Reef fish lose their way as environment turns hostile

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 fish ecologists Dr Monica Gagliano (AIMS and James Cook University) and Dr Martial Depczynski (AIMS, Perth), with Dr Stephen Simpson from the University of Edinburgh and James Moore from JCU in Townsville, has found that fish with asymmetrical ear bones struggle to return to the reef.

The implications could be profound for the survival of reef ecosystems, which depend upon a rich biodiversity for effective function and health.

The research has been published today (Friday 7 March 2008) in the prestigious UK scientific journal Proceedings of the Royal Society*.

The stresses causing ear bone 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. It is now known that 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. Fish are not the helpless victims of currents and tides – they actively navigate.

The project examined damselfish, which are abundant on many reefs, including Queensland’s Great Barrier Reef and Western Australia’s Ningaloo Reef. As a group, damselfish are well-understood and provide a good model for other kinds of reef fish.

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

At hatching, 41 per cent had symmetrical ear bones (otoliths) and 59 per cent asymmetrical. When the team examined the otoliths at the settlement stage 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 our opinion, ear bone asymmetry in the early life stages of reef fish interferes with their capacity to find and settle on coral reefs,” Dr Gagliano said.

Vertebrate animals make sense of sounds by comparing differences in the acoustic signal between their two ears. To do this well, their ear structures must be relatively symmetrical. Asymmetrical ear bones do not appear to make the fish deaf, but they may interfere with the ability of the fish to hear effectively.

Dr Gagliano said that fish otoliths were a sensitive tool for studying the effects of environmental stress in fish. “Asymmetry has been used as a stress indicator for a long time, although in some contexts it remains controversial,” Dr Gagliano said.

“In our case, it looks like it is very reliable. Preliminary data indicate that if we increase the stress, the asymmetry of the otoliths will increase,” she said.

“There is a degree of asymmetry that is acceptable in the population – some is natural,” co-author Dr Depczynski said. “Not all the babies are created equal and not all of them are going to make it, even in pristine environments.”

The problem now is that an already high mortality rate among reef fish hatchlings is likely to rise even higher if young fish can’t navigate by sound.

At least part of the problem is likely to be linked to ocean acidification, although much more research needs to be done to examine the link. Fish ear bones, like their skeletons and many other kinds of structures such as reef-building corals, are made from calcium carbonate. When seawater becomes more acidic, there is less calcium carbonate available for building any calcium-based structure, including ear bones.

Acidity appears to be having a two-fold effect, creating a hostile marine environment and also robbing the environment of the building blocks of calcium-based structures. This has a direct effect on fish development and on their food sources, as many creatures the fish eat are also dependent on calcium.

While stress is part of life for reef fishes, new stresses are now being piled on top of existing ones and fish are showing the effects, according to Drs Gagliano and Depczynski.

Australian Institute of Marine Science, 7 March 2008. Press release.

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