The geographic cone snail, Conus geographus, is native to tropical coral reefs in the Indian and Pacific oceans. (Image: Cone Snail/YouTube)
Cone snails may be too slow to chase down a school of fish, but they have a secret weapon: some of the deadliest venom on Earth. The most dangerous of all is the geographic cone snail, which can deliver a blend of several hundred toxins with a harpoonlike tooth it fires at up to 400 mph (644 kph).
That's impressive, but as a new study reveals, this snail has more tricks up its sleeve than we thought. Its venom includes a novel form of insulin, the study's authors report, the first time that hormone has been found in any venom. The snail releases it into the water, immobilizing nearby fish by making their blood-sugar levels drop. Once they're in hypoglycemic shock, it can eat at its own pace.
"This is a unique type of insulin," says study author Baldomero M. Olivera, a biology professor at the University of Utah, in a press release about the research. "It is shorter than any insulin that has been described in any animal. We found it in the venom in large amounts."
Along with the geographic cone snail, Conus geographus, the researchers also found insulin in the venom of another species, Conus tulipa. Most other known ingredients in cone-snail venom are neurotoxins, the researchers note, but insulin uses a "radically different mechanism" by disrupting energy metabolism. Once a fish slows down, the snail extends a stretchy mouthpart to grab it:
Not only have the snails weaponized insulin, but they've also tailored it specifically for fish. It bears a "much greater similarity to fish insulins" than to any mollusk hormone, according to the study, and even a synthetic version made by the researchers wreaked havoc with blood glucose and swimming behavior in lab fish. Although these snails clearly aren't short on venom varieties, the researchers suggest adding insulin might have helped them trap entire schools of fish, providing an evolutionary perk.
The study also involved other cone snails, including some that use their harpoon tooth for ambush hunting, but insulin venom is apparently unique to species that use the fish-trapping method. The insulin they make consists of just 43 amino-acid building blocks, which is fewer than any known form of the hormone. Along with other chemical quirks, the researchers say this stripped-down size might have evolved as a way to make the insulin more effective at causing hypoglycemia in prey.
All this is interesting, but since cone snails pose a well-known danger to people — especially C. geographus, whose sting has caused at least 30 recorded human deaths — it may seem like further reason to fear them. While it is wise to avoid the snails themselves, however, discoveries like this about their venom have begun to offer surprising benefits for human health care.
The snails' minimalist insulin, for example, might be useful in testing how the human body controls blood sugar and energy metabolism. Any new insights on the structure or function of insulin could help the fight against diabetes, a disease that kills about 1.5 million people worldwide every year.
Many of the "conotoxins" in cone-snail venom selectively target certain cells in the body, making them potentially valuable in treating an array of human diseases, including cancer. A class of compounds known as alpha-conotoxins seem especially promising, according to another recent study, since they target nicotinic receptors that play a key role in conditions ranging from Alzheimer's disease and schizophrenia to tobacco addiction and lung cancer. One species, Conus regius, has venom rich in alpha-conotoxins that show potential in detecting and treating certain types of cancer.
When isolated, some cone-snail compounds have even proven useful in developing painkillers such as ziconotide, which is at least 100 times more potent than morphine and lacks the risk of addiction. Conotoxins have also shown promise for treating Parkinson's disease, strokes and epilepsy, and that's only the ones scientists have studied so far. About 500 species of cone snails produce as many as 100,000 different conotoxin mixtures around the world, most of which remain a mystery.
Cone snails are relatively common, so those secrets might not face an imminent risk of disappearing. But many marine ecosystems are in rapid upheaval due to ocean warming and acidification, including coral reefs inhabited by cone snails. Humans are now some of the planet's most dangerous animals, yet research like this can still remind us of tangible reasons why nature is worth preserving. And, thankfully, it can also remind us that even the most dangerous animals have a good side.
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