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The Gulf toadfish is a talker. Like many other fish, this reclusive bottom fish produces sounds to attract mates. But a new study suggests the toadfish might be using smells to communicate as well. More specifically, the smell of its urine.
“You know how you always see dogs sniffing each other’s pee?” says Maria Cartolano, the lead author of the study and a biologist at the University of Miami in Florida. “It’s similar to that.”
Urine is essentially a vehicle to excrete metabolic waste. Unlike dogs, however, Cartolano points out that Gulf toadfish and most other fish steadily excrete the bulk of their metabolic waste through their gills, not in controlled releases through a urinary tract. Gulf toadfish, which live in the Gulf of Mexico, are one of the few fish that can switch between excreting two forms of metabolic waste—ammonia and urea. Based on previous research, Cartolano and her team knew that during the breeding season, toadfish release more urea, even though converting ammonia to urea is energetically costly. But unlike ammonia, which must be excreted continuously, urea can be released in a controlled pulse—giving it potential as a signal.
Cartolano and her colleagues think that to send messages through their urine toadfish must be changing the form of their waste.
To find out whether these urea pulses signal the fish’s desire to mate, the researchers brought 101 fish into the lab and recorded their excretions when they were exposed to seawater with different scents: the smell of prey, an alarm scent, the smell of a potential mate, or no scent.
They found that toadfish exposed to the odor of a potential mate were more likely to release urea—but so were toadfish exposed to prey. The scent of a potential mate also caused toadfish to release urea more frequently.
The scientists think this change implies the fish are communicating using their urine, but the results are not conclusive. They still need to show that the urine cues affect breeding behavior, and determine whether it’s the urea pulse that’s the signal or if there is some undetected pheromone in the mix.
Peter Sorensen, a University of Minnesota fish biologist who was not associated with the new study, is skeptical that urea is the signal. He points out that urea is a very common molecule, whereas pheromones are much more species specific. However, “there’s every physiological, genetic, and ecological reason to believe that marine fish use pheromones and other chemical cues,” says Sorensen.
As scientists learn more about fish communication, they’re also discovering ways in which humans are interfering with it. Studies show that toadfish in areas with noisy boat traffic have a harder time hearing each other, and vocalize less. Human impacts are fouling up scent communication as well. Ocean acidification affects fishes’ abilities to smell, as do a number of pollutants. This interference could have consequences for toadfish populations.
“If we’re taking away their ability to communicate vocally and chemically, how are they going to be able to find their mates?” Cartolano says.