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.

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

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.”

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.

Credit: Wayan Vota

If baby bottles leach the hormone disruptor BPA, what happens to the baby?

Credit: Rubbermaid Products

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.”

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…”