Rare Fish Performs "Virgin Births"—First Known in The Wild

The smalltooth sawfish, one of the world's most threatened marine fish species, may have come up with a plan B.

Populations of this Atlantic Ocean ray are so low that it's possible some are producing young through a process known as "virgin birth," in which the females can fertilize their eggs without sperm.

Seven immature smalltooth sawfish likely entered the world in this way, according to a new study published Monday in the journal Current Biology.

Virgin birth—also known as parthenogenesis—isn't unheard of in sharks, snakes, and even birds. But this is the first known instance of virgin births in wild members of the shark family. (See "World's Longest Snake Has Virgin Birth—First Recorded in Species.")

"If you can't find a mate and you need to reproduce," says lead author Andrew Fields, a geneticist at Stony Brook University in New York, then virgin births are one way of solving the problem.

The smalltooth sawfish, which can grow up to 20 feet (6 meters) in length, are listed as endangered by the United States. Though it's unknown how many individuals remain, scientists believe the population is less than 5 percent of its size at the time of European settlement in the U.S.

Males Need Not Apply

Essentially, in virgin birth, a female is able to fertilize her own egg and produce offspring. It occurs when material that's split off from an egg as it divides—called a polar body—fuses back with the egg. It's similar to how sperm fuse with an egg during fertilization. (Read about wild snakes that don't need males.)

Although the sawfish pups aren't exact clones of their mother, says Fields, they get all of their genes from her, rather than half from her and half from a father.

Fields found the sawfish virgin births by accident. He was looking through a database of 190 smalltooth sawfishes tagged in southwestern Florida between 2004 and 2013 when he found something odd: Seven animals whose genes suggested they only had one parent.

The geneticist was a little surprised at first, but figured it made sense that parthenogenesis would occur in a species with low population numbers like the smalltooth sawfish.

Having a homogenous gene pool is deadly for a species with low population numbers, says George Burgess, director of the shark-research program at the Florida Museum of Natural History. Low genetic diversity leaves a population more vulnerable to harmful mutations and less able to cope with environmental changes.

Special Sawfish?

The big question now, Fields says, is whether virgin births are more common among wild animal populations than researchers think.

Or, "is this a special case because we have an endangered species with very few individuals and they don't run into each other, or is it because they're sawfish and sawfish have something interesting going on?" (Read about a hotel shark that broke the record for virgin births.)

Burgess, who wasn't involved in the study, isn't surprised that sawfish can reproduce this way. But he's glad to have this added information about a species that scientists still know very little about.

For now, Fields and colleagues are keeping an eye on the sawfish genetics samples that come in, looking for more evidence of virgin births and to see if the seven immature rays are still out there.

"We're hoping to find out whether or not they can produce offspring," Fields says. The team will have to wait several more years for that kind of confirmation, though. It takes sawfish about seven years to become sexually mature, and the ones in the geneticist's study were born in 2011.

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