They found two sequences that looked surprisingly similar to that of the hormone insulin, used by humans and other vertebrate animals to regulate energy metabolism. The insulin genes were more highly expressed in the venom gland than genes for some of the established venom toxins.
One sequence proved very similar to that of fish insulin. Chemical analysis of venom confirmed that it contained abundant amounts of this insulin. The type of insulin found in venom glands (consists of 43 amino acid building blocks, fewer than any known insulin) seems to match the prey of a given cone snail.
Fish insulin was present in the venoms of Conus geographus and Conus tulipa, which both practice the same fish-trapping method. But the team found no evidence of fish insulin in the venom of five species of fish-eating cone snails that are ambush hunters that attack with a harpoon-like organ.
Nor did they find fish insulin in the venom of cone snails that prey on mollusks or worms. For more clear-cut evidence that snails use insulin as a weapon, the scientists came up with a fast way to synthesize enough of the insulin to directly test its effects on fish.
A synthetic form of the snail insulin, when injected into zebrafish ,
caused blood glucose levels to plummet. The insulin also disrupted swimming behavior in fish exposed through water contact, as measured by the percentage of time spent swimming and frequency of movements. The team proposes that adding insulin to the mix of venom toxins enabled predatory cone snails to disable entire schools of swimming fish with hypoglycemic shock.