Lurking among the underwater plants in Australia’s ponds and streams is a fish called the Almighty Mouth. The species gets its name from its impressive jaws, which catch passing prey. But the males also use their almighty mouths to gently carry up to hundreds of babies.
Dads do this oral care, called mouthbrooding, for two to three weeks at a time. Like other mouthbrooding fish, they do so at great personal cost. However, according to a study published Wednesday in the journal Biology Letters, all-powerful parents sometimes have babies that aren’t their own.
“If it’s true, it’s actually pretty clear,” said Tony Wilson, an evolutionary biologist at Brooklyn College who studies reproduction in fish and was not involved in the research.
The study’s lead author, Janine Abecia, is a Ph.D. She is a candidate at Charles Darwin University in the Northern Territory, Australia, where she has been studying the almighty mouth, or Glossamia aprion, as well as the blue catfish Neoarius graeffei. They both live in the freshwater environments of Australia. The parents of both species take the fertilized eggs into their mouths and carry them until the young have hatched.
Their research has suggested that these two species don’t eat anything when they’re on call: “I opened the stomachs of the mouthbrooders and they were empty,” said Ms Abecia.
Research in other types of mouthbrooders, which can be fathers or mothers, depending on the species, has shown that they do not eat either. Having a mouth full of pups can also make breathing difficult. And it appears to slow down the parent, which could make it harder for predators to escape, Ms Abecia said.
Given the costs, it makes evolutionary sense for fish parents to only engage in oral care for babies they are sure are theirs. However, scientists don’t know how often this is true. “It’s actually a question that has interested me for a long time,” said Dr. Wilson.
Mrs. Abecia collected gloomy parents from both the almighty mouth and blue catfish from the rivers of the Northern Territory. She collected additional adult fish, without pups in their mouths, for genetic comparison. She then selected about 10 eggs or babies from each parent’s mouth and analyzed their DNA to find out where they came from.
With the blue catfish, things went as expected. All nine daddies seemed to have their own young, and all of those baby fish had the same mother.
However, within the mighty jaws of the almighty mouth, things were a bit weird. The almightymouth species form seemingly faithful pairs in the lab, Ms. Abecia said. However, of the 15 batches of youngsters she studied in the wild, four did not quite fit this story.
Two batches of young had multiple mothers, suggesting that the male had courted a female when he already had eggs in his mouth. One batch had multiple fathers, perhaps because another male had surreptitiously fertilized some eggs before the brooding father fertilized and slurped them up. And in one batch, the young had no relation to the fish that was carrying them.
“It’s a very small study,” said Dr. Wilson, so it would be “premature” to draw any conclusions about how common these deceived parents are. He noted that although the blue catfish in this study appeared monogamous, there might have been problems that the researchers’ sample didn’t pick up on. “Personally, I would like to see more data,” he said.
But, he added, the genetic techniques used in this study are making it easier for scientists to ask these kinds of questions about the private lives of apparently monogamous animals. “Stories like this are probably just the beginning of understanding the complexities that exist in nature,” she said.
Scientists have already discovered other mouthbrooding fish that carry the wrong babies. In one type of cardinal fish, about 8 percent of the offspring included young from a second parent. A study of fish called silvery arowanas found that of 14 brooding parents, two had their mouths full of unrelated pups.
Because of their efforts, these parents will not pass on any of their genes. Why hasn’t evolution made them more careful?
One possibility is that a mouth full of goldfish makes them look sexy.
“Some females of other species are attracted to males who are already caring for their young,” said Ms Abecia. Males who get the wrong babies now might make up for it later; perhaps even more females are eager to fill those males’ scaly mandibles with eggs.
“This shows that it is not only the females that go out of their way to care for their young,” said Ms Abecia. “In a way, it’s inspiring.”