On first glance, the Irukandji jellyfish (Carukia barnesi) doesn’t seem to warrant a second one. Barely the size of a matchstick, tentacles and all, it’s so tiny that most swimmers wouldn’t notice one if it swam right past their face. But what the little jelly lacks in size it makes up for in potent toxins, which are stored in microscopic stinging cells that cover its body. The jellyfish’s sting won’t hurt at first, but 4 to 48 hours later, the venom from these tiny jellies—a species of box jellyfish—can cause a syndrome characterized by feelings of impending doom and life-threatening spikes in blood pressure. Researchers have pursued the secrets of venomous creatures like these for decades. And they’ve found that the weapons these animals use to inject their toxins, as much as the venoms themselves, reveal just how clever, and devious, evolution can be.
Although most people use the words poisonous and venomous interchangeably, they have very different meanings to toxinologists (the scientists who study toxins and the organisms that produce them). Poisonous species transmit toxins passively through inhalation, ingestion, or absorption. Venomous animals, in contrast, actively deliver their payload with bites and stings that carry painful, dangerous, even deadly, consequences. To be venomous, an animal must have toxin-delivering weaponry, plus a gland or tissues in which to store the toxins until they can be injected. These are the weapons that have long been the cause of human injury and death, but they have also been the source of inspiration and innovation—and invaluable evidence for how evolution works.
The Better to Bite You With
Some of the best-known venomous animals are, literally, armed to the teeth. Snakes, spiders, and others in this group use mouthparts to inject their toxins. Fangs come in a wide variety of shapes and sizes, not to mention injection methods.
A perfect example of that variety can be found in snakes. Most deadly serpent species belong to one of two families: the elapids (Elapidae) and the vipers (Viperidae). Elapids, such as cobras and mambas, have short, sturdy fangs built for durability. Early elapids fed on other reptiles, so their teeth had to punch through tough, thick scales. “Their fangs evolved to be like hole-punchers,” says Bryan Fry, head of the venom evolution lab at the University of Queensland in Brisbane. Modern elapids prey on anything from birds to fish, but the hole-punching fangs have remained.
Vipers, in contrast, possess long, slender fangs with a sharp cutting edge on the inside of the fang’s curve. These snakes ambush their prey with a quick snap of the head, quickly sinking their teeth into and back out of their victims like the jab of a hypodermic needle. Such teeth are well-suited for their primary prey, mammals, which have thin skin but thick fur—a combination that favors length over strength.
In contrast, some venomous snakes, such as those in the Colubridae family, have fangs at the back of their jaw rather than the front. And some members of another family, the Atractaspidinae, have an even stranger venom system. These so-called stiletto snakes, named for the stiletto-blade shape of their fangs, are burrowers that attack in confined spaces. Because they don’t have room to open their mouths widely, their fangs jut out of the sides of their mouths. “With a sideways flick of their head, they jab the fang into their prey,” Fry says.
Venomous weaponry is hardly limited to reptiles. There are more than 2,000 venomous fish in the sea. Most of these deliver their toxins through stingers, but reef-dwelling fish called fangblennies do so with their bite. As far as scientists can tell, fangblenny teeth evolved in a very different manner than those of venomous snakes. In snakes, toxins appeared before the animals developed a means by which to deliver them. But in fangblennies, it appears that the highly modified teeth evolved first.
Fangs aren’t the only way to deliver venom by mouth. Leeches use venom as a numbing agent, a way to mask their presence so as to feast on their victims’ blood unnoticed. These worm-like parasites suction themselves onto their victims with a strong oral sucker and make cuts using three, blade-like jaws studded with fine teeth to access their blood meal. The leaches’ venom contains chemicals that simultaneously anesthetize the bite area and prevent the blood from clotting. Over the centuries, medical practitioners have exploited those anticoagulants, intentionally placing leeches on patients to stimulate blood flow. Today, compounds from leech venom have been turned into life-saving medications, some used as anticoagulants during surgery and others in people with blood-clotting disorders.
