Telescopefish are small, deep-sea aulopiform fish

Fish

Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...

 comprising the small family

Family (biology)

In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...

 Giganturidae. There are just two known species

Species

In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

, both within the genus

Genus

In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

 Gigantura. Though rarely captured, they are found in cold, deep tropical to subtropical waters worldwide.

The common name of these fish are understandably named for their bizarre, tubular eye

Eye

Eyes are organs that detect light and convert it into electro-chemical impulses in neurons. The simplest photoreceptors in conscious vision connect light to movement...

s. The genus name Gigantura is in reference to the Gigantes

Gigantes

In Greek mythology, the Giants were the children of Gaia, who was fertilized by the blood of Uranus, after Uranus was castrated by his son Cronus...

, a race of giants

Giant (mythology)

The mythology and legends of many different cultures include monsters of human appearance but prodigious size and strength. "Giant" is the English word commonly used for such beings, derived from one of the most famed examples: the gigantes of Greek mythology.In various Indo-European mythologies,...

 in Greek mythology

Greek mythology

Greek mythology is the body of myths and legends belonging to the ancient Greeks, concerning their gods and heroes, the nature of the world, and the origins and significance of their own cult and ritual practices. They were a part of religion in ancient Greece...

—coupled with the suffix oura, meaning "tail", thus Gigantura refers to the ribbon-like lower half of the tailfin.

Physical description

The Giganturidae are slender, slightly tapered fish with large heads dominated by large, forward-pointing telescoping

Telescope

A telescope is an instrument that aids in the observation of remote objects by collecting electromagnetic radiation . The first known practical telescopes were invented in the Netherlands at the beginning of the 1600s , using glass lenses...

 eyes with large lenses

Lens (anatomy)

The crystalline lens is a transparent, biconvex structure in the eye that, along with the cornea, helps to refract light to be focused on the retina. The lens, by changing shape, functions to change the focal distance of the eye so that it can focus on objects at various distances, thus allowing a...

. The head ends in a short, pointed snout. The highly extensile mouth is lined with sharp, slightly recurved and depressible teeth and it extends well past the eyes. The body lacks scale

Scale (zoology)

In most biological nomenclature, a scale is a small rigid plate that grows out of an animal's skin to provide protection. In lepidopteran species, scales are plates on the surface of the insect wing, and provide coloration...

s but is covered in easily abraded, silvery guanine

Guanine

Guanine is one of the four main nucleobases found in the nucleic acids DNA and RNA, the others being adenine, cytosine, and thymine . In DNA, guanine is paired with cytosine. With the formula C5H5N5O, guanine is a derivative of purine, consisting of a fused pyrimidine-imidazole ring system with...

 which imparts a greenish to purplish iridescence

Iridescence

Iridescence is generally known as the property of certain surfaces which appear to change color as the angle of view or the angle of illumination changes...

 in life. The gas bladder

Gas bladder

The swim bladder, gas bladder, fish maw or air bladder is an internal gas-filled organ that contributes to the ability of a fish to control its buoyancy, and thus to stay at the current water depth without having to waste energy in swimming...

 is absent and the stomach

Stomach

The stomach is a muscular, hollow, dilated part of the alimentary canal which functions as an important organ of the digestive tract in some animals, including vertebrates, echinoderms, insects , and molluscs. It is involved in the second phase of digestion, following mastication .The stomach is...

 is highly distensible.

The transparent fin

Fin

A fin is a surface used for stability and/or to produce lift and thrust or to steer while traveling in water, air, or other fluid media, . The first use of the word was for the limbs of fish, but has been extended to include other animal limbs and man-made devices...

s are spineless

Spine (zoology)

A spine is a hard, thorny or needle-like structure which occurs on various animals. Animals such as porcupines and sea urchins grow spines as a self-defense mechanism. Spines are often formed of keratin...

; the deeply forked and hypocercal caudal fin is most striking, with the lower lobe extended to a length exceeding that of the body. The pectoral fins are large (ca. 30–42 rays), situated above the gill

Gill

A gill is a respiratory organ found in many aquatic organisms that extracts dissolved oxygen from water, afterward excreting carbon dioxide. The gills of some species such as hermit crabs have adapted to allow respiration on land provided they are kept moist...

 opening, and inserted horizontally. The anal fin (ca. 8–14 rays) and single dorsal fin

Dorsal fin

A dorsal fin is a fin located on the backs of various unrelated marine and freshwater vertebrates, including most fishes, marine mammals , and the ichthyosaurs...

 (ca. 16–19 rays) are both situated far back of the head. The pelvic fins and adipose fin are absent.

Also absent are: the premaxilla

Premaxilla

The incisive bone is the portion of the maxilla adjacent to the incisors. It is a pair of small cranial bones at the very tip of the jaws of many animals, usually bearing teeth, but not always. They are connected to the maxilla and the nasals....

