Barreleye Fish

Barreleye Fish

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 head, 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); no cone cells are present. 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 the barreleye is believed to steal 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. Dolichopteryx longipes is the only vertebrate known to use a mirror (as well as a lens) in its eyes for focusing images.

The toothless mouth is small and terminal, ending in a pointed snout. As in related families (e.g. Argentinidae), an epibranchial or crumenal organ is present behind the fourth gill arch. This organ—analogous to the gizzard—consists of a small diverticulum wherein the gill rakers insert and interdigitate for the purpose of grinding up ingested material. The living 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; Dolichopteryx has several along the length of its belly, and Opisthoproctus has 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, specifically, Photobacterium phosphoreum (family Vibrionaceae). 

The ventral surface of Opisthoproctus species is characterised by a flattened and projecting 'sole'; in the mirrorbelly (Opisthoproctus grimaldii) and Opisthoproctus soleatus, this sole may act as a reflector, by directing the emitted light downwards. The strains of P. phosphoreum present in the two Opisthoproctus species have been isolated and cultured in the lab. Through restriction fragment length polymorphism analysis, the two strains have been shown to differ only slightly.

In all species, the fins are spineless and fairly small; in Dolichopteryx however, the pectoral fins are greatly elongated and wing-like, extending about half the body's length, and are apparently used for stationkeeping in the water column. The pectoral fins are inserted low on the body, and in some species, the pelvic fins are inserted ventrolaterally rather than strictly ventrally.

Several species also possess either a ventral or dorsal adipose fin, and the caudal fin is forked to emarginated. The anal fin is either present or greatly reduced, and may not be externally visible; it is strongly retrorse in Opisthoproctus. A single dorsal fin originates slightly before or directly over the anal fin. A perceptible hump in the back begins just behind the head. The gas bladder is absent in most species, and the lateral line is uninterrupted. The branchiostegal rays (bony rays supporting the gill membranes behind the lower jaw) number two to four. The javelin spookfish (Bathylychnops exilis) is by far the largest species at 50 cm standard length; most other species are under 20 cm.