“Almighty Mouth” doesn’t care when its cheeks are full of another fish’s babies

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 massive jaws, which grasp passing prey. But males also use their omnipotent mouths to gently carry up to hundreds of babies.

Dads do this oral cure, called mouth hatching, for two to three weeks at a time. Like other mouth-hatching fish, they do so at great personal cost. Yet, according to a study published Wednesday in the journal Biology Letters, almighty fathers sometimes carry children who aren’t theirs.

“If that’s true, it’s actually pretty accurate,” said Tony Wilson, an evolutionary biologist at Brooklyn College who studies reproduction in fish and was not involved in the research.

The lead author of the study, Janine Abecia, has a Ph.D. she is a candidate of Charles Darwin University in the Northern Territory, Australia, where she studied the almighty mouth, or glossamia aprion, as well as the blue catfish Neoarius graeffei. Both live in the freshwater environments of Australia. The fathers of both species collect the fertilized eggs in their mouths and carry them until the young have hatched.

Her research suggested that these two species do not eat at all when on duty with their father: “I opened the stomachs of the breeding stock in my mouth and they were empty,” said Ms. Abecia.

Research on other types of mouth breeders – which can be fathers or mothers, depending on the species – has shown that they don’t eat either. Having a mouth full of offspring can also make it difficult to breathe. And it appears to slow down the parent, potentially making it harder to escape predators, Ms. Abecia said.

Given the costs, it makes evolutionary sense for fish parents to only engage in oral care of children they are confident are theirs. Yet 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.

Ms. Abecia collected fathers who hatched the mouths of both the almighty mouth and the blue catfish from the rivers of the Northern Territory. She collected other adult fish, with no babies in their mouths, for genetic comparison. She then selected about 10 eggs or babies from each father’s mouth and analyzed their DNA to figure out where they came from.

With the blue catfish, things were as expected. All nine dads seemed to be carrying their babies and those little fish all had the same mother.

Credit…Alison J. King

Inside the mighty jaws of the almighty mouth, though, things were a little weird. The all-powerful species of mouth forms seemingly faithful pairs in the laboratory, Ms. Abecia said. Yet, out of 15 groups of young people she studied in nature, four didn’t quite fit this story.

Two groups of juveniles had multiple mothers, suggesting that the male had courted a female while she already had eggs in her mouth. One batch had multiple fathers, possibly because another male had secretly fertilized some eggs before the hatching father fertilized and drank them. And in one batch, the young were totally unrelated to the fish that was carrying them.

“It’s a very small study,” said Dr. Wilson, so it would be “premature” to draw conclusions about how common these deceived dads are. He noted that even though the blue catfish in this study looked monogamous, there may have been a handkerchief that the researchers’ sample didn’t catch. “Personally, I’d 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 privacy of seemingly monogamous animals. “Stories like this are probably just the beginning of understanding what complexities exist in nature,” she said.

Scientists have already discovered other fish from hatching in the mouth carrying the wrong babies. In one type of cardinal fish, about 8% of the broods included the young of a second dad. A study of fish called silver arowana found that out of 14 hatching fathers, two had mouths full of offspring that were totally independent.

For their efforts, these dads don’t pass on any of their genes. Why hasn’t evolution made them more careful?

One possibility is that a mouth full of minnows makes them look sexy.

“Some females of other species are attracted to males who are already caring for their young,” said Ms. Abecia. Boys who now get stuck with the wrong babies could make up for it later; perhaps even more females will be eager to fill the scaly jaws of those males with eggs.

“It shows that it’s not just females who go out of their way to care for their offspring,” said Ms. Abecia. “In a way, it’s inspiring.”

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