Lanternfish are any of the species of marine fish belonging to the taxonomic family Myctophidae. They are of deep-sea fishes that live in the upper bathypelagic (or just bathyal) zone during the day and move to the surface at night. Lanternfish are best known for their light producing structure (bioluminescence) from which they get their name. They are often mistaken for anglerfish, which also have a lantern-like structure on their head. They are the most widely distributed fishes in the middle of the deep ocean and are also the most abundant. The lanternfish family contains 230-250 species spread across 30-35 genera (numbers vary due to disagreement between researchers).
It is very difficult to gather data on the lanternfish family because of their vertical nature. The constant movement from place to place vertically and horizontally also makes it near impossible to state a definite habitat and territory for each species of lanternfish. These dramatic movements also introduce several factors that could affect the growth of a lanternfish, making the study of their development a challenge for researchers.
Lanternfish are generally small, ranging from 2 to 30 cm (0.79 - 11.81 in.) in length, and most are shorter than 15cm (5.9 in.) Lanternfish have a slender, compressed body covered in small scales. These scales are smooth and are uniform with each other, called cycloid scales. There are a few species that have rough, sharp, and rigid scales called ctenoid scales. They have a large head with huge elliptical or round eyes. Lanternfish have large mouths with closely packed rows of teeth on their jaws. Their jaws extend behind the eyes. In general they have small fins, one dorsal (top/backside), one forked caudal (rear), one adipose (top-rear) and one anal. The pectoral fins range from large and well developed, to small or even nonexistent. The species of the genus Lampanyctus (pictured in the gallery below) have greatly elongated fins when compared to other Lanternfish. Most lanternfish have a gas bladder, but it degenerates in some species as they mature.
Photophores (light-producing organs) are a major and distinct feature of lanternfish in all but one species, Taaningichthys paurolychnus. These photophores are usually arranged in certain patterns and orders. This is a simple way to tell the difference between one lanternfish species and another, and is how an organism gets classified. In some species the pattern of photophores differs in males and females. The photophores can emit the colors of blue, green, and yellow, but the light is very dim. For example, males usually have caudal patches of photophores above the tail, and the females have theirs below the tail.
Lanternfish start out as a planktonic organism and mostly use currents to move. As the adults move up and down several thousand meters every day, the planktonic babies must learn to do this as well, which is why they make use of currents. The babies develop their photophores when they are about 2 cm (.75 in.) long. This is also where the sexual differences start to become apparent.
They can live to be three years old. As lanternfish are mainly prey, the family has a very high mortality rate. Its annual mortality rate hovers around 79%, which is very high. The Lanternfish could live longer in theory, but because of this mortality rate, few fish ever live longer than 3 years. Despite this high death rate, myctophids make up around 65% of all deep-sea biomass. They account for 550 - 660 metric tonnes of biomass globally, and thrive greatly in their habitats.
Most species live at 300-1,500 meters (980-4,920 ft) deep during the day, and 10-100 meters (33-330 ft) deep at night. Lanternfish migrate vertically like this to avoid predation and to stay near their zooplankton prey. After feeding they return downwards into the protection of the dark before the sun rises. Most species live along the coast and stay near to the continental slope for easy access to deeper waters. Different species will gather in layers separate from other species, they are thought to do this to avoid competition. These layers created by the separate species become so thick that sonar scans show them as ocean floors. As they are not truly the ocean floor, these scans are called “false bottoms” and made the early mapping of the ocean floor very difficult. The migration patterns are determined by a myriad of factors. Season, latitude, sex, developmental stage, and living depth are all factors that drastically impact how far the species will move. Some of the deeper living species won’t even migrate whatsoever, while others do it every now and then, rather than every day like most.
Lanternfish play a major role in the food chain. Whales and dolphins are major predators of lanternfish, along with salmon, tuna, sharks, grenadiers, other lanternfish, pinnipeds (seals and walruses), penguins, large squid and other deep-sea marine life. Lanternfish also eat large amounts of plastic from ocean debris. One lanternfish in particular was found with 80+ plastic pieces in its gut while scientists were monitoring garbage patches. 
The pattern of photophores are different for each species, because of this they are thought to be used for communication. The light emitted from the photophores can also be used for shoaling (gathering a group) and for finding a mate. The photophores are also used as camouflage. Lanternfish use the lights to perform counterillumination. Counterillumination is the matching of lights to an object’s surroundings. Lanternfish do this by matching the light levels below and above them to make their silhouette nearly invisible when viewed from underneath the fish.
The photophores on males are larger than on females in some genera such as Diaphus. These particular photophores have been called “headlight organs.” lanternfish can use these photophores to respond to certain human objects. One researcher noticed that a lanternfish in an aquarium was flashing his photophores in response to his watch. The fish may have been repeating the pattern of flashes created by the ticking second hand.
A quick video on what exactly the photophores do and what lanternfish gain from having them.
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