Most animals have little use for their waste products. But even waste does not go to waste for the gulf toadfish. According to a new study, the animal cloaks itself in its own excretions to hide from predators.
A longstanding puzzle has been why the gulf toadfish (Opsanus beta)--native to the U.S. Gulf Coast--excretes urea as well as ammonia. Most fish dispose of their waste simply in the form of ammonia, which diffuses easily into the surrounding water. Some fish species, however, such as Kenyan Tilapia, have evolved a more energy-consuming process of excreting urea instead of ammonia. It's thought that they do this because the pH of the water in which they live is too high to dissolve ammonia, which can build up and kill the fish. But the water the gulf toadfish lives in dissolves ammonia easily, so it should have no need to expend energy on urea excretion.
Evolutionary physiologist John Barimo of Portland State University in Oregon and his colleague, marine biologist Patrick Walsh of the University of Ottawa in Ontario, Canada, wondered if urea somehow helps toadfish avoid one its deadliest predators, the gray snapper. To find out, the researchers placed a gray snapper in a tank housing a hollow plastic pipe that served as an artificial toadfish shelter. The team then added either urea, ammonia, or a urea-ammonia mix near the shelter. Each of the three compounds attracted the snapper to the shelter and caused it to attack a clay toadfish inside. But the urea and the urea-ammonia mixture were only half as successful at attracting the snapper as the ammonia alone, the team reports online today in the Journal of Experimental Biology.
The findings suggest that urea somehow masks the smell of the ammonia the toadfish also excrete, says Barimo. Excreting ammonia alone is too dangerous from a predator perspective, and excreting urea alone is too expensive, he says, so the toadfish's strategy appears to be a trade-off between energy conservation and survival.
"I think this is a huge breakthrough," says fish physiologist Patricia Wright of the University of Guelph in Ontario. This study has successfully "hammered home the real reason" toadfish excrete urea, she says. Just how much control the toadfish has in regulating its urea-ammonia mix in response to predators still needs to be tested, says Barimo.