# Stomiidae

Stomiidae is a family of deep-sea ray-finned fish, including the barbeled dragonfishes. They are quite small, usually around 15 cm, up to 26 cm. These fish are apex predators and have enormous jaws filled with fang-like teeth. They are also able to hinge the neurocranium and upper-jaw system, which leads to the opening of the jaw to more than 100 degrees. This ability allows them to consume extremely large prey, often 50% greater than their standard length.

## Evolution

Due to their extreme habitats, stomiids are very rare in the fossil record. The oldest known genus is Azemiolestes Prokofiev, 2001 from the Middle Eocene (Lutetian) of Georgia. Other fossil genera include Abruzzoichthys Carnevale, 2002 from the Middle Miocene of Italy and Mrazecia Pauca, 1929 from the Early Oligocene of Romania.

## Habitat

The family Stomiidae can be found in all oceans. They also exist at a wide range of depths between the surface and thousands of meters deep into the bathypelagic zone, depending on the water's ideal feeding and breeding conditions. There is also some evidence that certain species within the family Stomiidae exhibit migratory behavior. Temperature, salinity, oxygen, and fluorescence profiles of an area can affect some species' (like Sloane's viperfish Chauliodus sloani) preferred habitat changes from day to night with DVM.
Brian Coad, ichthyologist from the Canada Museum of Nature once observed that there are "64 [species of Dragonfishes] reported from Canada, 5 of which reach the Arctic". These species are most commonly found in the mesopelagic to bathypelagic regions at a depth of 1000m-4000m, and in the Arctic, most samples of these species have been captured along the Davis Strait. The average temperature in these waters is approximately 3–4 °C Some examples of species discovered in that region are: Astronesthes cf. richardsoni; Borostomia antarcticus; Chauliodus sloani; Malacosteus niger; Rhadinesthes decimus; Stomias boa.

## Features

It is one of the many species of deep-sea fish that can produce their own light through a chemical process known as bioluminescence. A special organ known as a photophore helps produce this light. The deep-sea dragonfishes have large heads, and mouths equipped with many sharp fang-like teeth. They have a long stringlike structure known as a barbel, with a light-producing photophore at the tip, attached to their chin. They also have photophores attached along the sides of their body. A specific species of Stomiidae, the Chauliodus, cannot luminesce longer than 30 minutes without adrenaline. However, in presence of adrenaline, it can produce light for many hours. They produce blue-green light, the wavelengths of which can travel the farthest in the ocean. The deep-sea dragonfish waves its barbel back and forth and produces flashing lights on and off to attract prey and potential mates. Many of the species they prey upon also produce light themselves, which is why they have evolved to have black stomach walls to keep the lights concealed while digesting their meal in order to stay hidden from their predators.

### Jaw morphology of Adults and Larvae

The jaw of members in the Stomiidae family is adapted extremely well for survival and predation in the deep sea. Although small in size, the dragonfish jaw is adapted to capture large prey that are up to 50% the body mass of themselves. The long "loosejaw" of the dragonfish exhibits increased resistive forces to lower jaw adduction compared to fish with shorter jaws; however, due to decreased surface area of the lower jaw, dragonfish are able to lower the mechanical advantage of adduction and increase adduction velocity through the reduction of resistive forces. Additionally, it is seen that the adductor mass of the lower jaw of deep-sea dragonfish is significantly decreased, allowing for increased ability to attain high adduction velocity. This makes the deep-sea dragonfish significantly more competitive when hunting for prey due to its ability to capture large prey quickly and efficiently.
An important distinction in jaw morphology between an adult dragonfish and its larvae are the shape of the mouth. The adult fish have an elongated snout-like face with a protruding jaw, while the larvae have a rounder shaped mouth and a lower jaw that does not protrude.
Additionally, members of this family have a unique head joint that contribute to its ability to open its 'loosejaw' so wide. Deep-sea dragonfish have a flexible connection between the base of the skull and first vertebrae called the occipito-vertebral gap where only the flexible notochord is present. In some taxa the first to tenth anterior vertebrae are reduced or entirely absent. This gap is the result of notochord elongation in this specific area. Functionally, the gap allows deep-sea dragonfish to pull back their cranium and open their mouths up to 120°, which is significantly farther than other taxa that lack such a head joint. This is what allows deep-sea dragonfish to engulf such large prey, resulting in improved survival through the ability to consume more organisms in an extremely food limited environment.
On top of an extremely well adapted jaw, members of the Stomiidae family also have teeth that are adapted for hunting in deep sea. Their teeth are sharp, hard, stiff, and transparent when wet, making their teeth dangerous weapons as these teeth become basically invisible in the light absent deep sea. This means the refraction index of their teeth is nearly identical to that of the sea water they inhabit. The transparency is due to a nanoscale structure of hydroxyapatite and collagen, while the tips of the transparent teeth of deep-sea dragonfish were found to emit more red light in seawater which further contributes to its transparency as red light is close to invisible at the depths that the deep-sea dragonfish reside due to a lack of light penetration.

