These little shiners can get mighty confused when they are exposed to chemicals that mimic hormones, resulting in cross-species mating. (Photo: Creative Commons: brian.gratwi)

Fish are directed by hormones to select their mates and new research shows that chemicals that mimic hormones are confusing the swimmers into mating with other species.

Bisphenol A (BPA), an organic compound used as a hardening agent for polycarbonate and other plastics, mimics estrogen and disrupts a fish's endocrine system, which controls the release of hormones. This can change their behavior and appearance, which in turn can lead them to mistake a newly introduced species as a potential mate.

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"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," said Ward. "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 research, led by Dr Jessica Ward from the University of Minnesota, focused on the impact of BPA on Blacktail Shiner (Cyprinella venusta) and Red Shiner (Cyprinella lutrensis) fish which are found in rivers across the United States.

The researchers collected both species of fish from Georgia streams and kept them separated for 14 days in separate tanks, some of which contained BPA. Then they introduced the fish to each other, watching for changes in size, color, courtship displays or mate choice.

The shiners were susceptible to the effects of BPA, showing altered size, coloring and behavior of some fish. According to Mother Nature Network, 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.

"Our research shows how the presence of these manmade chemicals leads to a greater likelihood of hybridization between species," said Ward, according to Futurity.org. "This can have severe ecological and evolutionary consequences, including the potential for the decline of our native species." 

As invasive species move into new areas, Ward told Pioneer Press, BPA and other hormone-mimicking chemicals can escalate the loss of native biodiversity by breaking down species barriers and promoting the invader.