Map of Barreleye – The Full Wiki

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Barreleyes, also known as spookfish (a name also applied several species of chimaera), are small, unusual-looking deep-sea osmeriform fish comprising the family Opisthoproctidae. Found in tropical-to-temperate waters of the Atlanticmarker, Pacificmarker, and Indian Oceansmarker, the family contains thirteen species in six genera (four of which are monotypic).

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“).

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,
but 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 to translucent heads; this presumably allows the eyes
to collect even more incident light, likely protects the sensitive
eyes from the nematocysts (stinging cells) of the siphonophores
from which it is believed the Barreleye steals food, and
necessarily either serves as an accessory lens (modulated by
intrinsic or peripheral muscles) or is composed of tissue with a
refractive index very close to that of seawater. A recent study
disclosed that the 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
scale; 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.

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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, and are
apparently used for stationkeeping in the water column. 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
meters 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
spawner; 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 shallower depths than the adults—and upon metamorphosis into adult form they
descend 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.

Species

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There are fourteen species in six genera:

References

External links

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