Venomous fish are species of fish which produce strong mixtures of toxins harmful to humans (called venom) which they deliberately deliver by means of a bite, sting, or stab, resulting in an envenomation. As a contrast, poisonous fish also produce a strong toxin, but they do not bite, sting, or stab to deliver the toxin, instead being poisonous to eat because the human digestive system does not destroy the toxin they contain in their bodies. Venomous fish do not necessarily cause poisoning if they are eaten, as the digestive system often destroys the venom.
There are at least 1200 species of venomous fish,  with catfishes alone possibly contributing 250–625 species to that total.  The former number accounts for two-thirds of the venomous vertebrate population. There are more venomous fish than venomous snakes and indeed more than the combined total of all other venomous vertebrates. Venomous fish are found in almost all habitats around the world, but mostly in tropical waters. Encounters with these species injure over 50,000 people every year.
Venomous fishes carry their venom in venom glands and use various delivery systems, such as spines or sharp fins, barbs, spikes and fangs. The most common venom delivery system is via dorsal spines. Venomous fish tend to be either very visible, using flamboyant colors to discourage predators from attacking them, or skillfully camouflaged and possibly buried in the sand. Apart from the value of improved self defense or capacity to kill prey, venom helps bottom dwelling fish by killing bacteria that could otherwise invade their skin. Few of these venoms have been studied. They are a yet-to-be tapped resource for bioprospecting to find drugs with medical uses.
Venomous fish are very often involved in human injury, usually by accident, but few studies have been done to investigate the constituents of fish venom. Even fewer studies have been done to create antivenom.
The only commercially available antivenom is for the Indo-Pacific stonefish, Synanceja trachynis Stonefish Antivenom (SFAV).
It has been found that the mucous and sting venom of Potamotrygon cf. henlei, a species of stingray that is found in Brazil, is toxic to mice that have nociceptive, edematogenic, and proteolysis activities. Two peptides were isolated from the stingray venom, orpotrin, which causes vasoconstriction, and porflan, which causes inflammation. Knowing how these peptides are structured could lead to the development of a neutralization technique that could effectively act as an antivenom.
Of all the piscine venoms studied, they all produce profound cardiovascular alterations, both in vivo and in vitro. These changes stimulate the release of nitric oxide from the endothelial cells, smooth muscle contractions, and other effects of the atria. Piscine venoms also produce neuromuscular activity effects- depolarization of nerve and muscle cells. In addition, piscine venoms have strong cytolytic activity. In experimental models and in Western immunoblotting analysis, all tested piscine venoms showed structural similarity, which could lead to the advent of an overarching antivenom or other novel uses.
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