Cone snails have diversified into hundreds of species and each makes a distinct repertoire of venoms – mixtures that have evolved to target particular prey. But how did these slow-moving gastropods gain the wherewithal to hunt and kill fish? University of Utah biologist Baldomero Olivera and colleagues have tracked down a key evolutionary step. The venom probably first evolved for defense but then facilitated the exploitation of vertebrate prey. The scientists say that the ‘smoking gun’ is a peptide found in the venom of a worm-hunting cone snail that poisons an important nerve signaling channel in vertebrates:

This smoking gun is δ-conotoxin TsVIA, a peptide from the venom of Conus tessulatus that delays inactivation of vertebrate voltage-gated sodium channels. C. tessulatus is a species in a worm-hunting clade, which is phylogenetically closely related to the fish-hunting cone snail specialists. The discovery of a δ-conotoxin that potently acts on vertebrate sodium channels in the venom of a worm-hunting cone snail suggests that a closely related ancestral toxin enabled the transition from worm hunting to fish hunting, as δ-conotoxins are highly conserved among fish hunters and critical to their mechanism of prey capture…

Olivera came up with great nomenclature for cone snail venoms: ‘lightning-strike cabal’ for the mixture they inject via a harpoon-like organ to rapidly paralyze; ‘nirvana cabal’ for the mixture that some species release into the water to sedate fish. Both C. geographus and C. tulipa (the one starring in the video above) include a form of insulin in their venom that can induce hypoglycemic shock in passing fish, Olivera’s team reported earlier this year.

Cone snails actively adjust their venom mix. Hunting venom is prey-specific and mostly inactive against humans, for example, but the defensive sting of many species is full of toxins lethal to people. The most deadly cone snail, Conus geographus, can inject a venom dose about 200 times more than necessary to kill an adult human.

Source:

Insights into the origins of fish hunting in venomous cone snails from studies of Conus tessulatus, by Joseph W. Aman, Julita S. Imperial, Beatrix Ueberheide, Min-Min Zhang, Manuel Aguilar, Dylan Taylor, Maren Watkins, Doju Yoshikami, Patrice Showers-Corneli, Helena Safavi-Hemami, Jason Biggs, Russell W. Teichert and Baldomero M. Olivera. PNAS (2015)