Cone Snails Are Liars and Murderers

a cone snail swallows a fish whole
Paulo Oliveira / Alamy

The aptly named cone snail wears a house that resembles a Ben & Jerry’s receptacle, filled not with ice cream but with a squishy mollusk that sports an extendable, trunklike proboscis. The snails are superficially docile creatures, and can be painfully shy; sometimes they go weeks in a lab without taking a single bite of food, cringing at the slightest change in temperature, lighting, or human supervision. “These are not racing snails,” says Eric Schmidt, a biochemist at the University of Utah.

But when the snails eat, they feast, filling their digestive tract with a glut of battered bodies in various states of drug-induced disarray. Depending on the snail species at hand, some of the corpses might be fish, limp from hypoglycemic shock; they could be worms, sexually stimulated and hot to trot. These poor souls are among the many victims of cone-snail venom—one of the deadliest and most dizzyingly complex substances ever described in an invertebrate.

Some 1,000 species of cone snails exist, each, as far as scientists can tell, with its own unique recipe for venom. Several ingredients seem to closely resemble chemicals naturally made in fish, worms, and other mollusks; in manufacturing them, cone snails can turn their prey’s bodies against themselves. This sort of predatory mimicry is a devious strategy for an otherwise lethargic hunter: If you can’t beat them with speed, narc them and murder them. Had Agatha Christie been a mollusk, she might have been proud.

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Scientists have been enthralled by cone snails’ devious antics for decades. These mollusks can lance their prey in a fraction of a second, instantaneously incapacitating even the speediest fish; their cavernous mouths can balloon to chasmic widths and swallow their doomed targets whole. Their venoms make their modus operandi possible, and in recent years, researchers have made big strides in identifying several of these chemical cocktails’ most intriguing ingredients.

Some are straightforward paralytics released from a hooked tooth held at the proboscis’s end, which certain snails will launch like a harpoon at hapless fish. First, a fast-acting bolus of toxins temporarily freezes the fish; a second wave induces irreversible paralysis. The snail can then slowly reel its immobilized dinner into its billowing mouth. Baldomero “Toto” Olivera, a biochemist at the University of Utah who’s been spying on these snails for the past five decades, calls this the “taser and tether” tactic. It works best near coral reefs, where snails can sometimes ambush unsuspecting fish up to twice their size. “They can’t even get back into their shell when they harpoon a fish that big,” Olivera told me. A couple hours later, after the snail has liquefied the soft parts of its grub, it will belch the scales and bones back out. “You’ll find a ball of all the hard parts,” Olivera said.

Other snails, such as the geographer cone snail, hunt schools of fish that are snoozing out in the open. These creepers douse their prey in a concentrated cloud of benumbing chemicals that Olivera delightfully calls the “nirvana cabal.” Among the ingredients is insulin, the same hormone humans, fish, and many other animals make to keep their blood-sugar levels in check. The chemical seeps in through the fish’s gills and floods their bloodstream, triggering hypoglycemia and leaving them disoriented and defenseless. The fish “have no energy to swim away,” Olivera said. “It’s like they’re in an opium den.” The victorious snail then deploys its enormous mouth like a net to engulf the comatose fish one by one.

The fish are so zonked that they look “hypnotized, like they don’t even see the predator,” says Joshua Torres, a biochemist at the University of Copenhagen who studies cone snails. That might mean some fish “don’t even know what’s happening to them,” Helena Safavi-Hemami, Torres’s postdoctoral adviser, told me. Paralyzed fish of the tased variety are probably less blissfully unaware. “I think it’s painful,” Safavi-Hemami said. “They know, but then it’s too late, and they can’t do anything about it.” (For good measure, some insulin-ejecting cone snails, including geographers, will also sting their prey while engulfing them.)

A third strategy, “strike and stalk,” might be more macabre still. Many fish, wise to the danger of cone snails, might “gang up and viciously attack” one, Olivera said. So some snails will spend most of their time hiding, emerging only to sting a fish without tethering it. The snipe is so fast that some fish might never notice they’ve been hit. But within minutes, the toxins of these particular venoms will have worked their insidious magic: The fish will crumple, sluggish and quivering, allowing the snail to approach from behind and gulp it down gleefully.

Most of what’s known about these gastropods has been learned from studying fish hunters, arguably the flashiest ones of the bunch. But most cone snails prefer to munch on marine worms, and seem to engage an entirely different set of toxin-based strategies to do so. Torres, Schmidt, Olivera, Safavi-Hemami, and their colleagues recently discovered that one such tactic might involve tricking worms into thinking they’re about to get laid—a promise that rouses the wriggly creatures from their hidey-holes, only to end their life.

Torres’s team found that worm-hunting cone snails have evolved to produce molecules that resemble the pheromones of their prey. One chemical, conazolium A, looks like a pheromone that makes female worms swim in tight, tail-chasing circles before spewing their eggs; another is reminiscent of uric acid, which prompts males to eject gobs of sperm. Spritzed onto marine worms in the lab, the snail-made pheromones whipped most of the specimens into a sexual frenzy.

The pheromones “are a lure,” says Mandë Holford, a cone-snail-venom chemist at Hunter College. “I don’t think the worms even see it coming.” These cone snails are chemical saboteurs; they have weaponized another species’ appetite for sex. Because many marine worms die after reproducing, some of the ones consumed by cone snails likely die as duped virgins, their biological imperative unfulfilled.

Like the insulin-producers, these worm-hunting cone snails simulate chemicals naturally found in their prey. But while the insulin cohort seems to tame its targets, the pheromone-makers rile theirs up; both behaviors seem to bring hunter and huntee into close contact. “It’s incredible what these animals can do,” says Fiona Cross, a biologist at the University of Canterbury who studies spiders that also imitate other animals’ sexual attractants. “They’re so in tune with the biology of their prey.”

Torres and his colleagues still aren’t sure how the snails use these pheromones in natural contexts. They might inject them, like paralytics, or exude them to bathe their unsuspecting prey in eau de worm. Zhenjian Lin, a biochemist at the University of Utah, thinks the worm hunters might use their venom in multiple ways, first to attract, then to immobilize.  

Fundamentally, both insulin- and pheromone-producing snails are copycats, a trait that humans can adapt for our own purposes. Holford and other researchers think the venomous mollusks will become templates for humans making mollusk-inspired painkillers or insulin-based treatments. “Cone snails have mastered chemistry,” Torres told me. “They are smarter than we are.”

The beguiling allure of cone-snail venom is double-edged. Some cone-snail venoms are formidable enough to take down a human; over the years, a few dozen such deaths have been documented (typically after the victim has aggressively mishandled or frightened the mollusks). The cone-snail-savvy sector of the internet jokes that geographer cone snails are “cigarette snails,” because a sting would leave their victim time for only a few drags. Olivera chuckled grimly at this, but noted that it’s an exaggeration: Snail-driven death “takes longer than that,” he said. That’s great if medical care is on the way, but very bad if it’s not. Numbness spreads from the injection site, eventually reaching the diaphragm, which, once paralyzed, makes breathing impossible.

Olivera and Torres both assured me that they’ve never been stung—a streak that Torres in particular is intent on not breaking. When he goes diving for venomous-snail specimens, he picks them up with tongs.