If you have scoffed at urban myths of drug dealers injecting unsuspecting people who crossed their paths, think again. Such things do happen, but the culprits are fang blennies, a group of coral reef fish.
Most venomous fish deliver their dose using spines on their back, but members of the genus Meiacanthus have grooved teeth, often attached to venom glands, enabling them to deliver a toxic shock when they bite their victims. This alone makes them interesting to scientists, who conducted a study of the relationships within the genus and the venom itself.
In Current Biology, a team led by Dr Brian Fry of the University of Queensland announce their results, including the entirely unsuspected discovery that among the many molecules in the blennies' venom, there are a wide variety of opioids. These resemble opium, morphine, and heroin, but differ slightly from them.
Blennies use these molecules to discourage larger fish from eating them and to deter fish of similar size, including other blennies, from invading their territory. It seems to work, with descriptions of fang blennies disappearing into the mouth of a much larger fish, before reappearing while the potential predator appears shaken.
Most venoms used against predators, rather than those that serve to subdue prey, use pain as a deterrent. Fry describes being stung by a sting ray as “the second worst” pain he has ever felt, after breaking his back. Opiods, on the other hand, are famous for their role as painkillers.
However, there is a reason why competitive swimmers caught in drug scandals never seem to be doing heroin – it's not exactly performance enhancing. In a statement, Fry describes the effects on the victim fish as making them “slower in movement and dizzy”. Not only does this deter them from eating blennies, but it makes it difficult for rivals to claim their territory. The doses the blennies inject are insufficient to cause other fish to OD, but may induce disorientation, increasing vulnerability to predators.
Fang blenny's look a lot scarier without their scales and flesh. Bryan Fry