Barreleyes, also known as spook fish (a name also applied to several species of chimaera), are small deep-sea argentiniform fish comprising the family Opisthoproctidae. Found in tropical-to-temperate waters of the Atlantic, Pacific, and Indian Oceans.
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 are 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 heads; this presumably allows the eyes to collect even more incident light and likely protects the sensitive eyes from the nematocyst (stinging cells) of the siphonophores from which it is believed the Barreleye steals food. It may also serve as an accessory lens (modulated by intrinsic or peripheral muscles), or refracts light with an index very close to seawater. A recent study disclosed that Dolichopteryx longipes is the only vertebrate known to use a mirror (as well as a lens) in its eyes for focusing images. Via Barreleye
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