Not to be left out, there are even two species of mammal with venomous fangs: the insectivorous solenedons. These two endangered species found only on the Caribbean islands of Cuba and Hispaniola deliver toxic bites through slotted lower incisors. Why these animals have evolved this venom system remains a mystery. According to Nicholas Casewell, a toxinologist at the Liverpool School of Tropical Medicine in the U.K. who is studying solenodon venoms, the long-snouted mammals eat invertebrates and have big teeth, so venom doesn’t really seem necessary for predation. Nor are they using it for defense. “They don’t have any natural predators, apart from certain owl species,” he says. “We just don’t know.” Unfortunately, because fewer and fewer solenedons are found each year, scientists are scrambling to understand even the most basic aspects of their natural history—not an easy task, Casewell says. “They’re nocturnal predators. They use burrows. It’s very difficult to actually observe what they’re doing.”
A wide variety of venomous species use toxins strictly as a deterrent against potential predators. When it comes to venom delivery, these animals rely on the predators to do the work for them. Like forts prepared for a siege, their weapons are stationary spikes, meant to prevent attackers from doing irreparable harm.
Sea urchins are the epitome of this kind of defense, their small round bodies covered in protective spikes. Their phylum name—Echinodermata—literally means “spiny skin.” Not all urchins are venomous, but those that are would be a meal best skipped. Urchins in the Diadematidae family have two kinds of spines: long thin ones that are the first line of defense and shorter, even thinner spines that have poisonous tips. Both types of spines are covered in barbs that break off easily and remain embedded even after the spine has been pulled out. In the case of the short spines, these barbs continue to leach out painful toxins long after the predator has been impaled.
Unlike the banded urchin and other Diadematidae, flower urchins (in the family Toxopneustidae) don’t use their spines for venom delivery. These colorful urchins are covered in pedicellariae, tiny pincer-like appendages that are outfitted with their own venom glands. When another animal comes into contact with the pedicellariae, the urchin’s interior muscles squeeze, causing the pincers to clamp shut and inject venom. Even the least toxic members of this family pack a venom that, drop for drop, is more potent than most snakes’—just four milligrams could kill a grown human. (Fortunately, it would take a lot of urchins to deliver that dose.) The most toxic members, however, are best avoided altogether. While scientists haven’t yet tested the potency of the most venomous member, Toxopneustes pileolus, the species is rumored to have killed divers and may, in fact, wield the most toxic venom on the planet.
Most venomous fish—of which there are around 3,000—also use toxins exclusively for defense (fang-blennies excluded, of course). Depending on the species, venomous spines can jut from fins, gill covers, and many other body parts. Any predators attempting to close their jaws around these species learn quickly just how excruciating their toxins can be.
The deadliest of these are the stonefishes, so named for their cryptic appearance. They can be nearly impossible to spot, the color pattern of their skin mimicking the rocky ocean floor they rest upon. Such camouflage is key to their strategy of ambushing prey, as well as avoiding detection by their own predators. When camouflage fails, the stonefish’s venomous spines keep predators away.
Stonefish spines are among the most specialized venom-delivery systems in fish. While most fish spines are wrapped in venom-laden tissues, the stonefish has evolved stout, hollow spines that act like hypodermic needles to deliver a large volume of concentrated venom from an internal gland. When a predator attempts to close its mouth on a stonefish, the spines’ sharp tips pierce the predator’s flesh. The pressure of the bite squeezes the venom up through the stonefish’s spines and into the attacker. The pain of stonefish stings is so intense that, according to one report, even “hardened fishermen were often reduced to tears.” Stonefish stings can lead to nausea, difficulty breathing, irregular heartbeat, even death.
On land, caterpillars in the genus Lonomia also carry impressive spikes. Lonomia young are juicy little grubs, just the sort that small birds and mammals might make a meal out of. But these caterpillars have evolved a particularly noxious defense. Beautifully complex spires of spines, each one filled with venom, cover their soft, fleshy bodies and make them appear as though they are covered in delicate, pulled-glass sculptures. When a hungry bird attempts to swallow a whole caterpillar, the larva’s sharp spines penetrate the soft flesh of the bird’s mouth and throat. The spine tips then break like ampules allowing venom to leach into the wound. Venom from the deadliest of these caterpillars can effectively interfere with an animal’s blood clotting system, leading to uncontrolled internal hemorrhaging. In Brazil, where these animals are native, they used to cause dozens of human deaths each year. Now, thanks to anti-venom, those numbers have been drastically reduced.
