DiscoverScroll article

Barreleyes, also known as spookfish (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 because of their barrel-shaped, tubular eyes, which are generally directed upwards to detect the silhouettes of available prey; however, the fish are capable of directing their eyes forward, as well.

The morphology of the Opisthoproctidae varies between three main forms: the stout, deep-bodied barreleyes of the genera Opisthoproctus and Macropinna, the extremely slender and elongated spookfishes of the genera Dolichopteryx and Bathylychnops, and the intermediate fusiform spookfishes of the genera Rhynchohyalus and Winteria. All species have large, telescoping eyes, which dominate and protrude from the head, but are enclosed within a large transparent dome of soft tissue. 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 nematocysts (stinging cells) of the siphonophores, from which the barreleye is believed to steal food. Dolichopteryx longipes is the only vertebrate known to use a mirror (as well as a lens) in its eyes for focusing images. 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 surfaces of Opisthoproctus species are characterised by a flattened and projecting 'sole'; in the mirrorbelly (Monacoa 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. 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. The javelin spookfish (Bathylychnops exilis) is by far the largest species at 50 centimetres (20 in) standard length; most other species are under 20 centimetres (7.9 in).

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), and their binocular vision allows the fish to accurately track and home in on small zooplankton, such as hydroids, copepods, and other pelagic crustaceans. MBARI press release, Researchers solve mystery of deep-sea fish with tubular eyes and transparent head, 23 February 2009.

Quick Facts

• These fish are named because of their barrel-shaped, tubular eyes, which are generally directed upwards to detect the silhouettes of available prey; however, the fish are capable of directing their eyes forward, as well.
• All species have large, telescoping eyes, which dominate and protrude from the head, but are enclosed within a large transparent dome of soft tissue.
• 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.
• The ventral surfaces of Opisthoproctus species are characterised by a flattened and projecting 'sole'; in the mirrorbelly (Monacoa grimaldii) and Opisthoproctus soleatus, this sole may act as a reflector, by directing the emitted light downwards.
• 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.

Source material: Wikipedia - "Barreleye". Adapted and summarized for DiscoverScroll. Original contributors are credited through the linked Wikipedia article. Read original on Wikipedia. CC BY-SA 4.0. Changes were made from the original.