This endangered species gives new meaning to ‘single mom’

Sawfish are the first wild vertebrates shown to produce viable offspring without mating


Sawfish numbers are dwindling worldwide. A new study finds a few of the fish in Florida have reproduced without mates. But this won’t stave off the population’s decline for long, researchers worry.


A few female smalltooth sawfish have figured out how compensate for the low numbers of their kind. These sawfish have managed to reproduce without a male partner. This mode of reproduction is known as parthenogenesis (PAR-then-oh-JEN-eh-sis). It is common among many invertebrates, such as insects. And scientists have documented a few cases of non-mammal vertebrates reproducing this way in the lab. But this is the first evidence that a vertebrate in the wild has produced healthy, living offspring this way.

Every species of sawfish, a type of ray, is considered endangered or critically endangered. That means there is a real risk that the species might go extinct.

Sawfish numbers have fallen for two reasons. First, much of the habitat that nurtures and protects the juvenile fish has been lost. Second, sawfish can get caught up in fishing gear. Sometimes, fishermen have accidentally been hurt when this happens. So many fishermen who were unlucky enough to find a sawfish in their nets or lines used to kill it before dumping its body back into the ocean.

At one time, U.S. waters hosted two species of sawfish. But the largetooth type hasn’t been seen there in 50 years. (It’s still found in some other parts of the world.) Smalltooth sawfish still live off the tip of Florida. Studies indicate, though, that their numbers have dropped by at least 95 percent.

Andrew Fields works at Stony Brook University in New York. He and his colleagues were studying Florida’s smalltooth sawfish for signs of inbreeding. That’s when individuals that are too closely related mate, which tends to happen when populations are small. And that can be a problem. The offspring of inbred organisms can have a hard time surviving and reproducing successfully.

Between 2004 and 2013, these researchers caught 190 fish off of the coast of Florida. They snipped a small piece of fin off of each fish to analyze its DNA. Then they released the fish. They tagged some of them first with acoustic transmitters. This would allow the scientists to later track them.

The DNA can tell scientists if there had been inbreeding. It also can tell them if any offspring have been produced via parthenogenesis. Normally fish get half of their DNA from mom and half from dad. Twins will have the same DNA profile. An animal created through parthenogensis, though, will have no dad DNA. It also will have only half the genetic diversity of its mom, notes Demian Chapman. He studies shark populations at Stony Brook University. He was a coauthor on the study published June 1 in Current Biology.

While seeking evidence of inbreeding, the researchers found something surprising. About 3 percent of the fish resulted from parthenogenesis. This was totally unexpected. Normally, vertebrate offspring that are born this way die or are stillborn. But these sawfish all appeared fine.

Scientists don’t yet know how common parthenogenesis is in the wild. Chapman says it may be more common in very small populations, where females have trouble finding mates. This reproductive tactic may help a species avoid extinction — at least for the short term. “But only females are produced,” Chapman notes. “So it won’t work forever.” The only hope for a male to be produced is for a female to mate with a male, he says. And a healthy population ultimately will need males to survive.

Power Words

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acoustics  The science of sounds and hearing.

DNA  (short for deoxyribonucleic acid) A long, double-stranded and spiral-shaped molecule inside most living cells that carries genetic instructions. In all living things, from plants and animals to microbes, these instructions tell cells which molecules to make.

endangered   An adjective used to describe species at risk of going extinct.

extinct  An adjective that describes a species for which there are no living members.

genetic  Having to do with chromosomes, DNA and the genes contained within DNA. The field of science dealing with these biological instructions is known as genetics. People who work in this field are geneticists.

genetic diversity   The range of genes types — and traits — within a population.

habitat  The area or natural environment in which an animal or plant normally lives, such as a desert, coral reef or freshwater lake. A habitat can be home to thousands of different species.

inbreeding    The mating of animals that are too closely related, genetically. It is the opposite of genetic diversity. Animals that are inbred tend to become weak or sickly and often cannot reproduce successfully.

invertebrate   An animal lacking a backbone. About 90 percent of animal species are invertebrates.

mammal  A warm-blooded animal distinguished by the possession of hair or fur, the secretion of milk by females for feeding the young, and (typically) the bearing of live young.

parthenogenesis   An unusual form of reproduction where animals sometimes produce healthy offspring from an unfertilized egg.

population  A group of individuals from the same species that lives in the same area.

rays   (in biology) Members of the shark family, these kite-shaped fish species resemble a flattened shark with wide fins that resemble wings.

stillborn   Born dead.

tagging  (in biology) Attaching some rugged band or package of instruments onto an animal. Sometimes the tag is used to give each individual a unique identification number. Once attached to the leg, ear or other part of the body of a critter, it can effectively become the animal’s “name.” In some instances, a tag can collect information from the environment around the animal as well. This helps scientists understand both the environment and the animal’s role within it.

vertebrate  The group of animals with a brain, two eyes, and a stiff nerve cord or backbone running down the back. This group includes amphibians, reptiles, birds, mammals and most fish.

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