Keeping up with the cones: chased by evolutionary biologists and pharmacological researchers, a tropical mollusk redefines "a snail's pace."

Natural History, Feb, 2002 by Aparna Screenivasan

In the early 1900s a U.S. marine stationed in the Philippines found a beautiful shell near a coral reef. Thinking it would make a nice gift for his girlfriend back home, he carried it off, placing it on his shoulder to show his friends. As the young man walked toward his buddies, the animal inside the shell extended a needlelike organ and Stung him on the neck. Within minutes, the marine was dead.

The unfortunate American died from the sting of a cone snail, a member of the genus Conus. Collectors and beach-combers have long admired and coveted the snails' beautiful shells, risking a sometimes fatal sting to get them. The genus includes more than 500 species, whose shells are embellished with a seemingly endless variety of intricate patterns: repeating triangles or tiny cobblestone motifs, stripes, bands, spots. The length of the protoconch (the spiral on the wide posterior end of the shell) and the number of whorls differ by species, and the shells range from the size of a grape to that of a pear. To a collector, the allure of a mature, undamaged specimen--called a gem shell, and nearly impossible to find--cannot be underestimated. Despite the risk, people still pick them up.

"Cone snails don't have big jaws and sharp teeth, and they don't move very fast," says Baldomero Olivera, a University of Utah biology professor who studies these animals. "The only thing they have going for them is their venom." In a comparison among ten groups of carnivorous snails, Suzanne Lawrenz Miller, of the University of Washington, found that Conus are among the slowest, moving an average of 0.43 millimeters per second (some other meat-eating gastropods can move twice to ten times as fast). Thus, members of the genus must rapidly overwhelm their prey when they get the chance.

To detect prey, a cone snail uses its siphon, an organ that takes up water and directs it over the gills. Once the snail finds a target, it jabs the victim with its radula (a hollow tooth with a harpoon-like barb). Most mollusks use the radula to break up food, but the cone snail uses it to inject venom. A tether attached to the radula allows the predator to retract it quickly; the snail then draws the still-living prey through its proboscis and into its gut. If a fish were still able to twitch after the injection, it could escape before being swallowed, and another creature could swim up and eat it. And a worm-eating snail would be just a hungry snail if its prey could slip back into a burrow to die. The cone snail's venom must be quick-acting and extremely effective or, as with some molluscivorous species, be administered multiple times.

Cone snails have existed for 55 million years, a relatively short time in evolutionary terms. Yet the number of species of Conus--the largest genus in the animal world except for insects--has been doubling every 6.1 million years. (Other rapidly evolving snail types have been doubling their numbers every 10.3 million years.) "And these rates are underestimates" cautions biologist Tom Duda, of the Smithsonian Tropical Research Institute in Panama, because the method does not account for Conus species that are now extinct. "I call them the fastest snails in the West," says Alan Kohn, a biologist at the University of Washington. Possibly driving the rapid expansion of the genus are its venoms, which are mixtures of peptides known as conotoxins.

Like a chef who compulsively varies a recipe each time he prepares it, a cone snail might never shoot out the same toxin combination twice. The genes controlling such variability may be important targets for natural selection, but other pressures might also play a role. "One hypothesis is that conotoxins are evolving in response to changes in diet," says Duda. Conotoxins' effectiveness comes from being adapted to particular prey species. Duda is investigating whether snail species with specialized diets produce highly specialized venoms. Another factor driving the increasing diversity of Conus might be their capacity to survive a long-distance drift. Although adults generally do not move far from their coral-reef homes, the minuscule larvae, floating with the oceanic plankton, sometimes end up in unfamiliar territory (some species' ranges extend from Hawaii and Tahiti to Africa's east coast and India; others, however, occupy areas less than one-tenth that size). As the transplanted snails grow up surrounded by prey that their parents never encountered, natural selection may favor those with toxin cocktails and feeding patterns that fit conditions in their new home; hunger would rapidly eliminate those that failed to assimilate. Distance and time could, on their own, account for a far-flung colonist population becoming distinct from its ancestor, but these variables--even accompanied by a change in diet or habitat--do not guarantee that a new species will arise. Kohn reports that C. miliaris, for example, feeds on a wide variety of organisms at Easter Island, where it is the only ecologically important Conus species, but that in the central Indo-Pacific, where it shares space with many other members of the genus, its diet is very specialized.