Barreleye Fish
Fish with a Transparent Head



 
Barreleye Fish
Fish with a Transparent Head

For the first time, a large Pacific barreleye fish - complete with transparent head - has been caught on film by scientists using remotely operated vehicles at the Monterey Bay Aquarium Research Institute. The deep-sea fish's tubular eyes pivot under a clear dome.


Barreleyes, also known as spook fish, 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.


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.

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.

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 nematocysts (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.

The toothless mouth is small and terminal, ending in a pointed snout. As in related families (e.g. Argentinidae), there is an epibranchial or crumenal organ present behind the fourth gill arch.

This organ—analogous to the gizzard—consists of a small diverticulum (pouch) wherein the gill rakers insert and interdigitate for the purpose of grinding up ingested material.

In life, the body of most species is a dark brown covered in large, silvery imbricate scales; but these are absent in Dolichopteryx, leaving the body itself a transparent white. In all species a variable number of dark melanophores colour the muzzle, ventral surface, and midline.

Also present in Dolichopteryx, Opisthoproctus, and Winteria species are a number of luminous organs; in Dolichopteryx there are several along the length of the belly, and in Opisthoproctus there is a single organ in the form of a rectal pouch.

These organs glow with a weak light due to the presence of symbiotic bioluminescent bacteria.
 
Barreleyes inhabit moderate depths, from 400–2,500 metres down.

They are presumably solitary and do not undergo diel vertical migrations; instead, barreleyes remain just below the limit of light penetration and use their sensitive, upward-pointing tubular eyes—adapted for enhanced binocular vision at the expense of lateral vision—to survey the waters above.

The high number of rods in their eyes' retinae allows barreleyes to resolve the silhouettes of objects overhead in the faintest of ambient light (and to accurately distinguish bioluminescent light from ambient light).

Their binocular vision allows the fish to accurately track and home in on small zooplankton such as hydroids, copepods, and other pelagic crustaceans.