Stomiidae
Stomiidae is a family of deep-sea ray-finned fish, including the barbeled dragonfishes, that live in all oceans in a wide range of depths. They are quite small, usually around 15 cm, up to 26 cm, and they exhibit a strong sexual dimorphism. 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 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.Genera
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' preferred habitat changes from day to night with DVM.Brian Coad, ichthyologist from the Canada Museum of Nature once observed that there are "64 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.Sexual dimorphic features
Throughout their developmental life, the Stomiidae family exhibits a wide degree of sexual dimorphism. Female adult Stomiidae are much larger than the males. Some male species of Stomiidae like Idiacanthinae also lack teeth, a chin barbel, a functional gut, and pelvic fins in their larval stages in contrast to their female larval counterparts. Additionally, a significant visual detection gap exists between males and females. Males have larger eyes and lenses that contain more photophores than females. Thus, an increase in photophores enables males to detect female organisms at a greater distance than females can detect males.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 due to its length; however, the absence of a floor to the oral cavity decreases the surface area of the lower jaw, reducing resistive forces, and 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.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. The extent of the head movement exposes the gills and presents a wide opening to the oral cavity behind the head. 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.
An important distinction in jaw morphology between an adult dragonfish and its larvae is 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.