"We went back and said, Well if they are venomous, they should have structures [to facilitate] envenomation," Wheeler said. "We found that in almost all cases there was a specific structure that could be involved."
The study also raises intriguing questions about modern venomous fish.
Only some species, such as the lionfish, use their venom as offensive weapons the same way snakes do, according to the few fish studies that have been conducted.
Most use the venom for defense against predators.
"We don't know when stonefish evolved, but something really horrendous must have been out there to make these two-foot-long [half-meter-long] things develop such a strong toxin or venom," said study co-author William Leo Smith, a postdoctoral fellow in the museum's division of vertebrate zoology.
Rough Water Ahead
Venomous fish should not be confused with poisonous species, such as the infamous puffer fish, which harbor colonies of toxin-producing bacteria.
Venomous fish produce their own toxins. These fish also inflict their venoms with a delivery mechanism.
But unlike snakes, which employ fangs, fish generally boast sharp spines.
Most venomous species are ocean dwellers.
Typically, they are found near tropical shores. Indo-Pacific waters are notable for their abundance of venomous species.
But a number of freshwater species exist as well. Australia, for instance, is home to a deadly freshwater scorpionfish called the bullrout (map of Australia).
Southeast Asian waters harbor deadly freshwater catfish species (related photos: "Giant Catfish May Be World's Largest Freshwater Fish"). And a potentially lethal freshwater toadfish swims in the waters of the Amazon basin.
The most common North American examplessome species of catfish and Pacific rockfish (or rock cod)pose only a small threat to humans.
But deadlier lionfish, native to Australia and Indonesia, were introduced off the Florida coast several decades ago, probably when captive fish were washed into open water during a storm (map of Florida).
Their population is reproducing and the fish have been spotted as far north as Connecticut in summer.
Thousands of Victims
Though many people remain unaware of venomous fish, they can be a big problem.
More than 50,000 injuries are attributed to them annually. Some stings inflict only blisters, but others are strong enough to kill a human.
Most dangerous or deadly encounters occur between fishers and their prey.
"When you examine a place like Brazil that keeps good records, somewhere in the 40 to 60 percent range [of people encountering venomous fish] are fishermen," Smith said.
"But who knows how many people might die in rural areas of developing countries if they step on a stonefish in the mud flats? They may never be reported as a venomous-fish death."
Venoms That Heal?
What's more, little is known about fish venoms, Smith says.
"Stonefish are the most venomous as far as we know, but that's probably a big assumption," he said. "They are the only ones [for which] anyone has even bothered to make an antivenom."
But the new research could be a boon for scientists evaluating the human health benefits of toxic venoms.
Venoms, which affect the human muscular, nervous, and cardiovascular systems, have already shown applications for drug development.
Snake-venom research, for example, has produced at least six potential stroke and cancer treatment drugs, which may soon be reviewed for sale by the U.S. Food and Drug Administration.
The venom of gila monsters has shown promise for diabetes treatment. And reef-dwelling cone snails produce toxic venom that is being explored for a host of medical uses.
Despite the potential benefits, most fish venoms have never been examined for possible medical useseven though people have been aware of venomous fish for a long time.
"Some of the earliest writings about fish are about venomous ones," Smith said.
"They are mentioned in historic writings like those of Aristotlebut it's amazing how much has been ignored."
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