An Acidic Ocean Will Also Be a Silent One

These giant claws make some of the loudest sounds in the ocean, but fall silent as carbon dioxide concentrations rise. Tullio Rossi

Snapping shrimp, the creatures that make coral reefs noisy places, go quiet when living in water just a little less alkaline than the current ocean. Since many other species rely on the sounds shrimps make to find their way home, this is a disturbing and unanticipated discovery in a world where carbon dioxide is affecting the chemistry of the oceans.

Many coral reef fish disperse widely as larvae. Once transformed into juveniles they find their way to a reef through a combination of smell and sound, particularly the sound of snapping shrimp.

"Shrimp 'choruses' can be heard kilometers offshore and are important because they can aid the navigation of baby fish to their homes," said Adelaide University PhD student Tullio Rossi in a statement.

Last year Rossi confirmed a long-standing suspicion: Ocean acidification interferes with the hearing of many reef fish, making it harder for them to find their way to safety. The finding was expected because acidic conditions have been shown to affect the development of ear bones in many coral reef fish, interfering with their balance.

Rossi's latest finding is more of a shock. Recording sound at the frequencies at which the snapping shrimps are most audible he found the shrimps are largely quiet when living around volcanic vents, whose carbon dioxide balances the otherwise alkaline sea water around them.

 

 

Along with Dr. Ivan Nagelkerken, Rossi tested what happened when snapping shrimps were placed in water with raised carbon dioxide levels. The results have been reported in Proceedings of the Royal Society B, and make sobering reading.

Nagelkerken told IFLScience, "The shrimps could snap when threatened and made the same sound" compared to when placed in water with modern carbon dioxide concentrations. "But when we leave them alone in pairs they are much more modulated when communicating." The reduced snapping may not hurt the shrimps, although this is uncertain, but is disorientating for fish seeking reef sounds.

"For fishes at least high CO2 interferes with a neurotransmitter, and all normal behavior is altered," Nagelkerken said. "We think the same mechanism is in place for invertebrate species; it's the most likely explanation."

Rossi tested whether anything physical could be stopping the shrimps from snapping, but found their claws were of normal length, and they molted like healthy shrimps.

Nagelkerken told IFLScience that no work had been done to assess whether shrimps fall silent gradually, as carbon dioxide concentrations increase, or if there is a point where the problems start. He sees this as an important topic for future research. The experiment used concentrations matching those anticipated for 2100, lowering pH slightly below 8, from its current value of 8.14.

Besides the threat to fish, Nagelkerken said his team have shown coral larvae are also attracted to sounds.

When marine biologist Rachel Carson wrote "Silent Spring" to alert the world to the dangers of DDT, she probably never imagined that her beloved seas might one day also be silent.

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