March 10, 2015, by Tim Radford
Pollution plus heat produces many more male zebrafish than females.
Image: Peter Southwood via Wikimedia Commons
Scientists find that zebrafish exposed to hormone-disrupting chemical pollution produce abnormal numbers of male offspring, especially in increasingly warmer water. LONDON, 10 March, 2015 – Climate change seems to make everything worse – at least for some wild creatures. British scientists have just confirmed that higher temperatures could amplify the impact of hormone-disrupting chemicals that already pollute the environment. The world’s waterways are full of industrial pollutants with potentially damaging effects. They include industrial agents, the waste products of birth-control pills, herbicides, pharmaceuticals and even the residues of illegal narcotics. Altogether, more than 800 chemicals have been identified as having some hormone-disrupting capacity. Ross Brown, then with AstraZeneca Research and now at the University of Exeter, and colleagues report in the Proceedings of the National Academy of Sciences that they decided to look at the long-term effects of clotrimazole, a chemical commonly used in antifungal treatments and believed to disrupt hormones and interfere with the sex ratios of fish and amphibians. Conservationists – and others – have worried for decades about the build-up of such chemicals in the environment. They have cited them as possible threats to biodiversity, and have produced evidence that they could be implicated in sexual abnormalities in some species.
But these have been regarded as a separate problem, and not part of the mix of stresses that could accompany climate change. The British scientists tested a well-established laboratory and aquarium favourite, the zebrafish (Danio rerio). This is the first fish to have its entire genome sequenced – which means researchers already know a great deal about its life cycle, physiology and development. So the scientists observed normal spawning at the temperatures in which the fish evolved, and five degrees higher, at the 33°C projected for its home waters in 2100. In the tests, the water level of endocrine-disrupting clotrimazole was also at levels found polluting the world’s waterways today. Temperature plays a powerful role in determining the sex of some as yet unborn members of certain species. Warmer temperatures can make female status more likely for crocodilians, some lizards and turtles and tortoises. Higher temperatures, however, are likely to encourage higher ratios of male lizards, fish and amphibians. Since, in normal conditions, temperatures vary around an average, the numbers of males and females in a population tend to even out. But in reproduction, it’s the females that matter more. So a sustained tilt towards maleness could threaten a population’s survival.
The researchers found that the zebrafish exposed to the chemical pollutant produced an abnormally high percentage of male offspring. This ratio got even higher when the fish were confronted with the double whammy of clotrimazole and warmer waters. Fish that were inbred were the most likely to respond, while fish from a genetically-diverse heritage were somewhat less affected. The implication is that endangered species living in small populations in isolated waters could be at greater risk of extinction. This was a controlled laboratory experiment, conducted under very precisely-measured conditions, on one well-studied species. The real world is a messier place, and outcomes 80 years on for other freshwater fish and amphibians exposed to an unpredictable suite of stresses are harder to predict. But the zebrafish, a native of the Indian subcontinent and often a citizen of the flooded rice paddies, is also likely to experience a wide range of chemical pollutants. So conditions in the wild could be even worse. “Chemicals in the environment are usually looked at in isolation, but in reality animals are exposed to multiple stressful events at the same time,” says the report’s senior author, bioscientist Charles Tyler, of the University of Exeter. “They include changes in temperature, food scarcity, or harmful chemicals. “It is important that we understand how these pressures interact if we are to understand the real impact of rising global temperatures and increasing levels of pollution.” – Climate News Network
Tim Radford, a founding editor of Climate News Network, worked for The Guardian for 32 years, for most of that time as science editor. He has been covering climate change since 1988.