Venom

• 1. Introduction

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  Venom has evolved independently at least 104 times across 8 different phyla. That is an extraordinary case of separate animal lineages reaching the same solution from entirely different directions. At its core, venom is a toxin that an animal manufactures inside its own body. It is then actively delivered into another animal through a wound, made by a bite, a sting, or some other penetrating action. That delivery usually relies on a specially evolved structure, such as fangs or a stinger. Scientists call this process envenomation, and it separates venom from two closely related terms. A poison is delivered passively, through ingestion, inhalation, or absorption through the skin. Toxungen is actively transferred onto the outside of a target, but without a wound ever forming. Venom shows up in both terrestrial and aquatic animals, among predators and prey, and across vertebrates and invertebrates alike. It kills through four distinct biological routes. Necrotoxins and cytotoxins destroy cells. Neurotoxins disrupt nervous systems. Myotoxins damage muscle tissue. Haemotoxins interfere with blood clotting. What forces so many unrelated animals toward the same evolutionary answer? And what happens when the prey these toxins are built to kill develops the means to survive them?

• 2. Convergent Evolution

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  The multigene families that encode an animal's toxins sit under constant selective pressure. That pressure keeps generating more diverse compounds, each tuned to a specific function. Exactly how venom became so widespread, and so varied, across the animal kingdom is difficult to pin down. Some species do not build every toxin themselves. A number of them acquire venom components from associated microbes, organisms that can live directly inside a venom-delivery apparatus. Venom does not stay fixed once it appears. It keeps adapting to an animal's environment and to the specific victims it targets, evolving toward maximum efficiency against the precise ion channels found in its usual prey. Because of that, some venoms become narrowly specialized to match a single predator's standard diet.

• 3. Toxin Mechanisms

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  Ion channel toxins make up the neurotoxin class, and they turn up in animals as different as mambas, black widow spiders, scorpions, box jellyfish, cone snails, centipedes, and blue-ringed octopuses. Myotoxins take a different route, binding to a receptor and damaging muscle directly. These small, basic peptides appear in snake venoms, including that of the rattlesnake, and in the venom of some lizards. Cytotoxins kill individual cells outright, a mechanism present in the apitoxin of honey bees and in the venom of black widow spiders. A subclass of cytotoxins, the necrotoxins, goes further still, causing necrosis, the outright death of the cells and tissues they reach. Vipers and bees both produce complex venoms containing phospholipases. Viper venom often carries something extra: trypsin-like serine proteases.

• 4. Arthropod Venom

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  Spiders inject venom through fangs mounted on their chelicerae, while centipedes deliver it through forcipules, legs that have been modified into piercing tools. Scorpions and stinging insects rely on a sting instead, and in bees and wasps that sting is a modified ovipositor, the organ insects normally use to lay eggs. In the wasp Polistes fuscatus, females continuously release a venom laced with a sex pheromone that triggers copulatory behavior in males. Polistes exclamans wasps use their venom differently, as an alarm pheromone that rallies the nest and draws nearby wasps into attacking a predator. Parischnogaster striatula takes yet another approach, spreading venom over its entire body as an antimicrobial shield. Many caterpillars carry defensive venom glands tied to urticating hairs on their skin. These are usually just irritating, but the venom of the Lonomia moth can kill a person. Bees make an acidic venom called apitoxin to defend their hives and food stores. Wasps rely on a chemically different venom that paralyses prey rather than killing it, keeping the prey alive to provision their young. True bugs and many species of ant produce venom too, and the ant Polyrhachis dives applies its venom topically, using it to sterilize pathogens on its own body.

• 5. Sea And Amphibian Venom

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  Among the Cnidaria, box jellyfish, the Portuguese man-of-war, a type of siphonophore, and various sea anemones all carry venom. Sea urchins, in the phylum Echinodermata, are venomous too, as are cone snails and cephalopods such as octopuses, among the molluscs. In fish, venom turns up in roughly 200 species of cartilaginous fish, including stingrays, sharks, and chimaeras. About 1,000 species of catfish are venomous, alongside 11 clades of spiny-rayed fishes known as the Acanthomorpha. That group includes over 300 species of scorpionfish, more than 80 species of stonefish, and gurnard perches, blennies, rabbitfishes, surgeonfishes, some velvetfishes, some toadfishes, coral crouchers, red velvetfishes, scats, rockfishes, deepwater scorpionfishes, waspfishes, weevers, and stargazers. Among amphibians, some salamanders can extrude sharp, venom-tipped ribs when threatened. Two frog species in Brazil carry tiny spines around the crown of their skulls that deliver venom on impact.

