Sawfish Have Virgin Births: First in the Wild

The first known virgin births in the wild have been documented among critically endangered smalltooth sawfish in Florida waters.

The discovery, reported in the journal Current Biology, marks the first time that living offspring from virgin births -- births where the mother did not mate with a male -- have been found in a normally sexually reproducing animal in nature.

Lead author Andrew Fields explained to Discovery News that “with fragmented habitat and low abundance,” it is likely that female smalltooth sawfish, known for their iconic tooth-studded rostrum or “saw,” are having trouble findings males. Lack of mating can then trigger the unusual births among certain adult females, he and his team suspect.

Fields, a researcher at the Stony Brook University’s School of Marine and Atmospheric Science, and his colleagues were conducting routine DNA fingerprinting of the sawfish (a type of ray related to sharks) in a Florida estuary when they determined that about 3 percent of those studied arose due to virgin birthing, a process technically known as parthenogenesis.

This phenomenon is thought to occur when an unfertilized egg absorbs a genetically identical sister cell. The resulting offspring have about half of the genetic diversity of their mothers and often die. In this case, however, the “parthogens” looked to be in perfect health, so the scientists just tagged and released them as part of an ongoing study that they are conducting on sawfish movements.

As for how those individuals might then reproduce later in life, Fields said, “They may give birth sexually or asexually.”

It is likely that virgin births are more common than previously thought among sharks and rays, both in captivity and in the wild. It is harder to detect and confirm the latter, given that DNA testing would be needed. Such births over the years, though, have surprised many aquarium staff members, who have found baby sharks suddenly appearing in tanks inhabited for lengthy periods by a lone female or multiple females and no males.

Senior author Demian Chapman, also from Stony Brook University, told Discovery News, “We know of parthenogenesis in blacktip, bonnethead, whitetip reef, whitespotted bamboo, and zebra sharks in captivity, so (there are) 5 species total.”

More species are expected to be added to that list. Some birds and reptiles have also undergone parthenogenesis in captivity.

For the smalltooth sawfish, the discovery is a dire warning about the species’ dwindling population. Fields said that it and other sawfish “are on the brink of extinction.”

He says that, for the smalltooth sawfish, “there is no accurate estimated population size, but it seems to be at 1–5 percent of historical levels based on the best evidence available. The primary threats are entanglement during fishing and habitat loss.”

Read more at Discovery News