Perhaps even more surprising than lethal caterpillars are venomous frogs. There are many species of poisonous frogs, but only two that are venomous. Bruno’s casque-headed frog and its relative, Greening’s frog, have each evolved tiny projections from their bones designed to prick the skin of would-be predators, allowing toxins to enter. While the venom of these frogs is highly potent—just 1 gram of toxin from the Bruno’s casque-headed frog could kill up to 80 people—these small creatures produce very little toxin at any one time, and typically deliver only enough to act as a painful reminder for predators to steer clear.
Unlike animals armed with spiky weapons, which must sit and wait for a victim, those with venomous stingers actively engage their targets. Whether hunting meals, fending off attacking predators, or fighting rivals, these species brandish battle-ready weapons.
The small platypus is a bizarre mish-mash of body parts—a duck-like bill on an otter’s body—and it’s one of only five mammal species to lay eggs, rather than give birth to live young. But to scientists like Bryan Fry, a toxinologist at the University of Queensland in Brisbane, the platypus’s most intriguing peculiarity is its sting. The cuddly-looking critters are the only venomous mammal that stings, rather than bites. Males sport a spur on the heels of their hind legs that they can use to inject a venom an inflict excruciating pain.
The animals are usually quiet and reserved, but they go all in when trying to atract a mate. While platypus venom may not be as deadly as a Lonomia caterpillars’, it can still result in days of throbbing agony that traditional painkillers like morphine can’t touch. A war veteran jabbed on his thumb once described the pain as excruciating—far worse than anything he experienced as a soldier, including shrapnel.
A smaller but very potent sting can be found in the bullet ant, so named for the intense pain its stinger is said to cause. Unlike the platypus, it’s the females of this species that wield the weapons. And, like most venomous ants, bullet ants use their toxins to protect themselves and their colonies from potential threats. In humans, the intense pain of a bullet ant’s venom can last for hours, even days, depending on the number of stings. Some indigenous cultures in the Amazon incorporate these agonizing stings into their rights-of-passage rituals, requiring young men to endure dozens of stings on their hands to prove their manhood and their ability to lead.
Bullet ants are members of the insect order Hymenoptera—which includes ants, bees and wasps. All of the stinging members in this group use modified egg-laying structures to deliver their venom. One group of insects in this order, the parasitoid wasps, have evolved a particularly sinister use for their stings. These wasps use their venom to manipulate the behavior of other species, putting them into a zombie-like state and then laying eggs on their bodies. These host organisms then unwittingly provide a living meal—their own flesh—for the wasp larvae when they hatch.
To create these zombies, the wasps inject venom directly into the brains of their hosts with incredible precision. The stinger of the emerald cockroach wasp, for instance, can sense its exact location inside a cockroach’s brain. Scientists aren’t exactly sure how the wasp achieves this, but removing the wasp’s injection sites from a roach brain causes the wasp to search with her stinger for several minutes, seemingly frustrated that she can’t find what she’s looking for.
Sinister as this sort of parasitic behavior may be, in terms of sheer menacing appearance, few stingers are as fearsome as the scorpion’s. These venomous arachnids, with their spike-tipped tails, possess some of the most daunting weapons in the animal kingdom. And a number of scorpion species possess extremely potent venoms. About 25 of the 1,750 or so scorpion species have toxins strong enough to kill a human.
Interestingly, venom from one of those deadly species, ominously named the deathstalker, may one day be used in the treatment of cancer patients. Jim Olson, a pediatric oncologist at the Fred Hutchinson Cancer Research Center, discovered that a protein in deathstalker venom binds to cancerous cells and bypasses healthy ones. Olson took that protein, called cholorotoxin, and attached a fluorescent compound to it to create what he calls a “tumor paint.” When the paint is injected into a cancer patient, chlorotoxin targets tumor cells and binds to them, while the fluorescent marker lights them up. This allows surgeons to turn on a near-infrared lamp and see tumors with the naked eye, cancerous cells they might have missed otherwise. The compound is already in clinical trials.
