New research out of the University of Kansas shows the ability to produce venom has independently evolved 18 times among fresh and saltwater fish species.

The new study, published in the journal Integrative and Comparative Biology, offers a comprehensive catalog of the production of venom among fish species. Study authors say fish venoms may be an untapped resource for medicinal compounds.

"For the first time ever, we looked at the evolution of venom across all fishes," lead study author William Leo Smith, assistant curator at the University of Kansas Biodiversity Institute, said in a news release. "Nobody had attempted to look across all fishes. Nobody had done sharks or included eels. Nobody had looked at them all and included all fishes in an evolutionary tree at the same time."

Among freshwater fish, catfish species hold a monopoly on venom production, but in the ocean, venom is deployed by more species.

In addition to exploring the species that produce and use venom, researchers also cataloged the effects of known fish venoms.

"Fish venoms are often super complicated, big molecules that have big impact," Smith said. "Venom can have impacts on blood pressure, cause local necrosis, breakdown of tissue and blood, and hemolytic activity -- it prevents clotting to spread venom around prey."

"Venom is a neurotoxin," Smith added. "The average response is incredible pain and swelling."

The fish that make these neurotoxins have developed immunity. Molecules in the venom help protect the fish from its own weaponry. These molecules could be isolated and used to derive disease therapies.

Unlike poison, venom usually is only useful when deployed externally. Ingesting a venomous fish isn't typically harmful to the eater, whether that is a human or a bigger fish.

Most fish use venom as a defensive tool, but a few species go on the attack.

"Invasive lionfishes will orient themselves in a strange way and ram themselves at people," Smith said. "One-jawed eels have lost the upper jaw, but with the lower one they slam prey up into a modified fang. Their venom gland sits right above the brain."