Virgin Births: Endangered Sawfish Can Reproduce Without Sex

A baby smalltooth sawfish (Pristis pectinatus). These animals are capable of "virgin births," reproducing without sexual activity, in the wild. NOAA's Fisheries Collection

The smalltooth sawfish can grow up to 25 feet long, and possesses an impressive "saw" that can sense the weak electrical currents produced in the bodies of the small fish and crustaceans upon which it feeds. It also appears to have another impressive feature: the ability to reproduce without sex.

The sawfish has sadly been exterminated from much of its original range, throughout the Atlantic waters of the U.S. Southeast and the Caribbean, a result of overfishing and habitat destruction. For this reason, scientists have worried about the genetic diversity in the fish that remain; when only a small group of animals survive in isolated groups, this can create problems with inbreeding that drag down the population's health and fitness.

To that end, scientists recently set about testing the genetic diversity of sawfish (Pristis pectinata) in southwest Florida, where the population appears to have stabilized. That's when they discovered something shocking: Seven of the fish lacked the normal genetic diversity associated with sexual reproduction; the only explanation, the scientists write in a study published today in the journal Current Biology, is that the fish are "virgin births," products of a phenomenon called parthenogenesis.

Parthenogensis involves reproducing without sex, and it is seen in insect and arthropods. It has also been witnessed in some birds, amphibians and fish in captivity. This is the first time such virgin births have been demonstrated in a vertebrate—an animal bearing a backbone—in the wild. (In 2012 researchers found that two snakes that became pregnant via parthenogenesis, although it's unknown if these offspring would have survived as they were never born.) In other words, this is the first vertebrate "virgin birth" seen in the wild, according to the study authors, who hail from Stony Brook University, Chicago's Field Museum of Natural History and the Florida Fish and Wildlife Conservation Commission.

The study suggests that this may be a viable way of reproducing among some animals. The seven fish that were begot this way were an average of about one year old and were of normal size and appeared healthy, although it's unclear if they will be fertile and able to reproduce.

"Overall this shows that [virgin birth] from parthenogenesis is not just a weird process which happens in an aquarium, but actually in nature as well," says study lead author Andrew Fields, a researcher at Stony Brook University.

The scientists suspect such virgin births may be more common in small or declining populations. While "we don't know why they are reproducing like this…it's possible that the fact that they are critically endangered makes this type of reproduction more likely, simply because males and females fail to find one another when it's time to mate and then some of female's eggs develop without sperm," Fields says.

The finding doesn't change the fact that the fish are on the brink of extinction—and parthenogenesis isn't likely to help them survive. The phenomenon reduces the diversity of the offspring and "makes them less able to deal with changes in environments," Fields says. "So in a long term, it's not a great strategy."