Viperfish

A viperfish is any species of marine fish in the genus Chauliodus. Viperfishes are mostly found in the mesopelagic zone and are characterized by long, needle-like teeth and hinged lower jaws. A typical viperfish grows to lengths of 30 cm. Viperfishes undergo diel vertical migration and are found all around the world in tropical and temperate oceans. Viperfishes are capable of bioluminescence and possess photophores along the ventral side of their body, likely used to camouflage them by blending in with the less than 1% of light that reaches to below 200 meters depth.

Although it may appear to be covered in scales, viperfishes do not possess scales. Rather, they are covered by a thick, transparent coating of unknown substance. Extremely large, fang-like teeth give the fish a slightly protruded lower jaw.

Habitat
Viperfishes live in meso- and bathypelagic environments and have been found dominating submarine calderas such as the Kurose Hole, which is the site with the highest Chauliodus density known in the world. Viperfishes also engage in diel vertical migration, meaning they migrate up into more productive waters during the night to feed. However, it is likely that only part of the total population of viperfishes engages in diel vertical migration on any given night, which could be due to their slow metabolism, i.e. they likely do not have to feed every night. Temperature is another restricting factor in viperfish's vertical distribution in the ocean. Depth is restricted by temperature, and the upper thermal limit of viperfish is 12° to 15 °C. In tropical waters, viperfish tend to stay in the deep layers and not migrate much, while in temperate waters viperfish are more actively migrating and even interacting with epipelagic predators.

Body plan
Chauliodus species are recognized by their large, fang-like teeth. They are so long that they would pierce the brain of the fish if misaligned.

One species of viperfish, C. sloani, have a sampled standard length of 64.0 to 260.0 mm, with a mean SL of 120.3mm. The same species has a mean weight of 5.66 grams. Representatives from Chauliodus pammelas and Chauliodus sloani display a size-based depth differential. Individuals of a lesser mass are found at shallower depths and individuals of larger mass are found at deeper depths, below 500 meters. However, at nighttime larger viperfish can be found in shallower depths.

The eyes of Chauliodus sloani maintain a constant size and proportion throughout growth of the fish. In the retina, several rows of rod cell "banks" grow upon each other, increasing in number with size of the fish. This opposed the typical vertebrate retina, which only has one layer of receptors.

The first dorsal ray of Chauliodus is elongated, hinged, and connected via musculature; allowing it to swing forward. The tip of this ray has light organs. This fish lack scales, and instead is covered with hexagonal pigment patterns covered in an opalescent, slimy substance.

Bioluminescence
Chauliodus species utilize their capability of bioluminescence for two distinct purposes: attracting prey and avoiding predators. They show distinct anatomical adaptations for the two functions. Chauliodus possesses a bioluminescent lure located at the tip of its first dorsal ray, which it uses to attract prey by swinging it forward in front of its mouth. This allows the fish to lure prey directly in front of its mouth for feeding.

Chauliodus has photophores along the ventral side of its body that emit light through adrenergic nervous control. The distribution of this light closely matches the distribution of light in mesopelagic and bathypelagic ocean zones, making it difficult for predators to see the fish. This allows the fish to swim undetected by predators, aiding survival. This type of camouflage is called counter-illumination.

The presence of photo-microbes in the visceral organs of Chauliodus sloani indicates that bioluminescent microbes are likely responsible for the Chauliodus's ability to luminesce.

Feeding
Viperfishes, depending on the species, prey on other pelagic fishes and crustaceans. Stomach contents of captured individuals have contained lanternfishes, bristlemouths, copepods and krill. Based on the diel vertical migration of its prey, viperfish are assumed to be epipelagic migrants that search surface waters for food. The prey for viperfish, specifically the viperfish species C. sloani, are highly specific and of high abundance but feeding events for viperfish have low levels of occurrence.

Viperfish are able to maximize energy input by consuming few but large prey. In order to support the specificity of feeding, the viperfish has multiple adaptations such as a large-toothed mouth, modifications in its skull to allow for a wide opening of its mouth, and elastic stomach and body skin to compensate for large prey.

Migratory patterns
Vertical movements of viperfish are influenced by temperature. It was observed that the upper limit of distribution is restricted by temperature (12-15 °C). That is observed to affect vertical habitat and trophodynamics. In most tropical waters, it is likely that the viperfish exists full time below 400 meters. In temperate regions, viperfish trophically interact with epipelagic predators at superficial waters.

Many sub-species in the Stomiidae family participated in diel vertical migration. In migrating to the surface (400m depth) at night, they prove their ability to withstand large temperature changes of up to 7°C daily. They have been recorded in waters ranging from 4 to 14.5°C, highlighting the wide range of temperatures viperfish are capable of surviving in.

Viperfish have previously been recorded in the Italian waters off the western Mediterranean Basin, the Adriatic Sea, the Greek waters of the Aegean Sea, and in the Turkish waters of the Levant Sea. Viperfish have rarely been seen off the Algerian coast by Dieuzeide. They have been reported to occur off the northern Tunisian coast. m

Reproduction
Despite the abundance of viperfish in the meso- and bathypelagic, their reproductive ecology is widely unknown. This is due to research surveys rarely being able to catch mature adults, as well as the general lack of research on fish reproductive ecology in the deep sea. It is likely, however, that viperfish share a similar reproductive ecology to other dragonfishes which have been studied more extensively (under Stomiidae family).

Viperfish are gonochoristic, meaning that they don't exhibit both testicular and ovarian tissue simultaneously in their gonads. They reproduce through spawning, with a study on dragonfishes indicating that males are able to spawn sperm continuously whereas females display asynchronous oocyte development and batch spawning. That same study showed a skewed 1:2 sex ratio favoring females in their collection of over seventy Chauliodus sloani viperfishes in the Gulf of Mexico.

Two Chauliodus macouni eggs were recovered in the Columbia River in Oregon (likely displaced by strong Pacific currents), indicating a potentially long incubation period for viperfish eggs.

Species
There are currently nine extant recognized species in this genus:
 * Chauliodus barbatus Garman, 1899
 * Chauliodus danae Regan & Trewavas, 1929 (Dana viperfish)
 * Chauliodus dentatus Garman, 1899
 * Chauliodus macouni T. H. Bean, 1890 (pacific viperfish)
 * Chauliodus minimus Parin & Novikova, 1974
 * Chauliodus pammelas Alcock, 1892
 * Chauliodus schmidti Ege, 1948
 * Chauliodus sloani Bloch & J. G. Schneider, 1801 (Sloane's viperfish)
 * Chauliodus vasnetzovi Novikova, 1972

At least two more species are recognized from Late Miocene-aged fossils:
 * Chauliodus eximus, (Jordan, 1925), originally Eostomias eximus, from Late Miocene California
 * Chauliodus testa, Nazarkin, 2014, from the Late Miocene of Western Sakhalin Island