Fish-hunting sea snails need a few good tricks if they want a meal. For instance, some species of venomous cone snails do their angling with a potent toxin that wreaks havoc on a fish’s nervous system. But scientists didn’t know the origin of this surprising adaptation, which allows the sluggish mollusks to catch much faster prey. Now, new research in the Proceedings of the National Academy of Sciences has unraveled this evolutionary mystery by uncovering a “smoking gun” signature present in the venom of fish-eating snails and their worm-loving cousins. To start, scientists isolated the active ingredient in the venom of a species that feeds primarily on worms and opportunistically on fish (Conus tessulatus). The toxic culprit was a particular chain of 27 amino acids that makes the nervous system go haywire, “as though the fish had been hit with a Taser,” the paper reports. When researchers compared this toxin to those in angling species, they saw a striking similarity. Coupled with a reconstruction of the cone snail family tree, the evidence points to the toxin’s emergence before the snails’ ancestors developed a taste for fish. The researchers suggest that venom may have been a defensive adaptation for snails to ward away competition for wiggly worms on the sea floor. When other adaptations arose—such as a harpoon to hold a fish in place—it allowed cone snails to start hunting their new prey.