Ambon damselfish (Pomacentrus amboinensis). Image: Monica Gagliano.

Environmental stresses, including warmer and more acidic seawater, may be affecting the development of the ear bones in young reef fish, causing the fish to get lost at sea during a crucial stage of their development.

Research by AIMS fish ecologists has found that fish with asymmetrical ear bones struggle to return to the reef. The stresses causing asymmetry may be closely linked to a combination of rising sea surface temperature and acidity, both caused by high atmospheric carbon dioxide levels, along with a range of more localised stresses.

Abnormalities in fish hearing structures may be interfering with a vital part of the animals' life cycle. Most reef fish spend some time in the open ocean after hatching, before finding a place on the reef to settle and breed. Researchers have only recently established how important sound is in guiding young fish to their homes; fish at the end of their ocean stage "home in" on reef-associated sounds, such as the gurgling of fish and the snapping of crustaceans. A sophisticated hearing system that enables fine distinction between frequencies is needed by young fish to determine where to go.

The scientists collected damselfish hatchlings at their reef of origin and later traced fish from the same cohort arriving on the reefs after the ocean phase, attracting them to traps broadcasting various sound frequencies.

At hatching, 41 per cent had symmetrical ear bones (otoliths) and 59 per cent asymmetrical. When the team examined the otoliths of the returning fish a few weeks later, far fewer asymmetrical individuals were found to have made their way back to a reef. The scientists also found that those with asymmetrical ear bones that did make it to the reef took longer to do so than their symmetrical counterparts.

In the future, this problem may be aggravated by ocean acidification. Fish ear bones, like their skeletons, are made from calcium carbonate. When seawater becomes more acidic, there is less calcium carbonate available for building calcium-based structures, including ear bones. Apart from the direct impact upon fish development, acidity may also reduce their available food sources, as many of the creatures that larval fish eat are also dependent on calcium.