With a face shaped like a chainsaw and a body 18 feet long, you might think smalltooth sawfish have little to worry about. These giant shark-like rays, one of five sawfish species worldwide, viciously thrash their long, tooth-lined snouts back and forth to impale small fish and fend off the occasional hungry shark.
Yet thanks to overfishing and habitat destruction, smalltooth sawfish are incredibly endangered. Their population numbers are so low – and good mates apparently so hard to find – that female sawfish may be resorting to a method of reproducing never seen before in wild animals with a backbone: sex-free "virgin births".
Yes, some independent lady sawfish appear to be going it alone, producing baby sawfish sans mating. This was the surprising conclusion made by scientists from Stony Brook University, The Field Museum, and the Florida Fish and Wildlife Conservation Commission in a study published this week in the journal Current Biology.
This no-mate-required breeding strategy is known as parthenogenesis – and you might just remember it from "Jurassic Park". If you need a quick recap, the all-female populations of dinosaurs were able to bypass the park's careful controls and hatch their own young thanks to parthenogenesis – because the mama dinos had been recreated using frog DNA (among wild amphibians, some species are entirely female and reproduce in this way). As Jeff Goldblum's character, Dr Ian Malcolm, puts it, life finds a way.
But for the endangered sawfish, parthenogenesis could be a dead-end street. According to The Field Museum's Kevin Feldheim, one of the researchers involved in the study, it's a “last-ditch effort for a female to pass on her genes” – one that could actually threaten sawfish populations in the long run.
“If this is Plan B for survival, then it should serve as a wake-up call that we need serious global efforts to save these animals.”
Normally, a female manufactures egg cells to be fertilised by sperm from a male, which also results in waste products of cellular goo and genetic material called polar bodies. In parthenogenesis, it's these polar bodies that do the job of fertilisation. According to Feldheim, this results in offspring either “genetically identical to the mother” or with “half of the genetic variation found in the mother” – a genetic dead end that often produces deformed or short-lived young.
Signs of this rare reproductive process in sawfish came as a surprise – especially because the researchers weren't even looking for them. Instead, the discovery came out a routine study of smalltooth sawfish living in coastal waters off southwestern Florida, one of their last strongholds.
Because their numbers have fallen so dramatically (to about 5 percent of historical levels), researchers have been studying these little-known fish for a decade, increasingly concerned about the potential for inbreeding in such a small population. After genetically fingerprinting sawfish in three estuarine areas of Florida, they examined select genetic markers to determine whether closely related sawfish were mating with each other.
The answer to that question is largely no, but in establishing that, scientists discovered that roughly 3 percent of the population showed such a glaring lack of genetic diversity that those seven sawfish were most likely the product of parthenogenesis (five of them were even born from the same mother in the same year!).
While these young sawfish appeared healthy, their ability to reproduce or otherwise function normally is a big unknown. “Parthenogenesis is even more of a threat (genetically speaking) compared to inbreeding,” warns Feldheim.
While inbreeding can allow uncommon, harmful genetic variations to surface, inbred individuals retain at least some level of diversity, and that means they're actually better off than the genetic weaklings born by parthenogenesis. And that's bad news for endangered sawfish with a shrinking gene pool.
For Stony Brook University student Andrew Fields, who discovered the study's genetic clues to parthenogenesis, these are warning signs we should pay attention to. If this is Plan B for survival, then “it should also serve as a wake-up call that we need serious global efforts to save these animals.”
Animal Behaviour
New Species
Human Impact
