A Tiny Fish With Terrifying Fangs and Opioid Venom

Think about a venomous fang, and you’ll probably conjure up an image of a snake or spider. But perhaps you should also spare a thought for group of unassuming reef fish that are appropriately called fangblennies. They are finger-sized, colorful, and rather cute—that is, until they open their mouths. Their lower jaws bear two upsettingly large canine teeth, capable of delivering a deep bite. And the evolution of those teeth, according to a new study by Nicholas Casewell at the Liverpool School of Tropical Medicine, took the fangblennies down a path of subterfuge, double-crossing, and chemical warfare.

Of the 33,000 or so species of fish, some 2,500 are venomous—far more than the number of venomous snakes. A surprising number of deep-sea sharks (including many that glow-in-the dark) have venomous spines at the base on their fins. Stingrays deliver venom with their ostentatious tail spikes, stonefish inject toxins with spines on their backs, lionfish use their flamboyant fins, and tangs deploy scalpel-like spines at the base of their tails. (So, yes, Dory is venomous.) But among this toxic underwater menagerie, only two groups have venomous bites—an obscure group of deep-sea eels, and the fangblennies.

Unlike snakes, spiders, and scorpions, the venomous blennies don’t use their poisons to hunt. “They’re mostly plankton-feeders,” says Casewell. Instead, their toxins are defensive. George Losey from the University of Hawaii demonstrated this in the 1970s by offering captive fangblennies to groupers. If the large predators swallowed the tiny fish, they would soon re-open their mouths and allow the morsels to swim out unharmed. But if Losey defanged the blennies first, the groupers readily devoured them.

But there are many kinds of fangblenny and not all of them are venomous. Casewell and his colleagues, including Bryan Fry from the University of Queensland, confirmed this by dissecting their way through several species. They showed that all of them share the disarmingly large canines, but only one group—Meiacanthus—has venom glands.

Casewell’s team extracted those glands from one species, and worked out which genes were being switched on. They found that the gland produces at least three types of toxin, none of which have been seen before in fish. The first—phospholipases—are common in the venoms of snakes, bees, and scorpions; they cause inflammation, and can damage nerves. The second—neuropeptide Y—is used by the lethal cone snail, and causes blood pressure to tank.

The third group—enkephalins—are opioid hormones. They’re similar to the natural endorphins that give you feel-good effects during exercise or laughter, and they work by targeting the same molecules as synthetic opioid painkillers like fentanyl or oxycodone. “But these substances have to be released in the brain to have that type of activity,” says Irina Vetter from the University of Queensland, who was involved in analyzing the blenny venom and is an expert on pain. “It’s unlikely that they would relieve pain when we’re bitten by a fish because they can’t get into the brain that way.”  So contrary to a press release that was issued about this study, it’s unlikely that the enkephalins “act like heroin or morphine, inhibiting pain rather than causing it.”

But even if blenny venom isn’t a painkiller, it’s also not a pain-causer. Luiz Rocha from the California Academy of Sciences was once tagged by a Meiacanthus in the Red Sea, and even though he says the bite was “surprisingly deep and drew blood immediately,” it wasn’t painful.

That’s really weird. Fish venoms are known for being extraordinarily painful, inflicting agony out of all proportion to the wounds that they seep through. (Fry says that the time he was stung by a stingray was the most painful experience of his life, second only to breaking his back.) And pain is such a powerful deterrent that it’s surprising the blennies don’t evoke it.

Instead, Casewell thinks that they, like neuropeptide Y, are responsible for crashing a victim’s blood pressure. That should be enough to make a predator feel faint or uncoordinated, which may is why Losey’s groupers became slack-jawed upon ingesting a blenny.

The non-venomous fangblennies like Aspidontus defend themselves by mimicking other reef fish. Some take the guise of cleaner wrasse, which are tolerated by predators because they remove parasites. The blennies, however, exploit this tolerance to feed on their “clients”—the close in, and use their fangs to nip off bits of skin and scale. These sneak attacks may be why they evolved fangs in the first place.

But wait, there’s more! The venomous fangblennies are themselves mimicked by a wide variety of reef fish, and their impersonators include other non-venomous fangblennies. These harmless species, like Plagiotremus, take the colors, patterns, and swimming behavior of their more well-armed cousins to deter predators with the false threat of venom. They also become bolder. “In some places, Plagiotremus is very cryptic, hiding in holes and waiting for its prey to swim by, at which point it darts and takes a bite,” says Rocha. “But when Plagiotremus mimics the venomous Meiacanthus, it doesn’t hide.” It simply swims up to other fish and takes a nibble. And this convoluted web of deception was probably triggered “by the evolution of venom,” Casewell says.

“This is a monumental amount of work and it’s remarkable that the team pulled it all together,” says Leo Smith from the University of Kansas. “My main hope with this exciting study is that it will encourage researchers to explore the other 2,500 venomous fishes,” which have been neglected in favour of more infamous groups like snakes and spiders.

That’s possible because of new technologies that allow researchers to study venomous animals by looking at the genes they switch on. “It allows us to go beyond the traditional snakes and scorpions and investigate species with hard-to-dissect venom ducts or small quantities of venom,” says Mandë Holford from Hunter College. “It’s really an exciting time to be a venom researcher.”