### Evolution of sensory organs

The deep-sea dragonfishes are part of the stomiidae family, making up a clade of 28 genera and 290 species. The dragonfish possess unique adaptations to help them thrive in the deepest parts of the ocean. This family species have been discovered to use certain long-wave and short-wave bioluminescence to communicate, lure prey, distract predators, and camouflage themselves. The stomiidae family has many unique adaptations to their sensory organs for the deep sea. Most deep-sea organisms have only a single visual pigment sensitive to the absorbance ranges of 470–490 nm. This type of optical system is commonly found in the stomiidae family. However, three genera of dragonfish evolved the ability to produce both long-wave and short-wave bioluminescence. In addition, deep-sea dragon fishes evolved retinas with far-red emitting photophores and rhodopsins. These far-red emitting properties produce long-wave bioluminescence greater than 650 nm. This unique evolutionary trait was first seen around 15.4 Ma and had a single evolutionary origin within the stomiidae family.

### Reproductive features

Dragonfish females exhibit two distinct cohorts oocytes, one which is a white cream color during the first growing stage and the other which is orange-reddish in vitellogenesis. The orange-reddish ovaries are released in the current spawning season, while the other batch is in the growing stage. Stomiids are gonochoristic, allowing them to increase their reproductive fitness by using their energy to produce gametes instead of reconfiguring the reproductive system. The female adult stomiids are also larger than the males.

## Behavior

Dragonfish are a type of teleost fish that inhabit the deep sea and use bioluminescence to detect prey and communicate with potential mates. They possess far-red emitting photophores and rhodopsins that are sensitive to long-wave emissions greater than 650 nm, and have adapted to the unique light conditions of the deep-sea environment.

### Reproductive behavior

Egg-laying, which predominantly occurs in October, is preceded by a distinctive whirling behavior driven by the male prodding the side of the female's abdomen. Additionally, dragonfish possess a unique adaptation of being able to see using chlorophyll in their eyes, which may allow them to detect the weak bioluminescence of their prey and navigate their dark habitats more effectively. This research sheds light on the reproductive behavior and early life stages of the naked dragonfish and contributes to our understanding of the ecology and behavior of dragonfish species.

### Evolution and adaptations of the visual system

One study focuses on the stomiid family, which includes loosejaws and dragonfishes, analyzing the genetic makeup of the visual pigments in these fish and how they have adapted to the unique light conditions of the deep-sea environment. The research helps us understand how dragonfish behavior and vision have evolved to allow them to thrive in the deep sea. Dragonfish use far-red emitting photophores and rhodopsins to detect prey and navigate their habitats. Additionally, dragonfish use chlorophyll in their eyes to detect the weak bioluminescence of their prey, which is an unusual adaptation for a vertebrate.

