Sloane's viperfish
Sloane's viperfish, Chauliodus sloani, is a predatory, mesopelagic dragonfish found in deep-open waters across the world. The species was first described by German scientists Marcus Elieser Bloch and Johann Gottlob Schneider in their 1801 book "Systema ichthyologiae: iconibus CX illustratum", volume 1. Its size can range from 64 mm to 260 mm, and it is an iridescent silver-blue color. It has two rows of photophores, one on each side of the ventral part of its body. It is believed that "C. sloani" adjusts the intensity of ventral photophores during diurnal migrations to camouflage itself  from predators that might see its shadow from below. Taxonomy. "C. sloani" is part of the Chauliodus genus, which includes all viperfish. Viperfish are deep-sea fish with photophores (light-producing organs), long teeth, and hinged jaws. There are nine other species in the genus, including "C. danae" and "C. pammelas", both of which appear alongside "C. sloani" in primary literature. "C. sloani" is distinguished from other "Chauliodus" species by its range and the size of its teeth, which are large relative to its body even for the daggerfish. Distribution. "C. sloani" is found in tropical and temperate waters. It  is widespread in the Atlantic Ocean and the western Mediterranean Sea, as well as the Indian and Pacific oceans. In the Atlantic, its latitudinal range is from 35°N - 55°N, with highest abundance around 45°N. In the Arabian Sea, its range is primarily south of 10°N. It generally stays at depths from 200 m - 1000 m, both day and night, in contrast to "C. danae", whose individuals exhibit diurnal migration. Some small individuals of "C. sloani" also exhibit some limited diurnal migration. Larger individuals tend to live farther north. In the central Mediterranean  Sea, "C. sloani" is eaten as a primary part of the diet of the Atlantic bluefin tuna. Analysis of several cetaceans' stomach contents found remnants of "C. sloani", including the pygmy sperm whale and Gervais's beaked whale. Both were found beached on the Canary Islands. Anatomy. Jaw anatomy. The primary feature of interest in "C. sloani" is its enormous teeth. When the jaw is closed, the teeth fit together to form a cage in which prey can be trapped. Its jaw can be unhinged, allowing it to open its mouth up to 90° to catch prey up to  63% its own body size. Its teeth, however, are fixed in place, attached firmly to the jawbone rather than being depressible like in some other fish species. The immobility of its huge teeth requires it to be able to open its jaw so wide. Also helping with jaw expansion is the fact that its mouth does not have a floor. The size, shape, location, and number of teeth are consistent across individuals of "C. sloani", and each fang is highly specialized. "C. sloani" likely lures prey to its mouth with its photophore, or by arching its long dorsal ray to  dangle in front of its mouth. The foremost premaxillary tooth, which is relatively straight and has sharp ridges, may then be used to wound larger prey. The curved second premaxillary tooth is thought to be used like a dagger, retaining larger prey. All teeth come together into a cage to retain smaller prey. A study in 2009 showed that the teeth of "C. sloani" have dentin tubules while another study, a decade later, showed the teeth of "Aristostomias scintillans", another member of the family Stomiidae, lack dentin tubules. The reason behind difference in presence of dentin tubules in two species  of the same family has yet to be addressed. The jaw musculature of "C. sloani" contains a third adductor facialis muscle ventral to the skull. Recent research suggests that it improves the mechanical advantage of the system by anteriorly displacing the intersegmental aponeurosis during adduction, which increases bite force and angular velocity. General anatomy. "C. sloani" has a forked caudal fin, an adipose fin, and a dorsal fin located immediately behind head. Almost all of its fins contain soft rays. "C. sloani" has a low lipid content (~2.4%). The body is enveloped in a thick, transparent, gelatinous casing. Like many  stomiiformes, its scales and caudal skeleton are poorly ossified and it lacks a gas-filled swim bladder. Diet. "C. sloani" is a major consumer of myctophidae, or lanternfish, having a significant effect on their population in the Arabian Sea. An average "C. sloani" individual probably consumes at least one lanternfish every twelve days and there is no evidence to suggest that they increase their intake of food the bigger they are. Consequently, the stomachs of smaller individuals have been found containing proportionally larger prey than the prey found in the stomachs of larger individuals. However, they are not picky in  their diet—they also eat other fish, eggs, and algae. This is another thing that distinguishes them from the "C. danae", which feeds mainly on crustaceans, however there has been evidence that shows smaller variations had crustaceans included in the stomach during stomach content analysis. "C. sloani" typically feed on large prey, and in some cases was documented to consume prey up to 50% of its own bodyweight, and the size of their prey allows them to feed in little frequency while maximizing energetic efficiency. This is consistent with the observation that only individuals under 120 mm in length were  found with crustaceans in their stomach contents. The fish is considered a specialist predator given the small list of prey that were observed in the stomach contents following analysis. In a study to determine the effect of seasonal changes on feeding behavior, no significant difference was determined for the "C. sloani", and all of the prey observed were fishes. Earlier research found that of the 28 fish species preyed on, 18 were myctophids. There was no observation of a diurnal feeding pattern, meaning their feeding was not affected by time of day. Bioluminescence. The photophores on the sides of the  viperfish allow it to display bioluminescence. The viperfish has bioluminescence at the end of its fins that it can move forward towards its mouth in order to attract prey. In a recent study, it was determined that the hormones adrenaline and noradrenaline both contribute to bioluminescence in the viperfish, with adrenaline levels being higher in the photophores. The study also observed that when the tail of the viperfish was squeezed, the viperfish emitted light for about 5–10 seconds.