There are some badass carnivorous cone snails who hunt fish by tethering them in place with a harpoon-shaped tooth and then injecting a venom that messes with their nervous system. Now, researchers studying worm-hunting snails have detected the presence of a venom toxin similar to those found in their fish-killing relatives. The findings, published in Proceedings of the National Academy of Sciences last week, suggest that venom enabled the evolutionary transition from eating worms to eating much bigger, faster vertebrates.
Predators who change their prey preference can ignite the creation of new biodiversity—but we rarely get to see how that shift occurs. One example that we’ve been able to reconstruct involves bees, who are derived from wasps: Sphecoid wasps (carnivores) feed arthropod prey to their developing larvae, and bees (vegetarians) feed pollen to their young. Bee diversity really took off when they starting utilizing different flowering plants, and now, there are thousands of species.
Meanwhile, there are over 700 species of marine cone snails, and they all capture their prey using venom. Of these, about 100 or so specialize on fish. But exactly how did slow-moving snails who can’t even swim start eating fish? To investigate, University of Utah’s Russell Teichert, Baldomero Olivera, and colleagues analyzed the behavior, molecular biology, and phylogenetics of a worm-hunting cone snail from the Pacific Ocean called Conus tessulatus. In the wild, these guys feed on polychaete worms with a long, fleshy proboscis (pictured right). And if you put them in a tank with fish, they’d happily attack them too (pictured below), even without a harpoon-shaped tooth. You can watch videos of the cone snail eating worms and attempting to envenomate a fish here.
The team discovered the presence of a venom protein—called δ-conotoxin TsVIA—which acts on the nerve cells of vertebrate species. Specifically, it targets sodium channels, and it’s similar to the δ-conotoxin found in fish-hunting cone snail species. Fish killers use it to trigger “extreme hyperexcitability” of the vertebrate nervous system, resulting in almost instant paralysis. It’s as though the fish had been hit with a Taser, the authors describe.
Within the cone snail family tree, the ancestral δ-conotoxin emerged before their forebears developed a taste for fish, Science reports. And the venom likely first served as a defensive adaptation to ward off competitors going after the same tasty worms. That toxin, the researchers say, pre-adapted the worm-hunting cone snail lineage—enabling a later shift to hunting fish.
Images: Wikimedia Creative Commons (top), J.W. Aman et al., PNAS 2015 (middle, bottom)