Barreleyes, also known as spookfish (a name also applied several species of chimaera), are small, unusual-looking deep-sea osmeriform fish comprising the family Opisthoproctidae. Found in tropical-to-temperate waters of the Atlantic, Pacific, and Indian Oceans, the family contains thirteen species in six genera (four of which are monotypic.
These fish are named for their barrel-shaped, tubular eyes which are generally directed upwards to detect the silhouettes of available prey; however, according to Robison and Reisenbichler these fish are capable of directing their eyes forward as well. The family name Opisthoproctidae is derived from the Greek words opisthe (“behind”) and proktos (“anus”).
All species have large, telescoping eyes which dominate and protrude from the skull, but enclosed within a large transparent dome of soft tissue. These eyes generally gaze upwards, but can also be directed forwards.The opisthoproctid eye has a large lens and a retina with an exceptionally high complement of rod cells and a high density of rhodopsin (the “visual purple” pigment); there are no cone cells. To better serve their vision, barreleyes have large, dome-shaped transparent to translucent heads; this presumably allows the eyes to collect even more incident light, likely protects the sensitive eyes from the nematocysts (stinging cells) of the siphonophores from which it is believed the Barreleye steals food, and necessarily either serves as an accessory lens (modulated by intrinsic or peripheral muscles) or is composed of tissue with a refractive index very close to that of seawater. A recent study disclosed that the Dolichopteryx longipes is the only vertebrate known to use a mirror (as well as a lens) in its eyes.
Researchers at the Monterey Bay Aquarium Research Institute recently solved the half-century-old mystery of a fish with tubular eyes and a transparent head. Ever since the “barreleye” fish Macropinna microstoma was first described in 1939, marine biologists have known that it’s tubular eyes are very good at collecting light. However, the eyes were believed to be fixed in place and seemed to provide only a “tunnel-vision” view of whatever was directly above the fish’s head.
A new paper by Bruce Robison and Kim Reisenbichler shows that these unusual eyes can rotate within a transparent shield that covers the fish’s head. This allows the barreleye to peer up at potential prey or focus forward to see what it is eating.