But when attacked by a predator, this fish, part of a group called fang blennies,does something even more strange. A predator that puts this fang blenny in its mouth would experience a “violent quivering of the head,” according to George Losey, a zoologist who observed this species up close in a series of feeding experiments in the 1970s. Then the predator would open its jaws and gills. The little blenny would swim away, unscathed.
A study published on Thursday in Current Biology now lays bare the details of the fish’s unusual defense mechanism: Unlike most venomous fish, which inject toxins through their fins, fang blennies deliver venom through their bite. Furthermore, fang blenny venom does not appear to produce potent pain, at least in mice. Instead, it causes a sudden drop in blood pressure, which might temporarily stupefy predators.
“This is one of the most in-depth studies of how venom functions in any particular group of fish,” said Matthew Davis, an assistant professor of biology at St. Cloud State University in Minnesota, who did not participate in the research.
The authors of the study took a multipronged approach to studying venomous fang blennies. First, they imaged the jaws of fang blennies collected from around the Pacific and Indian Oceans to confirm what scientists long suspected: Not all fang blennies have venom glands at the base of their teeth.
Out of 100 fang blenny species, only about 30 are venomous, said Nicholas Casewell, a lecturer at the Liverpool School of Tropical Medicine and an author of the study. This pattern suggests that fang blennies first evolved large teeth, which certain species then coupled with venom.
Analyzing venom extracted from one fang blenny species, the scientists identified three toxins: an enzyme, a molecule used in neuron signaling and an opioid, in the same class as heroin and some prescription painkillers.
Though the toxins have never been reported in fish before, other animals — including snakes, bees, scorpions and cone snails — have independently evolved to use similar ingredients in their venoms, Dr. Casewell said.
When his team injected small amounts of fang blenny venom into the paws of mice, the mice showed no significant signs of distress. However, their blood pressure plummeted by nearly 40 percent.
“If you had such a big crash in blood pressure, you would immediately feel faint and dizzy,” Dr. Casewell said. “We don’t know that fish get faint or dizzy, but it’s extremely likely such a large drop would impact coordination and swimming ability.”
It’s noteworthy that fang blenny venom does not cause “instant, severe pain, which is a hallmark of other fish venoms,” said Jeremy Wright, an ichthyologist at the New York State Museum who was not involved in the study.
Though it may not be acutely painful, fang blenny venom is unpleasant enough to send a serious message to fish predators. Up to 20 species — some nonvenomous fang blennies and some fish that aren’t blennies at all — copy the bright colors, patterns or cruise-and-dart swimming style of the venomous fang blennies to escape predation themselves. A handful even use mimicry to feast on the scales and skin of larger fish without being eaten.
Though venom has evolved 18 separate times in 2,500 venomous fish species, fish venom is understudied, said Leo Smith, an associate professor of ecology and evolutionary biology at the University of Kansas who was not involved in the research. This study has the potential to break the field “wide open,” he said. “It will serve as a blueprint for future work.”
According to nytimes.com