Meet Eight Species That Are Bending the Rules of Reproduction

Roughly 100,000 years ago, in a romantic lagoon near Tampico on Gulf side of Mexico, two distinct fish species—a sailfin molly male and an Atlantic molly female—came together in an unlikely union. The colorful pair gave birth to the Amazon molly: an all-female, asexually reproducing mini-carrot length fish named after the all-female tribes of Greek legend, according to Schlupp of the University of Oklahoma.

Yet while these Amazons need no male genetic material to reproduce, they're not entirely independent. To kickstart their reproductive systems, they still need sperm. In a bid to find a suitor into this kind of thing, Amazons will actually disrupt mating processes between sexually reproducing mollies they come across in an effort to steal the male's seed from his erstwhile mate—by literally squeezing in between the pair.

"They kind of butt in and then it's almost as if they're hoping to get the mating that was meant for another female," Schlupp says. "The males that these Amazon mollies are mating with really have to get up close and personal with the Amazon mollies. These fishes have a specialized fin that they use to transfer sperm—we're actually talking about real copulation. It's not like a mass spawning where some parasitic female swoops in and gathers some sperm."

Talk about too close for comfort.

In 2014, scientists at the National Aquarium facilities in Baltimore happened on something fishy. One of their female swellsharks had just laid eggs, which subsequently hatched into five baby sharks. Yet the mother shark in question had been isolated in captivity from males for at least three years. 

While at first researchers thought this might be a remarkable case of sperm storage—other specieshad been known to store viable sperm in their bodies—genetic testing later revealed the female had reproduced via parthenogenesis, which happens when an egg fuses with a byproduct of egg production to create a clone of the mother without any help from a male. Solo reproduction has been also been seen in sawtooth sharks, and is usually considered a last-ditch effort for a female to pass on her genes.

"There are so many things about sharks that are bizarre, unique and interesting," says David Gruber, a biologist at the City University of New York who has conducted research on biofluorescent swellsharks. Add one more thing to that list of novelties: Virgin births. Because apparently, glowing in the dark and inflating your body size to almost triple isn't enough to set you apart from your run-of-the-mill sharks. 

A small collection of species from the crustacean family—including shrimp, lobsters and crabs—can reproduce asexually. The marbled crayfish, popular with aquarium hobbyists, is one of these. But this all-female crayfish is also a little different: it can only reproduce asexually. 

Zen Faulkes, a professor of biology at the University of Texas Rio Grande Valley, says that asexual reproduction in general seems to be associated with poor conditions, but that the asexual species are so different from their sexual peers that it's difficult to generalize any one reason for it. What's certain is that these crayfish are an invasive problem in many parts of the world. "They are rapidly spreading across Europe and Madagascar," Faulkes said. "I am convinced it's only a matter of time before they are found in the U.S." 

And when it comes to outcompeting native species, asexuality seems to play to their advantage. That's because there's no Adam and Eve necessary here: You only need one marbled crayfish in a given area to start a population. But how do they continue to survive without dying of disease? “Unknown at this point,” says Faulkes. “There's no published research on marbled crayfish diseases, apart from confirming they are crayfish plague carriers. In theory, yes, you would expect that if they were susceptible to some nasty bug, they would all be susceptible.”

In 2012, a brownbanded bamboo shark mother produced an egg that hatched a minimum of 3.5 years after its last possible contact with a male—a biological record at the time. "They had a female shark for all these years, then suddenly it gave fertilized birth," Gruber says. That’s remarkable, given that living sperm can last for only up to five days in the human body. 

Welcome to the world of semen-hoarding. After all, why fill out questionnaires at the sperm bank when you can save a piece of Prince Charming in your body for months at a time? Bamboo sharks are just one of the species known to have this ability, however: The aforementioned swellsharks can also store sperm, and nomadic blue sharks and dusky sharks can do this for months or even years, according to another study.

Heck, who needs a sperm bank when you are the sperm bank?

For most species, reproducing asexually is something that doesn’t happen very often. For water fleas, however, it’s the norm. These aquatic insects reproduce large broods of all-female clones in normal environmental conditions, according to Gerald LeBlanc, a professor of environmental and molecular toxicology at North Carolina State University.

But when there isn't enough food to go around, or when there are too many female clones around for comfort, the situation changes. Then, these females will begin producing male offspring as well. The males will then mate with the females, which turn bright copper during these stressful times, LeBlanc said in a release based on a 2005 study. The females then lay more durable eggs, which are more resistant to difficult environmental conditions. 

Caucasian rock lizards defy any number of attempts at sexual categorization. First, these all-female reptiles that inhabit rocky outcroppings in northern Eurasia don't need a trigger to lay functional eggs. But the seven-plus different varieties of asexual rock lizards are also the result of inter-species couplings between the many different sexual varieties of rock lizards, according to Susana Freitas, a PhD student in Sheffield University's animal and plant sciences department in the U.K.

It gets even more complicated. The asexual female hybrid clones sometimes also have a bit of an Oedipus complex—that is, if they mate with males of their father species, they can produce sexual offspring. The asexual species may also outcompete the sexual species in some areas by pushing them out of prime habitat, since they tend to produce more offspring and dominate areas where their range overlaps with their maternal species. "Given it is not a marginal part of the distribution range, it seems parthenogens are pushing sexuals away," Freitas says.

Unisexual mole salamanders have spent about 6 million years perfecting a boycott on traditional reproduction. But, like Amazon mollies, they still need a little help kick-starting their cloning process. Rob Denton, a PhD student and research fellow at Ohio State University who recently conducted a study on these salamanders' fitness, says that these salamanders need to steal sticky sperm packets from related species in order to prompt their reproductive system into action.

Researchers still don't know why exactly this needs to happen; after all, mole salamanders’ offspring are usually clones without any recombinant DNA from the sperm itself. But sometimes, genes from the species sneak into the genetic code of the all-female species, giving them properties that can make them look different from their sexual peers. You might think that extra diversity could give them the advantage, but Denton’s research shows it also makes them a little less mobile than their sexual peers.

Which is to say: mystery still unsolved.

Some female whiptail lizards have learned to "be the man" in their relationships in order to reproduce. Researchers have found that some all-female hybrid clones actually go through the same motions as the males of the sexual variety, gripping a fellow female by the neck and then by the pelvic region. "The only difference between pseudo-copulation and true copulation is that the unisexual lizards are morphologically female (they lack hemipenes), and so intromission cannot occur between them," wrote David Crews in Scientific American in 1987.

So why do they do it? Apparently, this pseudo-sex is critical for ovarian development and females in different periods of their ovarian cycle will develop male-like behavior at different times. “By alternating sex roles they maximize fecundity and increase the efficiency of reproduction,” he wrote.