Bathylychnops
Dolichopteryx
Macropinna
Opisthoproctus
Rhynchohyalus
Winteria Barreleyes, also known as spookfish (a name also applied several species of chimaera), are small, deep-sea, odd-looking osmeriform fish comprising the family Opisthoproctidae. Found in tropical-to-temperate waters of the Atlantic, Pacific, and Indian Oceans, the family contains 11 species in six genera (four of which are monotypic).

These fish are named for their barrel-shaped, tubular eyes which in most species are fixed gazing upwards. The family name Opisthoproctidae is derived from the Greek words opisthe ("behind") and proktos ("anus").

Physical description

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 elongate 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 skull. In all but Winteria, the eyes are fixed gazing upwards; in Winteria, the eyes are directed forwards, but with a slight upward pitch. 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 transparent to translucent heads; this presumably allows the eyes to collect even more incident light. 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; 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. In all species the pectoral fins are inserted low on the body, and in some 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 emarginate. The anal fin is either present or greatly reduced, and may not be externally visible; it is strongly retrorse in Opisthoproctus. There is a single dorsal fin originating slightly before or directly over the anal fin. There is a perceptible hump in the back, beginning just behind the head. The gas bladder is absent in most species, and the lateral line is uninterrupted. The branchiostegal rays number 2–4. The javelin spookfish (Bathylychnops exilis) is by far the largest species at 50 centimetres standard length (SL; a measurement excluding the caudal fin); most other species are under 20 centimetres SL.

Life history

Barreleyes inhabit moderate depths, from the mesopelagic to bathypelagic zone, ca. 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 allow 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. The distribution of some species coincides with the isohaline and isotherm layers of the ocean; for example, in Opisthoproctus soleatus upper distribution limits coincide with the 400 metre isotherm for 8°C.

What little is known of barreleye reproduction indicates they are pelagic spawners; that is, eggs and sperm are released en masse directly into the water. The fertilized eggs are buoyant and planktonic; the larvae and juveniles drift with the currents—likely at much shallwer depths than the adults—and upon metamorphosis into adult form they descent to deeper waters. Dolichopteryx species are noted for their paedomorphic features, the result of neoteny (the retention of larval characteristics).

The bioluminescent organs of Dolichopteryx and Opisthoproctus, together with the reflective soles of the latter, may serve as camouflage in the form of counterillumination . This predator avoidance strategy involves the use of ventral light to break up the fishes' silhouettes, so that (when viewed from below) they blend in with the ambient light from above. Counterillumination is also seen in several other unrelated deep-sea families, which include the marine hatchetfish (Sternoptychidae). Also found in marine hatchetfish and other unrelated families are tubular eyes; cf. telescopefish, tube-eye .

See also

• List of fish common names
• List of fish families

References

• "Mesopelagic fishes". Encyclopedia of ocean sciences, Vol. 3, 2001. A. G. V. Salvanes and J. B. Kristofersen. February 2005 version. (PDF file.)
• "Restriction Fragment Length Polymorphism Analysis Reveals High Levels of Genetic Divergence Among the Light Organ Symbionts of Flashlight Fish" The Biolological Bulletin. 181: 135-143. (August, 1991). Connie J. Wolfe and Margo G. Haygood. February 2005 version. (PDF file.)
• "Bioluminescent signals and the role of reflectors" Journal of Optics A: Pure Applied Optics. 2 R29-R38. Peter J. Herring (2000). February 2005 version.
• Fishes: An introduction to ichthyology. Peter B. Moyle and Joseph J. Cech, Jr; p. 320. Printed in 2004. Prentice-Hall, Inc; Upper Saddle River, NJ. ISBN 0131008471