This Fish Is a Junkie by Nature

By Daniel Oberhaus on April 29, 2017

If the new Planet Earth series has taught us anything, it’s that nature is full of amazing animals—and that, despite all our knowledge, there are still crazier critters waiting to be discovered. Case in point is the fang blenny, a tropical fish which looks more like Dracula than Nemo and uses opioids to stun its enemies.

As reported in the scientific journal Current Biology in late March, it was discovered why these little brightly colored fish native to coral reefs of the Pacific Ocean were able to pack a larger punch than their puny body size would suggest. In the 1970s, a Hawaiian zoologist studying the fish had observed something strange about their behavior during a series of feeding experiments. When a predator would swallow a fang blenny, the predator’s head would begin quivering violently. After a little while, the predator would open its mouth and gills, and the fang blenny that had been swallowed would swim away unharmed. How this tropical fish managed to accomplish such a daring feat remained a mystery until last month.

As the new study demonstrated, the fang blenny delivers a dose of toxin to its would-be killer through its fangs. This in itself is pretty odd, considering most fish deliver toxins through their fins, but the fang blenny delivers its toxins like a snake. But even more strange is that this venom the blenny injects doesn’t cause its enemies pain—rather, it causes a sudden drop in blood pressure that stuns the predator. Both of these effects make sense when the nature of the fang blenny toxin is taken into account.

The blenny’s venom is chock full of opioid peptides, a molecule that is produced naturally in humans and targets the same receptors in the brain as opium or heroin. But whereas humans naturally produce opioid peptides for hormonal reasons and can derive effects such as pain relief and euphoria from their action on the brain, the fang blenny uses them for defense by quickly lowering predators’ blood pressure. According to experts, based on the response of fish on the receiving end of the blenny’s venom, it’s unlikely that it’s producing a pain-killing effect, as it might in humans. (Indeed, the blenny’s venom may lead to new types of pain medication). 

There are hundreds of species of fang blenny in the wild, but only a handful of them have toxins in their defense arsenal. This raises a few intriguing questions, such as why do non-toxic blennies have fangs, and why do only some have toxins? Regarding the first question, scientists think that the ancient relatives of the blenny used to pose as a harmless normal fish, and then when it would get close enough to its prey, swim up and gouge out part of it with its fangs for dinner. In other words, these blennies were the “assholes” of the seven seas.

Why some ended up with venom and not others is, like everything else in evolution, the product of chance and circumstance. Still, many biologists are in awe of these little fish, as they are some of the only creatures in the animal kingdom that developed toxins for defense purposes, rather than as a hunting tool. So maybe they’re not such assholes after all. 

Main photo credit Twitter; inside photo Wikipedia.

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