Chuck Bednar for redOrbit.com – Your Universe Online
With their slow traveling speeds and lack of obvious offensive weapons, cone snails might not seem like much of a predator, but a new Proceedings of the National Academy of Sciences study indicates that these creatures have a secret weapon: a toxic form of insulin.
As Helena Safavi-Hemami, a research assistant professor at the University of Utah, and her colleagues explain, the cone snail produces a plethora of different fast-acting toxins designed to target the nervous systems of their prey. Among those toxins is a newly discovered weaponized form of insulin that causes the blood sugar level of their prey to plummet.
A unique form of poison
Safavi-Hemami, the lead author of the study, said that it is “very unlikely” the substance is used for any other purpose. Senior author and biology professor Baldomero M. Olivera noted that it was “a unique type of insulin” that was “shorter” than any that had previously been found in any type of animal, and that “large amounts” of it were detected in the venom.
When the researchers injected a synthetic form of the snail’s insulin into the zebrafish, it caused the creature’s blood glucose levels to drop dramatically. The insulin also disrupted the swimming of fish exposed to it through water contact, based on measurements of the amount of time those fish spent swimming and the frequency of their movements.
They suggest that adding insulin to the blend of toxins allowed predatory cone snails, which are abundant in tropical marine waters and tend to inhabit coral reefs, to disable an entire school of fish by inducing hypoglycemic shock.
Venom is prey-specific
According to Safavi-Hemami and her fellow researchers, each species of snail produces its own unique blend of venom compounds designed and perfected over the years to target a specific type of prey.
For example, Conus geographus, a cone snail responsible for several human deaths in accidental encounters, traps fish by releasing a blend of immobilizing venom into the water. It causes those fish to become disoriented, then protrudes a stretchy, mouth-like part and engulfs the prey with it as it slowly advances.
In order to understand the mechanisms behind this behavior, the Utah researchers analyzed the gene sequences of all of the proteins expressed in the venom gland of Conus geographus. They discovered two sequences closely resembling the hormone insulin, which is used by humans and other types of vertebrates to regulate their energy metabolism.
“The insulin genes were more highly expressed in the venom gland than genes for some of the established venom toxins,” the university said, noting that one sequence proved to be extremely similar to that of fish insulin. The researchers conducted a chemical analysis of the snail venom which confirmed that it contained “abundant amounts” of this insulin.
The type of insulin found in the snail’s venom glands appear to match the prey of that given species. Fish insulin was present in the venoms of Conus geographus and Conus tulipa, both of which both practice the same fish-trapping method.
However, no evidence of fish insulin was found in the venom of five species of fish-eating cone snails that are ambush predators, attacking with a harpoon-like organ. Likewise, no trace of the substance was found in the venom of cone snails that prey on mollusks or worms. Instead, those snails expressed insulins similar to those used by mollusk and worms.
They believe that the snail insulin could become a tool that could be used to study the systems used by the human body to control blood sugar and energy metabolism. It consists of 43 amino acids, fewer than any known type of insulin, and its unusual chemical modifications and size likely evolved as a way to more effectively cause hypoglycemia in prey, the authors said.
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