Think `snails', and slime-oozing creatures munching innocently on a lettuce leaf probably spring to mind. But not all snails are innocuous; marine cone snails are lethal predators that are surprisingly quick on the draw when a tasty fish strays too close. Hungry cone snails harpoon their hapless prey using their proboscis, a tube-like appendage with a venom-filled tooth nestled near the end. Cone snails produce an astonishing array of peptides, some of which paralyse their victims in just a fraction of a second. But do snails of the same species inject identical poisonous blends(p. 2873)?
`There are hundreds of cone snail species, each with its own cocktail of hundreds of poisonous peptides', Jonathan Sweedler explains. But the diminutive size of these marine predators means that it can be a struggle to collect enough precious poison to identify the components of their venom. Sweedler and his colleagues at the University of Illinois at Urbana-Champaign use sophisticated mass spectrometry to analyse incredibly tiny samples, such as a single cone snail's minute venom droplet. Sweedler and his team joined forces with marine biologists William Gilly and Joseph Schulz at the Hopkins Marine Station at Stanford University to examine cone snails' poison profiles.
To take a closer look at cone snails' venom, Gilly and Schulz collected cone snails from coral reefs near Samoa and Hawaii and took them back to the lab. To coax the snails to deliver the same poisonous blend that the predators would inject into their unsuspecting prey, Gilly and Schulz `milked' the snails. They tricked the molluscs into thinking that plastic tubes were juicy morsels by placing a fish fin over the top of the tube. When they dangled the`fake fish' near the snails, the predators eagerly fired off their harpoons,squirting their venom into the tubes. After months of `milking' the snails in this way, Gilly and Schulz sacrificed some of them to analyse the toxic brew in the snails' venom ducts.
When the team analysed different snails' poison profiles in the milked venom samples using high performance liquid chromatography, they were astonished to find consistent differences between individual snails. Puzzled by these individual poison profiles, they decided to look for differences between a snail's injected venom and its duct venom.
Back in Sweedler's lab, the team identified the mini-proteins in each snail's injected venom and duct venom using liquid chromatography and mass spectrometry. To their surprise, they discovered that a snail's injected venom is a very simplified subset of the poisonous cocktail present in its venom duct, even though the venom duct contents of different snails were quite similar. Clearly, cone snails don't fire off their entire collection of venomous peptides when they harpoon a passing fish. `It appears that some mechanism has evolved that carefully selects a subset of peptides from the snail's duct to inject into its prey', Sweedler says. Why and how some poisons are chosen while others are rejected is still a mystery. But one thing is clear; cone snail venom peptide diversity is even more complicated than anyone anticipated.