Macropinna microstoma

The Pacific barreleye fish (Macropinna) is a genus of ray-finned fish belonging to Opisthoproctidae, the barreleye family. It contains one species, M. microstoma. It is recognized for a highly unusual transparent, fluid-filled shield on its head, through which the lenses of its eyes can be seen. It was originally believed that the tubular eyes of this fish were fixed in place, offering only tunnel vision above its head. However, in 2008, scientists discovered that its eyes were able to rotate both up and forward with its transparent dome. This feature allows for M. microstoma to observe prey while keeping its eyes protected from stinging tentacles of jellyfish-like organisms it feeds on.

M. microstoma, a small fish of about 15 cm, has a tiny mouth and most of its body is covered in large scales. The fish normally hangs nearly motionless in the water, at a depth of about to in the mesopelagic zone, using its large fins for stability and with its eyes directed upward. In the low light conditions it is assumed the fish detects prey by its silhouette. This is because the eyes have no lens in the diverticulum, so images would most likely occur through reflection. Due to the eyes facing upwards, light is able to enter the eyes and through the illumination, silhouettes are formed. MBARI researchers Bruce Robison and Kim Reisenbichler observed that when prey such as small fish and jellyfish are spotted, the eyes rotate like binoculars, facing forward as it turns its body from a horizontal to a vertical position to feed. Robison speculates that M. microstoma steals food from siphonophores.