Freaky fish flirting

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Plastic chemical fingered for fish fiasco

The chemical bisphenol-A (BPA) is a component of the polycarbonate plastics used in everything from eyeglass lenses to sports equipment to baby bottles. Because the body can mistake BPA for the hormone estrogen, the chemical has attracted fear and loathing from parents who don’t want their infants imbibing BPA from a baby bottle.

Credit (original files): Estradiol: Boghog; BPA: Calvero
Chemical structure of estradiol, one of three naturally occurring estrogens in women. Rollover: Bisphenol-A (BPA) is similar enough to “mimic” estrogen.

Amid a wave of concern in many countries, several U.S. states have banned BPA from baby bottles.

You might not want your fish mating in a broth of BPA, either. A study published Wednesday shows that low levels of BPA increased the rate of inter-species courting between two related fish species.

“BPA is an estrogen mimic; its structure is very similar to estrogen,” says study co-author Jessica Ward, an adjunct assistant professor of fisheries, wildlife and conservation biology at the University of Minnesota. “When ingested it is able to bind estrogen receptors on cells, and trigger a cascade of reactions that are normally triggered by estrogens.”

The study did not test for actual mating (in which the male deposits sperm onto eggs laid by the female), but it did suggest that BPA could contribute to fish declines, even extinctions.

lil black shiner
lil red shiner

In the Southeast United States, for example, the native blacktail shiner Cyprinella venusta is being displaced by the red shiner C. lutrensis.

A 3x2 grid of fish showing how controls and BPA affect fish coloration

Ward, J. L. & M.J. Blum. Exposure to an environmental estrogen… Evolutionary Adaptations, 2012.
Male red shiners show color variations after swimming in water with BPA. Secondary sexual characteristics are controlled by hormones and can be disrupted by hormone mimics, but the change may not explain the cross-species mating behavior.

“Hybridization is a real problem,” says Ward, adding that several papers have shown “an extensive species collapse [of the blacktail shiner] on the upper Coosa River.” Invading red shiners, she says, are “so aggressive that they can outcompete natives, and also hybridize quite extensively. This fuels their invasive spread, and accelerates native species decline.”


Coosa River runs southwest through Alabama and Georgia, meets Alabama River.

Credit: John Lambert
Blacktail shiners in the Coosa River are declining as red shiners invade and cross with them. Does the endocrine disruptor BPA feed this process?

When fishies go a-courting

To find out whether the hormone-mimicking BPA could feed this hybridization, Ward and Michael Blum of Tulane University exposed shiners to a dilute BPA solution or with plain water. Red shiner males lost most of their distinctive red coloring in the BPA bath, a change consistent with hormone disruption.

Baby holding and drinking from bottle

Credit: Wayan Vota
If baby bottles leach the hormone disruptor BPA, what happens to the baby?

Four plastic re-usable bottles of different colors

Faced with fear of endocrine disruptors, manufacturers are introducing BPA-free bottles.

BPA is in watery ecosystems across the United States, although at lower levels than the 1,280 micrograms per liter concentration used in the study. The level was chosen to be comparable to previous studies.

In fish mating, like the more familiar process seen in bars and night clubs around the world, females check out males. But while women can usually rest assured they are talking to Homo sapiens, fish have to look twice. “They will check out this male and that one,” Ward says, “and use a lot of cues to say, ‘This is my species, but that is not mine.'”

When the researchers tested female shiners, those that had imbibed BPA spent more time and effort flirting with alien males, Ward says. “Their discrimination ability is much lower after being exposed to BPA.”

Normally, however, the fishy-females can “assess pretty quickly and say ‘I am not interested, you are a different species,'” Ward says, “but … after BPA they spend more time hanging out with males of another species, are not so quick to turn them down.”


Top: blacktail shiner with black spot on tail; bottom: red shiner with very faint black spot on tail; middle: hybrid shows medium-size black spot on tail.

Credit: USGS

Although the researchers did not study whether eggs were cross-fertilized after this courtship, they did document a breakdown in “reproductive isolation,” that would allow new species to enter an ecosystem and create hybrids at the expense of the natives.

Something fishy about this courting!

Although hormone disruption has been a hot research field for a decade, “most studies are interested in what goes on with individuals,” Ward says. “We are … extending that to the population level.”

But what is the mechanism? “My hunch is that it has something to do with the fact that females like males that court with greater intensity,” Ward says. “After BPA, males show a greater tendency to court other species. Females are paying more attention to these males, because they are paying more attention to the females.”

We must say it’s a rather familiar “chemistry between the sexes…”

— David J. Tenenbaum


Terry Devitt, editor; S.V. Medaris, designer/illustrator; Molly Simis, project assistant; David J. Tenenbaum, feature writer; Amy Toburen, content development executive


  1. Ward, J.L, Blum, M.J, “Exposure to an environmental estrogen breaks down sexual isolation between native and invasive species”, Evolutionary Applications, Wiley-Blackwell, July 2012, DOI: 10.1111/j.1752-4571.2012.00283.x
  2. The NIH on BPA
  3. Consumer tips to avoid BPA exposure
  4. Food additives mimic hormones
  5. BPA and human exposure
  6. Defining a species
  7. The first biological species concept
  8. Fact sheet: Blacktail shiner
  9. Fact sheet: Red shiner
  10. Interspecies’ sex
  11. Other news on hormones in waterways