Macropinna microstoma has a peculiar head that allows it to capture, with its eyes, much more light than normal. Another peculiarity is that of being able to rotate the eyeballs inside the head, allowing it to see both the predators trying to surprise it from above and the preys.
This species is batipelagic: it populates the northern Pacific Ocean in the temperate and subarctic belts. It can be encountered from 500 to 4000 meters deep.
Scientists were able to film it, in 2021, by means of a remotely operated submarine.
It feeds on pelagic organisms such as jellyfish, siphonophores and crustaceans. It produces pelagic eggs. It is quite similar to the fish of the genus Opisthoproctus and like them it has tubular eyes turned upwards. In this species, however, there is a colorless hemispherical capsule which covers and protects the eyes and which can give the impression that the head is transparent.
The two indentations at the anterior end of the head are not actually eyes but olfactory organs. The eyes can be rolled forward. It has an adipose fin on the caudal peduncle. Adults are dark in color. The maximum known length is 15 cm.
At first glance it seems that its eyes are at the front of their head in the form of two black holes; these are its olfactory organs. Marine biologists used to believe that the barreleye's eyes were fixed in its head, which would only allow it to look upward.
Its large, flat fins allow it to remain nearly motionless in the water, and to maneuver very precisely. Most of the time, the fish hangs motionless in the water, with its body in a horizontal position and its eyes looking upward.
The green pigments in its eyes may filter out sunlight coming directly from the sea surface, helping the barreleye spot the bioluminescent glow of jellyfish or other animals directly overhead. Macropinna microstoma can get as long as 6 inches (15 cm).
It has a large, dome-shaped, transparent head. This protects its sensitive eyes from the nematocysts (stinging cells) of the siphonophores, from which it is believed to steal food. Through the dome, the entire inner part of the head can be seen, that is, their eyes, brain and all the nerve endings that make up their head.