Venom is so useful to fish that it evolved at least 18 times, according to a new study led by Dr. William Leo Smith of the University of Kansas Biodiversity Institute.
Deep-sea one-jawed eel (Monognathus) is shown with a venomous fang on the roof of the mouth. Image credit: W. Leo Smith.
In the study, which appeared in the journal Integrative and Comparative Biology, Dr. Smith and co-authors found that, across almost 3,000 extant venomous fish species, venom systems evolved independently 18 times: four times in cartilaginous fishes, once in eels, once in catfishes, and 12 times in spiny-rayed fishes.
“For the first time ever, we looked at the evolution of venom across all fishes,” Dr. Smith said.
He and his colleagues analyzed medical reports of people exposed to venom from fishes.
Then they assembled the family trees for those fish, using specimens from natural history museums to trace evidence of venom through closely related species.
The team found that the predominant function for venom in fishes is defense.
“Up to 95% of venomous fish use their toxins defensively, usually gathering venom within their dorsal spines, where it can be deployed in case the fish is crushed or another fish attempts to swallow it,” they said.
“Some, however, use venom offensively to debilitate their prey and can sometimes injure people.”
“Venom in freshwater is dominated by catfishes, as opposed to marine environments where it is widespread across many groups.”
“It is surprising how comparatively common venom is in deep-sea sharks (30% venomous sharks) compared to deep-sea bony fishes (5% venomous bony fishes).”
According to the scientists, very few fishes have evolved venomous fangs or teeth.
“Dorsal spines are the most common envenomed structures (95% of venomous fish species and 15 independent evolutions),” they said.
“In addition to envenomed spines, fishes have also evolved venomous fangs (2% of venomous fish species, two independent evolutions), cleithral spines (2% of venomous fish species, one independent evolution), and opercular or subopercular spines (1% of venomous fish species, three independent evolutions).”
“One-jawed eels have lost the upper jaw, but with the lower one they slam prey up into a modified fang,” Dr. Smith added.
“Their venom gland sits right above the brain.”
The 18 independent evolutions of venom each pose an opportunity for drug makers to derive therapies for a host of human ailments.
“Fish venoms are often super complicated, big molecules that have big impact,” Dr. 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. The average response is incredible pain and swelling.”
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W. Leo Smith et al. Evolution of Venomous Cartilaginous and Ray-Finned Fishes. Integr. Comp. Biol., published online July 3, 2016; doi: 10.1093/icb/icw070
