Barreleye

The barreleye is a family of deep-sea ray-finned fish belonging to the order Stephanoberyciformes. These unusual fish are characterized by their transparent heads, through which their tubular eyes can be seen. The scientific name for the family is Opisthoproctidae.

Barreleye are found in temperate and tropical oceans worldwide, typically at depths ranging from 200 to over 1,000 meters. Their transparent skulls are believed to be an adaptation for detecting prey in the dim light of the deep sea. The eyes are often pointed upwards to spot the faint silhouettes of organisms above them.

The most famous species, Macropinna microstoma, possesses a distinctive barrel-shaped, transparent dome on its head, filled with fluid. Its small, sensitive eyes are located within this dome and can rotate independently. This unique feature allows it to look forward and upward simultaneously.

Other members of the Opisthoproctidae family also exhibit adaptations for deep-sea life, such as large mouths and bioluminescent organs. Their diets generally consist of small crustaceans and plankton. Barreleye are relatively small fish, with most species measuring less than 15 centimeters in length. Due to their deep-sea habitat, they are rarely encountered by humans and much of their biology remains poorly understood.