many human activities that affect coastal fish stocks.

Questions people ask about Barreleye

Good reading and sources

• Opisthoproctus on WikipediaEncyclopedia
• More photos on Wikimedia CommonsPhotos
• Wikidata recordOpen data