### Visual communication and behavior

Teleost fishes exhibit a wide range of visual signals, including color, texture, form, and motion, that are used to find mates, establish dominance, defend territory, and coordinate group behavior. Dragonfish have specialized bioluminescent organs that produce red light to communicate with potential mates and prey. Understanding the visual communication and behavior of teleost fishes is essential to understanding the behavior of dragonfish in their natural habitats.

## Bioluminescence in Stomiidae

Dragonfish of the Stomiidae family are largely characterized by their bioluminescent barbels, which act as lures for prey and are a species-specific structure. These barbels extend anteriorly off the bottom jaw, and prey attracted to its bioluminescence include lanternfish and bristlemouths. It is proposed that the specificity of bioluminescent barbel structure to certain species allows for advantageous same-species recognition that promotes genetic isolation, in addition to allowing scientists to more easily identify distinct species due to anatomical barbel differences. The diversity of Stomiidae species is exceptional for their clade age thanks largely to the species-specific barbels. Further, sexual dimorphism of bioluminescence in dragonfish contributes to even greater diversity within the species, but the greater abundance of immature specimens within research collections makes studying sexual dimorphism challenging.
In addition to a bioluminescent barbel, members of the Stomiidae family have a blue light emitting photophore in the postorbital region. Some dragonfish, such as the Malacosteus niger, all Aristostomias species, Pachystomias microdon, and Chirostomias also have a unique red light emitting photophore in the suborbital region. It is thought that the mechanism of red bioluminescence produced by the suborbital photophore is facilitated by energy transmission and is chemically similar to the blue bioluminescence of the barbel. While suborbital photophores that emit red bioluminescence are particularly helpful for finding prey, since many organisms in the deep sea can only see blue light, it appears as though this red light emission by dragonfish is not directly associated with prey choice, and it is thus hypothesized that it may be used for intraspecific communication. This raises an interesting question of to what extent the red bioluminescence determines dragonfish prey choice.

### Lure bioluminescence

Species of the Stomiidae family use blue bioluminescence for communication, camouflage, and as a luring mechanism. They emit shortwave blue bioluminescence from postorbital photophores and from a long, slender appendage on the chin, called the barbel. The shaft of the barbel is composed of cylindrical muscles, blood vessels and nervous fibers, and the bulb of the barbel has a single photophore. The catecholamine adrenaline is found in the connective tissue within the stem. One hypothesis regarding barbel control is that adrenaline innervation may control both the movement of the barbel and its production of bioluminescence. Data from a study performed on specimens of the Stomias boa species agree with this hypothesis because the barbels of the dragonfish produced light emissions following exposure to external adrenaline.
The loose jaw dragonfishes, which include species from Aristostomias, Malacosteus, and Pachystomias, have the ability to detect and produce red bioluminescence. This is made possible by far-red emitting photophores located under the eye and rhodopsins that are sensitive to long-wave emissions. This red bioluminescence is used to illuminate prey and to detect other far-red dragonfishes, because it goes undetected by most other species. The species with far-red emitting photophores differ in morphology and behavior from most other dragonfish species. For example, the barbels of these species are more simple in structure than those of other dragonfishes. They also differ in foraging strategies. While most dragonfishes that produce shortwave blue bioluminescence undergo regular diel vertical migrations, this is not seen in those with far-red emissions. The foraging strategy they undergo involves remaining in the deep-sea and emitting far-red bioluminescence to illuminate a small area and search for prey. Although Malacosteus, Pachystomias, and Aristostomias all have suborbital photophores that produce red bioluminescence, there are differences in the suborbital photophores between these three genera, in their shape, color, flash duration, and maximum emission.

## Representative species gallery

* Astronesthes niger
* Bathophilus vaillanti
* Chauliodus danae
* Echiostoma barbatum
* Eustomias trewavasae
* Grammatostomias dentatus
* Idiacanthus atlanticus
* Malacosteus niger
* Melanostomias melanops
* Neonesthes capensis
* Photostomias guernei
* Pachystomias microdon
* Photonectes gracilis
* Photostomias atrox
* Stomias nebulosus