Venomous snail uses insulin to put prey into sugar coma

How do predatory snails, not known for their swiftness, catch their next meal? For one species of the sea-dwelling cone snail, lack of speed doesn't hinder them from successfully capturing prey. Instead, they use insulin to bring their victims down to their speed, according to a new study.

Researchers at the University of Utah have discovered in the snail species a unique type of insulin that acts as a key component of a venomous mixture used on prey. But while other toxins in the venom typically target the nervous systems of the hunted fish, the insulin disables the energy metabolism of victim, sending them into hypoglycemic shock.

The research team, led by biology professor Baldomero Olivera, sought to figure out just how the cone snail's hunting strategy works. The snail the research team studied is the owner of a distended false mouth, a tubular body part that the snail protrudes during the hunting process. Olivera and colleagues had earlier discovered, though, that unlike other similar species, this particular cone snail uses its false mouth like a net to engulf fish prey before ever injecting them with venom.

Some biologists suggested the species released toxins into the water to slow down and disorient victims. To better understand what this concoction is made of, Olivera's team analyzed the gene sequences of the venom's different chemical components. The researchers found that two sequences in the mixture closely resembled the type of insulin, a hormone that regulates metabolic functions by controlling the level of glucose in the blood, that is found in certain fish.

The scientists tested the effects of the insulin by injecting adult zebrafish with the hormone and subsequently saw significant drops in the fishes' blood glucose levels. While the fish are able to maintain healthy levels of glucose when left alone, the exposure to additional insulin can throw this proportion off balance.

Understanding that this cone snail species can catch prey without directly injecting prey, they also analyzed the behavior of fish when insulin was released into the water. In this scenario, the researchers still observed a notable decrease in the fish's movements.

A distinctive weapon, the fish insulin has so far been found in only two species of cone snail. Other cone snails that also eat fish but use different hunting methods did not appear to contain the insulin. 

The team did not find evidence of the same fish insulin in species of cone snail that feed on other sea-dwellers like mollusks or worms either. They did, however, discover sequences in these species that resemble insulin found in the creatures they prey on. These findings suggest that the presence of insulin may have evolved in these different species of snail to improve their ability to capture their targeted prey.

"We already knew that these animals make hundreds of neurotoxins in their venom and compounds that cause tissue degradation and affect cardiovascular function," said study co-author Helena Safavi, according to The Verge. "Now we can add yet another mechanism to this list: the disruption of the prey's energy metabolism."

As for other animals, scientists have not yet found any venomous species where insulin is present. But the gila monster, a lizard found in the US and Mexico, contains toxins in its venom that attack the digestive systems of its victims. In 2005, the FDA approved a diabetes drug derived from a hormone found in gila monster saliva. 

"Specialized insulin is used for chemical warfare by fish-hunting cone snails" appears in the current issue of The Proceedings of the National Academy of Sciences