June 29, 2008

Fish Getting Lost As Oceans Acidify

By Luntz, Stephen

Increasing ocean acidity as a result of carbon emissions may be making it hard for fish to form symmetrical otoliths (ear bones), creating a further threat to the health of coral reef ecosystems as fish with asymmetrical ear bones struggle to find their way to the safety of coral reefs. Dr Monica Gagliano of the Australian Institute of Marine Science (AIMS) was part of a team investigating damselfish otoliths. At hatching they found that 59% had asymmetrical otoliths.

The lack of balance between the ears didn't deafen the fish, but it did seem to take away the subtle directional hearing necessary to find their way back to a coral reef after a period spent in the open ocean early in their life.

Among damselfish arriving on the reef, the proportion with asymmetric ear bones was much lower than among the hatchlings, and those that did make it were later arrivals than their symmetrical siblings. "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," Gagliano says.

Otoliths, like the rest of the fish skeleton, are made from calcium carbonate. Acidic water offers less calcium carbonate from which to form fish bones or coral structures (AS, May 2008, pp.31- 33).

Slight increases in acidity, which is already occurring as a result of carbon dioxide absorption into the upper oceans, won't stop bone formation outright but might make it more difficult to create the perfectly balanced ear bones needed to detect the high frequency sounds associated with a healthy reef.

Gagliano's co-author, Dr Martial Depczynski of AIMS, says that other forms of stress can also interfere with otolith symmetry, but asymmetry is particularly likely if less calcium carbonate is available for bone formation as a result of increased acidity.

Given the strong disadvantage experienced by those hatchlings with asymmetrical otoliths, 59% might seem a surprisingly high proportion. However, Depczynski says: "Asymmetry has been well- studied in bird wings and human feet," and the figure is not unusual.

It is not possible to tell whether the rate of asymmetry has already been inflated by human influences. "Given that 95% of larval fish don't make it to a reef they have to be an evolutionary superhero to make it back," Depczynski says.

The research was published in the Proceedings of the Royal Society.

The survival of these damselfish depends in large part on perfectly balanced ear bones to guide them to the safety of a reef.

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