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Wasp sting, with a droplet of venom

Venom is a secretion containing one or more toxins produced by an animal to cause harm to another.^[1] Venom has evolved in a wide variety of animals, both predators and prey, and both vertebrates and invertebrates.

Venoms kill through the action of at least four major classes of toxin, namely necrotoxins and cytotoxins, which kill cells; neurotoxins, which affect nervous systems; myotoxins, which damage muscles.

Venomous animals kill tens of thousands of humans per year. However, the toxins in many venoms have potential to treat a wide range of diseases.

Evolution

The use of venom across a wide variety of taxa is an example of convergent evolution. It is difficult to conclude exactly how this trait came to be so intensely widespread and diversified. The multigene families that encode the toxins of venomous animals are actively selected, creating more diverse toxins with specific functions. Venoms adapt to their environment and victims and accordingly evolve to become maximally efficient on a predator's particular prey (particularly the precise ion channels within the prey). Consequently, venoms become specialized to an animal's standard diet.^[2]

Mechanisms

Phospholipase A2, an enzyme found in bee stings.

Venoms cause their biological effects via the toxins that they contain; some venoms are complex mixtures of toxins of differing types. Among the major classes of toxin in venoms are:^[3]

• 
  Necrotoxins, which cause necrosis (i.e., death) in the cells they encounter.^[4]Rattlesnake and viper venom contains phospholipase and trypsin-like serine proteases.


• 
  Neurotoxins, which primarily affect the nervous systems of animals.^[5] These include ion channel toxins that disrupt ion channel conductance. Black widow spider, scorpion, box jellyfish, cone snail and blue-ringed octopus venoms (among many others) function in this way.


• 
  Myotoxins, which damage muscles by binding to a receptor, are small, basic peptides found in snake (such as rattlesnake) and lizard venoms.^[6][7][8][9]
  


• 
  Cytotoxins, which kill individual cells, are found in the apitoxin of honey bees and the venom of black widow spiders.^[10][11]
  

Taxonomic range

Venomous animals

Venom is widely distributed taxonomically, being found in both invertebrates and vertebrates; in aquatic and terrestrial animals; and among both predators and prey. The major groups of venomous animals are described below.

Arthropods

Venomous arthropods include spiders, which use fangs — part of their chelicerae — to inject venom; and centipedes, which use forcipules — modified legs — to deliver venom; along with scorpions and stinging insects, which inject venom with a sting.

In insects such as bees and wasps, the stinger is a modified egg-laying device — the ovipositor. In Polistes fuscatus, the female continuously releases a venom that contains a sex pheromone that induces copulatory behavior in males.^[12] In Polistes exclamans, venom is used as an alarm pheromone, coordinating a response with from the nest and attracting nearby wasps to attack the predator.^[13] In Dolichovespula arenaria, the observed spraying of venom out of their sting has been seen from workers in large colonies.^[14] In other cases like Parischnogaster striatula, the venom is applied all over their body as an antimicrobial protection.^[15] The venom from Agelaia pallipes has inhibitory effects on processes like chemotaxis and hemolysis which can lead to organ failure.^[16]

Many caterpillars have defensive venom glands associated with specialized bristles on the body, known as urticating hairs, which can be lethal to humans (e.g., that of the Lonomia moth), although the venom's strength varies depending on the species.^[17]

Bees synthesize and employ an acidic venom (apitoxin) to cause pain in those that they sting to defend their hives and food stores, whereas wasps use a chemically different alkaline venom designed to paralyze prey, so it can be stored alive in the food chambers of their young. The use of venom is much more widespread than just these examples. Other insects, such as true bugs and many ants, also produce venom. At least one ant species (Polyrhachis dives) has been shown to use venom topically for the sterilisation of pathogens.^[18]

Other invertebrates

There are venomous invertebrates in several phyla, including jellyfish such as the dangerous box jellyfish^[19] and sea anemones among the Cnidaria,^[20]sea urchins among the Echinodermata,^[21]
and cone snails^[22] and cephalopods including octopuses among the Molluscs.^[23]

