[skipwords]Bisphenol-A is blurring the lines between species of fish, a new study has found, potentially sparking a boom in interspecies sex that could upend some ecosystems.

Better-known as BPA, the industrial chemical mimics estrogen, allowing it to disrupt hormones in a wide range of animals. It's used as a hardening agent in many types of plastic, but its widespread use also makes it a common pollutant in nature.

"Chemicals from household products and pharmaceuticals frequently end up in rivers, and BPA is known to be present in aquatic ecosystems across the United States," says lead researcher Jessica Ward, an ecologist at the University of Minnesota, in a press release about the study. "Until now studies have primarily focused on the impact to individual fish, but our study demonstrates the impact of BPA on a population level."

The study focuses on how BPA affects blacktail shiners and red shiners, two fish found in rivers and streams across the U.S. Both are native to North America, but the red shiner can become invasive when venturing beyond its native central U.S., even if it's just in a neighboring region of the country. That's the case in Georgia, for instance, where the fish in this study came from — and according to the results, native blacktails and non-native reds seem to be getting along a little too well.

"Our research shows how the presence of these manmade chemicals leads to a greater likelihood of hybridization between species," Ward says. "This can have severe ecological and evolutionary consequences, including the potential for the decline of our native species." The U.S. Geological Survey has been studying this issue for years, and a 2002 report offered this look at how they hybridize:

SHINER SHIFT: When the blacktail shiner (1) and red shiner (2) breed, their offspring (3) can be recognized by an intermediate spot near its tail and by its intermediate body shape. (Photos: USGS)

In the new study, published this week in the journal Evolutionary Applications, Ward and co-author Michael Blum of Tulane University collected both shiner species from two Georgia streams. They then kept various individuals separated for 14 days in tanks, some of which contained BPA. On the 15th day, they introduced fish from the different tanks to each other, watching for any physiological or behavioral changes such as size, color, courtship displays or mate choice.

Because it so closely resembles estrogen, BPA can trigger a wide array of changes to an animal's body that are normally regulated by hormones. In the shiners, it altered the size, coloring and behavior of some fish; male red shiners exposed to BPA, for example, showed less intensity in body color than unexposed males. Such changes apparently make the fish look like other species of shiners, which can "break down sexual isolation between native and invasive species," as the researchers put it.

Previous research has shown other reproductive oddities in U.S. fish, including a 2009 report on the rise of intersex bass — mostly males with eggs in their testes, but also some females with male traits — that may have been affected by "endocrine-active compounds" like pesticides, PCBs, soaps and pharmaceuticals. In the shiner study, the authors note that recent work has shown how endocrine disruptors such as BPA can "feminize" male fish, "potentially resulting in the loss of populations."

"This process poses long-term ecological consequences, especially in areas threatened by the introduction of invasive species," they write. "BPA and other hormone-mimicking chemicals can escalate the loss of native biodiversity by breaking down species barriers and promoting the invader." The researchers also note that red/blacktail hybrids have been known to dominate shiner habitats, suggesting that exposure to BPA could indirectly doom some native species.

"Our results contribute to a growing body of evidence demonstrating that the effects of human-mediated environmental alteration can extend well beyond individual-level reproductive success," they add, "with significant evolutionary consequences for populations and species."

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