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If you swallow this tropical blenny, you’re likely to have bitten off more than you can chew. It has two prominent fangs on its lower jaws, which it uses to inject a unique venom that sends predators into a limp mess.

When a predator engulfs a blenny, the tiny fish bites the inside of the predator’s mouth. The bigger fish’s blood pressure plummets, its coordination goes hopelessly awry and its mouth gapes involuntarily, allowing the tiny prey to swim out unscathed.

“The predators would shake and quiver, and open their jaws and gills really wide,” says Nick Casewell of the Liverpool School of Tropical Medicine, UK, and joint leader of a team that has established the ingredients of blenny venom. “What’s more, they never eat blennies again, so whatever the effect is, it seems to be very unpleasant for predators.”

The researchers have now tested venom from 11 species found in the reefs of the western Pacific Ocean, but many of the insights came from Meiacanthus grammistes (the striped poison-fang blenny) and Meiacanthus atrodorsalis (the forktail blenny).

Don’t suck it and see

Casewell and his colleagues in Australia and elsewhere have found three main components, all of them previously identified in other animals. The first of these are enkephalins – seen in some scorpion species – which act on opioid receptors, suggesting they have an analgesic effect.

“What’s really unusual are these opioid-like neuropeptides called enkephalins, which don’t induce pain,” he says. “Most animals that produce venom use it to inflict pain, yet we found no evidence of that with the blenny venom.”

The second, neuropeptide Y, has been found in cone snails, and causes blood pressure to drop. Casewell thinks the enkephalins and neuropeptide Y act together to cause a fast drop in blood pressure of as much as 37 per cent in 4 minutes.

“If this happened in a human, you would feel faint, dizzy and quite sick,” says Casewell. “We don’t really know what sensations fish experience, but they clearly suffer such an adverse sensation that they avoid ever eating blennies again.”

The third main ingredient called PLA2, a phospholipase – a fatty substance commonly made by snakes, lizards, bees and scorpions – probably causes inflammation.

Sincerest form of flattery

Casewell hasn’t yet been bitten by a blenny, but says that people who have report no pain, but sometimes develop inflammation lasting a day or so. “It’s possible this does cause pain in fish, but we didn’t detect a pain response in mice,” he says.

The team hopes to test blenny venoms further in fish, to explore whether this is the case. They’ll also look at venoms from other blenny species to see if they contain different ingredients.

They don’t think the pressure-drop agents would be of any clinical use in people with high blood pressure because the drop is fast but fleeting.

Whatever causes the venom’s dramatic effect, there are many other species of fish that have benefitted from it, by evolving to look, swim and appear like the blennies, but don’t produce any venom – an evolutionary phenomenon called Batesian mimicry.

“They are protected just by looking like a blenny,” says Casewell.