Macropinna

The Pacific barreleye fish (Macropinna microstoma) is ray-finned fish belonging to the Opisthoproctidae, the barreleye family. The only species in the genus, it is recognized for a highly unusual, transparent, fluid-filled shield on its head, through which the vivid green lenses of its eyes can be seen. The tubular eyes of this fish originally were believed to be fixed in place, offering only tunnel vision above its head, but in 2008, scientists discovered that its eyes were able to rotate both up and forward within the transparent dome. This feature allows for M. microstoma to observe prey while keeping its eyes protected from stinging tentacles of jellyfish-like organisms on which it feeds. 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 around 600 to 800 m (2,000 to 2,600 ft) in the mesopelagic zone, using its large fins for stability and with its eyes directed upward. In the low-light conditions, the fish apparently detects prey by its silhouette; because the eyes have no lens in the diverticulum, images most likely are seen 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 the fish turns its body from a horizontal to a vertical position to feed. Robison speculates that M. microstoma steals food from siphonophores.

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