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  Stress gives reef fish wonky ears
Reef fish who are stressing about environmental changes are creating irregular offspring, says a new Australian study.

The study, published in the latest edition of Oecologia, shows a clear link between mothers producing the stress hormone cortisol, and the development of asymmetrical ears in the offspring of the common coral reef fish, Pomacentrus amboinensis.

Most animals, produce the hormone cortisol as a response to stress. Reef fish typically produce it in response to an encounter with a predator.

"Cortisol is needed for development," says study lead author Dr Monica Gagliano of James Cook University in Townsville.

But the researchers found that if cortisol is produced in excess it can have harmful effects on the development of the embryo.
Asymmetrical ears

The researchers bathed reef eggs in different concentrations of cortisol.

According to Gagliano, who is also a research fellow with the Australian Institute of Marine Science, the levels chosen were based on the amount of cortisol that eggs would be exposed to in calm and stressful reef environments.

The researchers then viewed the embryos under a microscope as they grew, focussing on the development of the ear bones.

Gagliano says the "ears of the babies are essential."

"They need to have their ears working, so they can hear the sound of the reef," she says. After the fish hatch, they must find the reef in order to fully develop and eventually reproduce.

The researchers found that embryos exposed to high doses of cortisol had a higher proportion of asymmetrical ears. "The higher the level of cortisol the more wonky their ears are," says Gagliano.

She says, 59% of the embryos exposed to high doses of cortisol developed asymmetrical ears, compared to only 28% of the embryos that were exposed to no cortisol.

The study found that hatching success was significantly affected by the level of cortisol the embryos were exposed to.

Only 44% of the embryos exposed to high doses of cortisol survived compared with 83% of those that were not exposed.

The researchers also found embryos exposed to high doses of cortisol hatch sooner.

"If there are a lot of predators around mum, might get stressed because she is worried she will lose the babies. She will put more cortisol in the babies and they will hatch earlier to avoid the dangerous environment," says Gagliano, "But then [the fish] will pay the cost later."

According to Gagliano, the study highlights the potential impact climate change could have on reef fish due.

She says, if stress levels increase it could negatively affect the population of reef fishes.
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