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Fish-eating snails stun their prey with insulin

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Study finds what could be the first ever use of insulin in animal venom

Evolution can be an arms race between predator and prey — as predators develop new killing tactics, prey respond by evolving camouflage abilities, or developing defense mechanisms. A team at the University of Utah has discovered that two species of cone snail have a seemingly unique way to hunt — by releasing fish insulin into the water to slow down the metabolism of the fish they seek to eat.

According to a study published in the Proceedings of the National Academy of Sciences, the two fish-hunting cone snails that have evolved this unique ability are Conus geographus and Conus tulipa. These snails use a specially evolved fish insulin to induce hypoglycemic shock in their prey. Instead of pursuing their meals, they wait until one swims by and then flood the surrounding waters with the insulin. That causes a suppression of glucose to the fish's vital organs (like the brain), rendering the snails' targets lethargic and easy to catch.

Cone snail
Weaponized insulin helps the snails draw in their prey

"We already knew that these animals make hundreds of neurotoxins in their venom and compounds that cause tissue degradation and affect cardiovascular function," says Helena Safavi, a researcher at the University of Utah and co-author of the study. "Now we can add yet another mechanism to this list: the disruption of the prey’s energy metabolism."

Researchers at the University of Utah were using DNA and protein sequencing to examine the components of snail venom, in the hopes that it might have pharmaceutical potential. While doing that, it was discovered that one compound wasn't a neurotoxin at all — it was insulin. Not only was it found in abundance, it "looked considerably more like fish insulin than endogenous snail insulin." The team synthesized the hormone and tested it on fish finding that the insulin lowered glucose levels in the blood and caused hypoactivity after being absorbed through the gills of the fish.

Cone snails can be grouped into fish-, mollusk-, and worm-hunting varieties, and the team at the University of Utah studied several representatives of each. Early research into these other, non fish-hunting types shows that they produce insulin specific to their prey — which suggests they produce a weaponized insulin as well.

Cone snail insulin
On the left, a control fish. On the right, a fish that has been affected by the venomous insulin.

The newly discovered tactic is unique for a few reasons. While the other 100 or so species of cone snail also create insulin, theirs is an insulin typical to invertebrate mollusks. The specialized insulin produced by Conus geographus and Conus tulipa, however, is more characteristic of insulin found in vertebrate fish — 90 percent of the amino acids that make it up are similar to that of the zebrafish and white sucker fish, for example.

Additionally, insulin has never been described as part of an animal venom, according to Safavi. Inducing hypoglycemic shock in prey has not been reported for any other venomous animal.

That said, there are other predators that have venom components that target the metabolism of their prey, like the Heloderma suspectum species of gila monster. But as Safavi sees it, the importance of the study is that it shows how evolution can turn good compounds into bad ones. It's another weapon in a long line of clever techniques used by venomous animals when it comes to capturing prey.

Additional reporting by Arielle Duhaime-Ross