No Males Required: Shark Switches To Asexual Reproduction

S. fasciatum have stripes like a zebra as babies, leading to their name zebra sharks. However, as adults they have spots and are sometimes called leopard sharks. Rich Carey/Shutterstock 

For the first time, a shark that previously reproduced in the usual manner has been observed using parthenogenesis, or asexual reproduction, to produce healthy offspring. The surprising event has given marine biologists a chance to learn about the evolutionary basis for doing without sex.

A zebra shark (Stegostoma fasciatum) named Leonie has been kept at Townsville's Reef HQ Aquarium since 1999 after being caught in the wild. Zebra sharks are endangered, so when Leonie was put in a tank with a male of her species, aquarium staff were pleased that it led to baby sharks for six successive years.

After more than a dozen babies, however, space was getting low and the aquarium separated the pair, putting Leonie in with one of her daughters. Although that stopped Leonie's breeding for a year, in 2014 Leonie laid eggs. None hatched, but in the 2015/16 breeding season, Leonie produced three healthy daughters.

Female sharks can sometimes store sperm for long periods of time after mating, but genetic analysis of these offspring published in Scientific Reports shows that they are identical to Leonie, with no input from her former mate. On the other hand, the sharks born when Leonie was housed with the male have been confirmed as combining the genetics of the two parents.

Although such a shift to asexual reproduction has been confirmed before in an eagle ray and a boa constrictor, this is the first known case in sharks. First author Dr Christine Dudgeon of the University of Queensland said in a statement: “This has big implications for conservation and shows us how flexible the shark’s reproductive system really is. Leonie adapted to her circumstances and we believe she switched because she lost her mate. What we want to know now is could this occur in the wild and, if so, how often does it?” 

The authors noted that some invertebrates switch their reproduction strategy based on temperature, but that could not have been the case here.

Parthenogenesis is used by plants and many invertebrates. It is never seen in mammals, and while it does occur in reptiles and fish, it is thought to be rare among vertebrates because the loss of genetic diversity makes species that rely on it vulnerable to parasites and changing environments. However, Dudgeon and her co-authors note the capacity to switch might be useful for cases where females have expanded into new territory and can't find mates, allowing them to produce offspring for a generation or two while waiting for the males to catch up.

They note this may occur more often than we realize, since it would be difficult to detect in the wild, and some cases in captivity might have been wrongly assumed to involve sperm storage. Nevertheless, Dudgeon told IFLScience that the only case where the “signature of parthenogenesis” has been seen in the genetics of wild sharks or rays has been in the sawfish, where overfishing may have made it hard for females to find mates.

Dudgeon told IFLScience the team are very keen to see whether the daughters produce offspring and, if so, how. “There's never been a documented case of a vertebrate species where young produced through this sort of parthenogenesis went on to reproduce sexually,” she said.

Reef HQ has no plans to rename the shark Mary.

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