Biologist Leo Smith held an unusual job while an undergraduate student in San Diego. Twice a year, he tagged along on a chartered boat with elderly passengers. The group needed him to identify two particular species of rockfish, the chilipepper rockfish and the California shortspine thornyhead. Once he’d found the red-orange creatures, the passengers would stab themselves in the arms with the fishes’ spines.
Doing so, the seniors believed, would relieve their aching arthritic joints. Smith, now at the University of Kansas in Lawrence, didn’t think much of the practice at the time, but now he wonders if those passengers were on to something. Though there’s no evidence that anything in rockfish venom can alleviate pain most fish stings are, in fact, quite painful themselves some scientists suspect fish venom is worth a look. Studying the way venom molecules from diverse fishes inflict pain might help researchers understand how nerve cells sense pain and lead to novel ways to dull the sensation.
Smith is one of a handful of scientists who are studying fish venoms, and there’s plenty to investigate. An estimated 7 to 9 percent of fishes, close to 3,000 species, are venomous, Smith’s work suggests. Venomous fishes are found in freshwater and saltwater, including some stingrays, catfishes and stonefishes. Some, such as certain fang blennies, are favorites in home aquariums. Yet stinging fishes haven’t gotten the same attention from scientists as snakes and other venomous creatures.
But thanks to Smith’s recent work, scientists can now see how venomous fishes fit within a tree of all fishkind. The tree shows that venom arose multiple times throughout history. Understanding which fishes are venomous is the crucial first step to working out the nature of the venoms, Smith says. Researchers are exploring how different fish venoms affect their victims and are discovering extraordinary diversity among fishes’ chemical weaponry. The scientists hope the powerful molecules in the venoms might yield insights that could be turned into medicines. One newly described venom appears to act on opioid receptors, perhaps to stupefy its victims. And venom molecules that stall cell division and others that calm inflammation are inspiring new treatment ideas that go beyond pain relief.
While fish-venom studies are rare, fish stings are not. An old estimate says about 40,000 to 50,000 people are stung by fish each year. But the number is probably much higher, Smith says, since many people don’t bother to report their experiences.
The most noticeable effect of a venomous fish sting is immediate pain, ranging from the mild sting of those rockfish from Smith’s scouting days to a feeling much more excruciating.
The most pain that I’ve ever been in was my first stingray envenomation, says venom researcher Bryan Fry of the University of Queensland in Brisbane, Australia. He was trying to collect a sample from a roughly 1½-meter-wide smooth stingray when it stabbed him in the thigh. The pain is immediate and blinding.
Smith’s first painful run-in was with a fuzzy dwarf lionfish at a pet store where he worked in his late teens. Later, at the library, he found no reports of that species having venom. In fact, medical records of fish stings documented only about 200 fish species as venomous. The experience helped set his career.
As his research progressed, Smith began building fish family trees to get a better handle on which fish spew venom. He presumed that fish related to known venomous ones could also be venomous. So he checked their anatomy for venom-delivery structures, like grooved spines. He reported a partial tree in 2006 and published a more complete version last year in Integrative and Comparative Biology. To assemble the latest tree, Smith and colleagues examined eight locations in the genetic instruction books, or genomes, of 388 species of fish, then used a computer program to work out, based on differences and similarities in those genomes, how the animals are probably related. He also examined museum samples of 90 types of fish for spines or fangs and venom glands. Based on what’s known about fish diversity, Smith’s lowball estimate is that, of about 35,000 fish species, 2,386 to 2,962 are venomous.
Based on his new tree, Smith estimates that there were 18 distinct instances in which nonvenomous fish evolved a venom apparatus give or take a few, since venom might have been lost from some groups, or evolved multiple times in others, he says. Jeremy Wright, curator of ichthyology at the New York State Museum in Albany, who has studied venom in catfish family trees, says Smith’s methods were sound and the data support the tree. However, Wright’s research suggests venom arose separately two or more times in the catfish lineage, while Smith’s tree says all stinging catfishes share a single common, venomous ancestor.
Whether fish venom arose 18 times, or 15, or 20, that’s a big contrast to other animals that use venom: In snakes, venom appears to have evolved only once. The same is true for the venom in bees and ants. To have venom evolve multiple times within a group is extraordinary, says Fry, who’s studied a range of venomous critters.
Fish experts say the distinct origins of fish venoms make sense because, unlike snakes, which always use their teeth, fish deliver venom in diverse ways. Spines with venom glands are most commonly found in fins atop the fish’s back, but not always. In many venomous catfishes, the pectoral fins contain the barbs and venom glands. Weever fish spines sit on the operculum, a bony flap that protects the gills on the fish’s cheeks. In stingrays, the flattened spine protrudes just above the tail. And in fang blennies, the venom glands sit at the base of enlarged lower canines, calling to mind tiny vampires of the sea.
Even within one fish genus, the venom-delivery apparatus can vary. Ichthyologists Jacob Egge, now at Pacific Lutheran University in Tacoma, Wash., and Andrew Simon of the University of Minnesota analyzed pectoral stingers of 26 species of madtom catfish, found in eastern North American freshwater. Some had smooth spines with a venom gland in the shaft, the two reported in 2011. Others had serrated spines, the better to cause injury, with a gland at the shaft and glands spread along the serrations. One species had no venom gland at all.
The effects of venom from fishes and other creatures vary widely, but in fishes, the goal is usually the same: to stop an attack. For most fish venoms, pain is key, but some cause numbness, too. All affect the cardiovascular system in some way, by lowering blood pressure, for example, which would probably startle and debilitate a predator, Smith says.
In people who have been stung, skin reddening, swelling, itching or temporary localized paralysis might also occur. In some cases, the venom can kill the tissues near the sting site. In rare cases, a combination of low blood pressure, failure of circulation or weak breathing can lead to death.
Just within the catfishes, venom effects differ between species. Wright injected venom from nine different species of catfish into largemouth bass, which are typical predators. It was clear that it was an uncomfortable experience for them, Wright says of his unlucky subjects. Many venoms caused loss of color and bleeding, some induced jerky muscle contractions or loss of balance, and one simply killed the bass outright, he reported in BMC Evolutionary Biology in 2009.
Society for Science & the Public 2000 – 2017.