Killer Snail Hunts with Weaponized Insulin

Even among a group of animals as weird as snails that hunt with venomous teeth they can launch like harpoons, the geography cone snail (Conus geographus), above, is a bit of a freak. 

Cone snails are a large group of colorful sea snails found mostly in the tropical waters of the Indian Ocean. They break two rules that a lot of people think snails abide by: that they’re 1) docile and 2) slow. These guys are carnivores and formidable hunters of fish, worms and mollusks. And while they’re generally slow moving, their attacks are lightning fast. After sneaking up on its prey, a cone snail ejects a barbed, venom-filled tooth from a long tube, called a proboscis, that surrounds its mouth. The venom paralyzes (and sometimes kills) the prey, and once it’s subdued, the snail retracts the tooth and drags the meal along with it into its mouth. The whole ordeal takes only a few seconds. 

The geography cone snail—the most venomous member of the group, responsible for several human deaths—takes a different approach. Instead of spearing a victim and then reeling it in like its cousins do, it first engulfs prey in a net-like false mouth (which leads to its proboscis and real mouth) and only injects it with venom after it's trapped. In other words, it grabs-and-stabs instead of stabs-and-grabs. 

Getting its false mouth around a fish can be a little bit of work, so the snail also releases a blend of toxins called the “nirvana cabal” into the water to sedate and disorient fish before trapping them. And it turns out that the snail's drug-grab-envenomate combo is even weirder than anyone had thought. While most of the ingredients of the nirvana cabal are neurotoxins that attack a victim’s nervous system, researchers have discovered that it also contains a specialized form of insulin that targets the prey’s metabolism. 

The discovery came while biologist Helena Safavi-Hemami, from the University of Utah, and her team were analyzing the genes at work in the snail’s venom gland. They found a compound that looked a lot like insulin, a hormone that’s essential to animals for regulating blood sugar. Instead of being the same form that the snails use in their own bodies, though, Safavi-Hemami found it was closer to the insulins found in fish.

The researchers figured that the fishy insulin, which they dubbed Con-Ins G1, was part of the nirvana cabal. By subverting insulin’s normal function and mixing it into its opening venom salvo, they thought, a cone snail could drive down a fish’s blood sugar, making it lethargic and easier to ensnare. They tested that idea by exposing fish to Con-Ins G1 in the water. Sure enough, the fish absorbed the insulin through their gills and became weak and sluggish. When the researchers injected the insulin into fish, their blood glucose levels became dangerously low.

After finding the fish-like insulin in the fish-hunting geography cone snail, the researchers looked at other cone snails that hunt mollusks and worms with the same mouth-netting tactic and found insulins that matched those produced by their preferred prey. In each case, the weaponized insulin was smaller than the natural version produced by the victims and appears to be stripped down to only the essential components, allowing it to act rapidly and perhaps dodge any safeguards that protect against overdoses.

As far as the researchers know, insulin has not been found in any other animal venom, but that doesn’t mean that snails are the only ones that use insulin as a weapon or target their prey’s metabolism. In 1982, Claus von Bülow was accused of trying to kill his wife with an insulin overdose, leading to two sensational trials. The gila monster, a lizard native to the American Southwest, turns its prey’s own hormones against it with a protein in its venom that promotes insulin secretion by the victim’s body.