War-related shrapnel injuries, with their embedded weapons, can cause pain that lasts for years. The same is true of venomous shrapnel: poisonous pieces left behind can yield painful wounds with prolonged healing times.
The shrapnel used by Irukandji and other box jellyfish, those with cube-shaped bodies in the class Cubozoa, has made them some of the deadliest animals on Earth. To inject their venom, these animals use a remarkable biological innovation: stinging cells called nematocysts. Each box jelly can send hundreds to thousands of these cells shooting into its victim at once.
Each stinging cell contains an armed capsule that’s ready to launch a venomous barb when triggered by contact from a potential predator or prey. Coiled up inside the nematocyst is a tiny tubule. When the nematocyst is fired, it sends that tubule—which is covered with microscopic spikes—into its victim. The tubule injects a miniscule amount of one of the most potent venoms on Earth, and its spikes ensure that it remains embedded. Many of these cells can discharge at once and, like venomous buckshot, leave a box-jelly victim with hundreds to tens of thousands of tiny pieces of toxic shrapnel embedded in its tissues.
Inserting these spine-laden tubules takes ballistic-level force. Researchers have found that when triggered, nematocysts discharge their tubule in less than a millionth of a second—around 1/500,000th the time it takes to blink an eye—making it the fastest biological action ever discovered. The tube tip can reach speeds of 83 miles per hour before hitting its target, exerting pressure equivalent to that of some bullets. And even after the venom has worn off, the embedded tubules can cause painful skin reactions for weeks or even months after the sting.
Although the threats some jellyfish pose are well known, most of us never think about deadly marine snails. But they do exist. Most snails have modified teeth fused together into a scraping device called a radula, which grinds up algae or plant matter and pulls it toward the snail’s mouth. But evolution has shaped cone snail “teeth” into disposable, venom-injecting harpoons. The tips of their radular teeth have arrow-like barbs that anchor the harpoon when it hits its target. When a snail is ready to attack, it loads one of these harpoons into a tubular, extendable mouthpart called the proboscis, and then begins to stalk its much speedier prey. When the snail strikes, it fires its radular tooth into the body of is prey, injecting a venom so toxic that it can instantly paralyze a fish. The snail uses the harpoon to reel in its meal, then envelops and ingests its prey.
The microscopic harpoons of cone snails may be potent, but they are far less intimidating than the jagged barbs wielded by another group of venomous marine animals. Stingrays are flat, bottom-dwelling fish armed with a defensive stinger of hardened cartilage located near the base of their tails. When attacked, a stingray can whip its stinger into action, using the serrated blade to wound its assailant. Once embedded, the stinger then breaks off and remains, leaching agonizing venom. (In human victims, the stinger is often so deeply embedded it requires surgery to remove.) Stingray blades are so visually intimidating that humans have long co-opted them and fashioned them into weapons of their own. Polynesian and indigenous Australian warriors brandished spears made of stingray spines. And the great warrior of Greek mythology, Odysseus, survived mythic beasts and unimaginable hardships only to eventually be felled by a spear tipped with a venomous stingray barb.
For centuries, the intrigue of venomous creatures has inspired people to take a closer look. And the weapons of these species have helped us understand some of the intricacies of evolution, ecology, and medical science. We learn more from them every day. And there are thousands more venomous species that scientists haven’t yet had the chance to study. Earth’s vast venomous biodiversity, and toxinologists the world over stand at the ready.
Additional image credits:
Header image of Irukandji box jellyfish (Carukia barnesi) with prey by Jurgen Freund
Closeup of Echinothrix diadema spines © Rich Mooi
Corythomantis greening frog © Carlos Jared
CT scan of Aparasphenodon brunoi © Carlos Jared
Death stalker scorpion (Leiurus quinquestriatus) by Chris Mattison
Irukandji box jellyfish (Carukia barnesi) with prey by Jurgen Freund
Southern stingray (Dasyatis americana) by Alexander Mustard