• 6. Reptile And Mammal Venom

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  Roughly 450 species of snake are venomous, producing their toxin in glands below the eye, the mandibular glands, before delivering it through tubular or channeled fangs. Snake venom is a mix of peptide toxins: proteases that break down protein bonds, nucleases that break down the phosphodiester bonds of DNA, and neurotoxins that disrupt signalling in the nervous system. A bite can cause pain, swelling, tissue necrosis, low blood pressure, convulsions, haemorrhage, respiratory paralysis, kidney failure, coma, and death, varying by species. Snake venom itself may have originated from the duplication of genes once expressed only in an ancestor's salivary glands. Venom also appears in a few other reptiles, including the Mexican beaded lizard, the gila monster, and some monitor lizards such as the Komodo dragon. Mass spectrometry has shown that the protein mixture in their venom is just as complex as that found in snakes. Some lizards, together with snakes, form a hypothetical clade called Toxicofera, spanning the suborders Serpentes and Iguania and the families Varanidae, Anguidae, and Helodermatidae. Among mammals, the extinct genus Euchambersia, a therocephalian, is hypothesized to have carried venom glands attached to its canine teeth. Living venomous mammals include solenodons, shrews, the European mole, vampire bats, male platypuses, and slow lorises. Shrews produce venomous saliva, a trait thought to have evolved along a path similar to that of snakes. Tarsal spurs like those of the platypus turn up in many non-therian Mammaliaformes groups, suggesting venom was once an ancestral trait among mammals generally. Research on platypuses shows their toxin first formed through gene duplication, though its later evolution appears to rely less on that mechanism than once believed. Modified sweat glands evolved into the platypus's venom glands.

• 7. Medicine And The Human Toll

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  Envenomation caused 57,000 human deaths in 2013, down from 76,000 in 1990. Scientists have found venom in over 173,000 species, and it has been the subject of more than 5,000 scientific papers. In medicine, snake venom proteins are already used to treat thrombosis, arthritis, and some cancers. Gila monster venom contains exenatide, a compound used to treat type 2 diabetes. Solenopsins, extracted from fire ant venom, have shown biomedical promise ranging from cancer treatment to psoriasis. A dedicated field, venomics, now studies the proteins in venom and how each component might serve a pharmaceutical purpose.

• 8. Resistance And Beyond

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  Venom often serves as a trophic weapon, and the tug-of-war between venomous predators and their resistant prey has been described as a chemical arms race. These pairs are expected to coevolve over long stretches of time, with predators killing off susceptible individuals until only resistant prey remain. Resistance typically climbs as predators grow less able to subdue the survivors, though building that resistance carries a high cost for both sides. The California ground squirrel shows this dynamic directly: it has varying resistance to the venom of the Northern Pacific rattlesnake, built through toxin scavenging that scales with local rattlesnake density. In response, rattlesnakes in those denser populations have locally evolved more effective venom. Kingsnakes of the Americas, constrictors that prey on many venomous snakes, carry a resistance that does not change with age or exposure. They are immune to the venom of copperheads, cottonmouths, and North American rattlesnakes, but not to that of king cobras or black mambas. In the ocean, eels resist the venom of sea snakes, mixtures of neurotoxins, myotoxins, and nephrotoxins that vary by species. Eels are especially resistant to the sea snakes that specialize in hunting them, while other fish outside that prey relationship show little resistance at all. Clownfish live permanently among the tentacles of venomous sea anemones, an arrangement only about 10 known anemone species can host, and only certain anemone-clownfish pairs are compatible. The anemone's venom, delivered through nematocysts and mucous secretions, is built to cause pain and tissue damage in predators. Clownfish survive it with a protective mucus that mimics the anemone's own chemistry closely enough to avoid triggering a sting, sometimes acclimating that mucus to match a particular species. Venom, in this active sense, is not limited to animals at all. Every domain and kingdom of life has evolved some version of it. Bacteria carry secretion systems that inject a toxic payload into another cell; Pseudomonas aeruginosa uses a type VI secretion system to attack competing microbes. Bacteriophages inject their contents in a way that is arguably similar to venom. Stinging plants deliver toxins directly into a target, and the parasitic plant Cuscuta goes further, using a haustorium to inject digestive enzymes and extract nutrients from its host. Phytopathogenic fungi use structures called appressoria to break into plants and deliver cell-killing toxins, and entomopathogenic fungi use the same structures to penetrate insects. Nematophagous fungi capture and penetrate nematodes through several mechanisms, then produce toxins of their own. Even among single-celled protists, Coleps use organelles called toxicysts to inject toxins into prey, while other protists rely on structures called extrusomes.