Snapping shrimps silenced as seas acidify: study

Weakened coral bleached bone white, oysters and clams dying by the millions – there’s no doubt ocean acidification can spell doom for marine species.

Now, it appears it will deaden the natural underwater cacophony too. A new study suggests one of the sea’s noisiest animals – the snapping shrimp – will be silenced, or at least subdued, as pH drops.

In the Proceedings of the Royal Society B, a trio of biologists from the University of Adelaide in Australia recorded the crackle of Alpheus shrimp near naturally acidic underwater vents and found fewer and quieter snaps.

Not only does this affect the shrimp’s communication skills, it also confuses the animals that rely on the snapping sounds for navigation.

“Coastal reefs are far from being quiet environments – they are filled with loud crackling sounds,” says staudy lead author Tullio Rossi.

“Shrimp ‘choruses’ can be heard kilometres offshore and are important because they can aid the navigation of baby fish to their homes. But ocean acidification is jeopardising this process.”

If you’ve ever noticed a click-clacking underwater while swimming in the sea, a bit like popping corn, you’ve probably heard some of the hundreds of species of snapping shrimp.

Even though they’re only around four centimetres long, they’re capable of generating a snapping sound as loud as 210 decibels – far louder than a 150-decibel gunshot – and second only to sperm whale clicks.

Their secret lies within their disproportionate larger snapping claw, which can be as long as half their body.

Instead of a normal pincer, it contains a hammer-like component, which, like a gun, fires into the other segment of the claw, producing a powerful wave of bubbles that can warn off predators and stun prey.

The sonic beacon that guides other marine animals will dim.

Ocean acidification, caused by dissolved carbon dioxide reacting with water and carbonate molecules to produce acidic bicarbonate ions, has already been shown to affect the behaviour of marine animals. It changes the way some species escape predation, for instance. Could it affect the sounds of the snapping shrimp?

So the Adelaide biologists eavesdropped on snapping shrimp in naturally acidic parts of the ocean – carbon dioxide-rich volcanic vents off the coast of Italy and New Zealand – during the “dusk chorus” when the invertebrates are most active.

The water around the vents was acidified to a similar extent as the world’s oceans are predicted by the end of the century.

They found snapping frequency halved. And those snaps were quieter, dropping five decibels on average. These figures were confirmed with lab experiments.

The reduced snaps stem from behavioural changes, rather than impairments to the snapping claw, says co-author Ivan Nagelkerken.

And with fewer, quieter snaps, the sonic beacon that guides other marine animals will dim. Some fish larvae, for instance, rely on snaps to guide them to safe coastal areas.

“If human carbon emissions continue unabated, the resulting ocean acidification will turn our currently lively, noisy reefs into relatively silent habitats,” Nagelkerken says.

“And given the important role of natural sounds for animals in marine ecosystems, that’s not good news for the health of our oceans.”

Hear the snapping shrimps’ normal chorus compared to that from a volcanic vent below: