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.