Vertebrates

Fish

Venom is found in some 200 cartilaginous fishes – stingrays, sharks, and chimaeras; the catfishes (about 1000 venomous species); and 11 clades of spiny-rayed fishes (Acanthomorpha), containing the scorpionfishes (over 300 species), stonefishes (over 80 species), gurnard perches, blennies, rabbitfishes, surgeonfishes, some velvetfishes, some toadfishes, coral crouchers, red velvetfishes, scats, rockfishes, deepwater scorpionfishes, waspfishes, weevers, and stargazers.^[24]

Amphibians

Among amphibians, some salamanders can extrude sharp venom-tipped ribs.^[25][26] The poison dart frogs (Dendrobatidae) secret toxic alkaloids including batrachotoxins, histrionicotoxins, indolizidines, pumiliotoxins, decahydroquinolines, gephyrotoxins, 2,6-disubstituted piperidines, 2,5-disubstituted pyrrolidines, pyridyl-piperidines, indole alkaloids, azatricyclododecenes and amidine alkaloids from the granular glands in their skin.^[27]

Reptiles

The venom of the prairie rattlesnake, Crotalus viridis (left) includes metalloproteinases (example on the right) which help digest the prey before the snake eats it.

Some 450 species of snake are venomous.^[24]Snake venom is produced by glands below the eye (the mandibular gland) and delivered to the victim through tubular or channeled fangs. Snake venoms contain a variety of peptide toxins, including proteases, which hydrolyze protein peptide bonds, nucleases, which hydrolyze the phosphodiester bonds of DNA, and neurotoxins, which disable signalling in the nervous system.^[28] Snake venom causes symptoms including pain, swelling, tissue necrosis, low blood pressure, convulsions, hemorrhage (varying by species of snake), respiratory paralysis, kidney failure, coma and death.^[29] Snake venom may have originated with duplication of genes that had been expressed in the salivary glands of ancestors.^[30][31]

Venom is found in a few other reptiles such as the Mexican beaded lizard,^[32] the gila monster,^[33] and some monitor lizards including the Komodo dragon.^[34] Mass spectrometry showed that the mixture of proteins present in the venom is as complex as the proteins found in snake venom.^[34][35]
Some lizards possess a venom gland; they form a hypothetical clade, Toxicofera, containing the suborders Serpentes and Iguania and the families Varanidae, Anguidae, and Helodermatidae.^[36]

Mammals

Euchambersia, an extinct genus of therocephalians, is hypothesized to have had venom glands attached to its canine teeth.^[37]

A few species of living mammals are venomous, including solenodons, shrews, vampire bats, the male platypus and the slow loris.^[24][38] Shrews are known to have venomous saliva and most likely evolved their trait similarly to snakes.^[39] The presence of tarsal spurs akin to those of the platypus in many non-therian Mammaliaformes groups suggests that venom was an ancestral characteristic among mammals.^[40]

Extensive research on platypuses shows that their toxin was initially formed from gene duplication, but data provides evidence that the further evolution of platypus venom does not rely as much on gene duplication as once was thought.^[41] Modified sweat glands are what evolved into platypus venom glands. Although it is proven that reptile and platypus venom have independently evolved, it is thought that there are certain protein structures that are favored to evolve into toxic molecules. This provides more evidence as to why venom has become a homoplastic trait and why very different animals have convergently evolved.^[42]

Venom and humans

Venomous animals resulted in 57,000 human deaths in 2013, down from 76,000 deaths in 1990.^[43]

Venoms, found in over 173,000 species, have potential to treat a wide range of diseases, explored in over 5,000 scientific papers.^[33] Snake venoms contain proteins which can be used to treat conditions including thrombosis, arthritis, and some cancers.^[44][45]Gila monster venom contains exenatide, used to treat type 2 diabetes.^[33]

See also

• Poison


• Envenomation


• Schmidt Sting Pain Index


• Big Four (Indian snakes)


• List of venomous animals


• Venomous mammals


• Venoms in medicine

References