, orbitosphenoid, parietal

Parietal

Parietal may refer to:*Parietal placentation*Parietal lobe of the brain*Parietal bone of the skull*Parietal scales of a snake lie in the general region of the parietal bone*Parietal cell in the stomach*Parietal pleura...

, symplectic, posttemporal, and supratemporal bone

Bone

Bones are rigid organs that constitute part of the endoskeleton of vertebrates. They support, and protect the various organs of the body, produce red and white blood cells and store minerals. Bone tissue is a type of dense connective tissue...

s; the gill raker

Gill raker

Gill rakers in fish are bony or cartilaginous processes that project from the branchial arch and are involved with filter feeding tiny prey. They are not to be confused with the gill filaments that compose the bony part of the gill. Rakers are usually present in two rows, projecting from both the...

s; and the branchiostegal rays. The loss of these structures is attributed to neoteny

Neoteny

Neoteny , also called juvenilization , is one of the two ways by which paedomorphism can arise. Paedomorphism is the retention by adults of traits previously seen only in juveniles, and is a subject studied in the field of developmental biology. In neoteny, the physiological development of an...

; that is, the retention of larva

Larva

A larva is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle...

l characteristics.

Gigantura indica is the larger of the two species in terms of length at ca. 20.3 centimetres standard length (a measurement excluding the caudal fin). However, Gigantura chuni (at ca. 15.6 cm standard length) is slightly more robust in build.

Life history

Telescopefish are presumed to be solitary, active predators, frequenting the mesopelagic

Pelagic zone

Any water in a sea or lake that is not close to the bottom or near to the shore can be said to be in the pelagic zone. The word pelagic comes from the Greek πέλαγος or pélagos, which means "open sea". The pelagic zone can be thought of in terms of an imaginary cylinder or water column that goes...

 to bathypelagic zones of the water column, from ca. 500 metres down to ca. 3,000 metres. By using their tubular, large-lensed eyes—which are adapted for optimal binocular light collection, at the expense of lateral vision—telescopefish are likely able to spy their prey's weak bioluminescence

Bioluminescence

Bioluminescence is the production and emission of light by a living organism. Its name is a hybrid word, originating from the Greek bios for "living" and the Latin lumen "light". Bioluminescence is a naturally occurring form of chemiluminescence where energy is released by a chemical reaction in...

 from a distance, as well as (by looking skyward) resolve the outlined silhouettes of prey against the gloom above. Their eyes may also help telescopefish to better judge distance of prey; these visual adaptations are typical of deep-sea fish (cf. barrel-eye, tube-eye

Tube-eye

The tube-eye or thread-tail, Stylephorus chordatus, is a deep-sea Stylephoriformes fish, the only fish in the genus Stylephorus and family Stylephoridae....

). Common prey items include bristlemouths, lanternfish

Lanternfish

Cooper Lanternfishes are small mesopelagic fish of the large family Myctophidae. One of two families in the order Myctophiformes, the Myctophidae are represented by 246 species in 33 genera, and are found in oceans worldwide. They are aptly named after their conspicuous use of bioluminescence...

, and barbeled dragonfish. Owing to the telescopefishes' highly extensile jaws and distensible stomachs, they are able to swallow prey larger than themselves; this is also a common adaptation to life in the lean depths (cf. sabertooth fish

Sabertooth fish

Sabertooth or sabretooth fish are small, fierce-looking deep-sea aulopiform fish comprising the family Evermannellidae. The family is small, with just eight species in three genera represented; they are distributed throughout tropical to subtropical waters of the Atlantic, Indian, and Pacific...

, black seadevil

Black seadevil

Black seadevils are small, deep-sea lophiiform fish comprising the family Melanocetidae. There are five known species , all within the genus Melanocetus...

).

Much less is known of their reproductive habits. They are presumed to be non-guarding pelagic spawn

Spawn (biology)

Spawn refers to the eggs and sperm released or deposited, usually into water, by aquatic animals. As a verb, spawn refers to the process of releasing the eggs and sperm, also called spawning...

ers, releasing eggs

Roe

Roe or hard roe is the fully ripe internal egg masses in the ovaries, or the released external egg masses of fish and certain marine animals, such as shrimp, scallop and sea urchins...

 and sperm

Spermatozoon

A spermatozoon is a motile sperm cell, or moving form of the haploid cell that is the male gamete. A spermatozoon joins an ovum to form a zygote...

 indiscriminately into the water. The fertilized eggs are buoyant and become incorporated into the zooplankton

Zooplankton

Zooplankton are heterotrophic plankton. Plankton are organisms drifting in oceans, seas, and bodies of fresh water. The word "zooplankton" is derived from the Greek zoon , meaning "animal", and , meaning "wanderer" or "drifter"...

, wherein they and the larvae remain—likely at much shallower depths than the adults—until metamorphosis into juvenile or adult form.