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Climate change worsens endocrine disruptors’ effects - Conservation

Climate change worsens endocrine disruptors’ effects

Aquatic animals face a lot of threats these days, from warming waters and unpredictable rainfall to endocrine disrupting chemicals that mess with their reproductive systems. But while such threats are widely recognized, few studies have examined their effects in combination, especially at a population level.

In a study published yesterday in the Proceedings of the National Academy of Sciences, researchers manipulated laboratory-reared groups of zebrafish (Danio rerio) to show that climate change and pollution together can radically restructure the sex ratios of fish populations, leaving them vulnerable to reproductive collapse.

Today, zebrafish are found in large numbers in scientific laboratories around the world, where they are widely used in studies of genetics, development, and toxicology. But as it turns out, the fish are also ideal for modeling the effects of large-scale environmental change.

In the wild, zebrafish inhabit small pools on the Indian subcontinent that are periodically connected by monsoon rains, enabling populations to intermix and ensure the genetic health of the species as a whole.

Researchers collected zebrafish from ponds around the Brahmaputra River in Bangladesh, and bred the fish in the lab over several generations to produce either outbred or inbred families. The inbred families simulate the small populations, lacking in genetic diversity, that are often characteristic of endangered species.

The researchers housed these families – 760 fish in all – in tanks kept at either 28° C, the average water temperature of zebrafish spawning ponds today, or 33° C, the average temperature predicted for the year 2100.

For many species of fish, amphibians, and reptiles, male and female are more fluid qualities than they are among humans: An individual’s sex may depend in part on the temperature at which its egg incubated or at which it grew up. Previous studies have shown that higher water temperatures result in more male zebrafish.

But no one knew how this pattern might interact with the effects of endocrine disruptors. So the researchers also dosed some of their study tanks with clotrimazole, a commonly prescribed anti-fungal medicine that has found its way into many waterways around the world and blocks production of the female sex hormone estrogen. Similar chemicals are known to increase the proportion of zebrafish males.

In combination, these two environmental challenges can have striking effects. A whopping 97 percent of inbred zebrafish raised in warm-water tanks and exposed to a high dose of clotrimazole developed into males, the researchers found.

Genetically healthy populations are somewhat shielded from these effects. For example, inbred fish exposed to high-dose clotrimazole alone developed into males 80 percent of the time, but only 60 percent of fish from outbred populations exposed to the same treatment were male.

The researchers also conducted computer modeling to show that an excess of males results in lower population growth rates and therefore an increased chance that the population will wink out altogether.

The results suggest that scientists can’t predict the effects of climate change without taking into account the other environmental challenges a species may face at the same time, the authors say. Conservation planners will have to take into account the fact that for fish, ‘a perfect storm’ may well become the new normal. – Sarah DeWeerdt | 3 March 2015

Source: Brown A.R. et al. 2015. Climate change and pollution speed declines in zebrafish populations. Proc. Natl. Acad. Sci. DOI: 10.1073/pnas.1416269112


Header image: A female zebrafish, a creature made rarer by climate change and endocrine disrupting chemicals, especially in combination. Credit: Azul, via Wikipedia.