Shrimp, bubble and pop



Snapping shrimps are the noisiest creatures in the shallow ocean, capable of drowning out submarine sonar by the "snap, crackle and pop" of bubbles generated by their claws. That is the verdict of researchers who have been studying how the tiny marine organisms make such a din.

[The shrimps] disturb underwater communication between submarines - and hostile submarines use colonies of them to hide

Prof Detlef Lohse Scientists had always believed that the distinctive crackling noise of shrimp colonies came from mechanical contact between two parts of the creatures' claws. But a new study, reported in the journal Science, has found that the noise actually originates from the collapsing of what are known as cavitation bubbles generated when the shrimps' claws slam shut. "When the shrimp closes its snapper claw, the water that is in between the claw-halves is squeezed out with great speed (30 metres per second), which is fast enough to generate a cavitation bubble," said Dr Michel Versluis, of the University of Twente in The Netherlands. "So, the sound of snapping shrimp is produced by this collapsing cavitation bubble and not by the claw halves hitting each other during claw closure - as was always believed," he told BBC News Online. Noisy ocean Life beneath the ocean wave is far from peaceful. Waves, rain and vocal marine mammals like dolphins and whales contribute to an underwater cacophony in the shallow ocean.

But the dominant source of background noise is created night and day by colonies of snapping shrimps. The noise seriously interferes with military and scientific sonar used to detect underwater objects. "They disturb underwater communication between submarines - and hostile submarines use colonies of them to hide," said Professor Detlef Lohse, also based at the University of Twente. Snapping shrimps have one normal claw and one over-sized claw, which can grow to up to half the body size of the 5-cm-long creature. The two parts of the claw are normally cocked open, but they close with lightening speed when a muscle contracts, ejecting a jet of water.

The scientists, based in the Netherlands and Germany, used an ultra-speed camera and a hydrophone in an aquarium to record what happened when seven individual shrimps closed their claws in response to being tickled by a paintbrush. The photographs showed bubbles forming between the two parts of the closing claws. The bubbles were dissipated, along with the water jet, when the claws snapped shut. Sound recordings revealed that the snapping sound occurred well after the claw itself had closed, when the bubble collapsed. The scientists used a mathematical equation to prove that the sound arose from imploding bubbles rather than physical contact between the claws. When water velocity is high, the pressure within the liquid drops to below the vapour pressure of water, allowing tiny air bubbles to expand. When the pressure builds back up, the bubbles implode, making a popping noise. The shrimps use cavitation to stun their prey - small crabs, fish and worms - or to communicate with other shrimps. Lord Rayleigh was the first to investigate cavitation in 1916. The British Royal Navy asked the physicist to find out why cavitating bubbles were causing damage to the propellers of its vessels. Although Lord Rayleigh was unable to come up with a solution, he developed a set of equations that are still being used today and formed the basis for the team's work on shrimps.