LURKING in the oceans are creatures that can create balls of plasma almost
hot as the surface of the Sun, a Dutch researcher has found. No, they aren't
the stars of a monster movie, but shrimps that live on coral reefs.
Snapping shrimp (Alpheus heterochaelis) have an outsized claw that they use
to fire jets of water at their prey and each other. The noise that they make
pervades the ocean, drowning out the sounds of ships and other animals.
"Submarines can use the shrimp to hide," says Detlef Lohse of the University
of Twente in the Netherlands.
To understand the source of this cacophony, Lohse and his colleagues filmed
the shrimp with a high-speed camera and found that the snapping sound comes
before the claws shut. The noise is generated not by the clicking claws, but
by tiny bubbles in the water jet that expand and then collapse violently--a
process known as cavitation (Science, vol 289, p 2114).
But the researchers also suspected that these cavitating bubbles might be
producing light as well as sound through a phenomenon called
sonoluminescence. When the bubbles collapse, any gas or water vapour inside
them is compressed, which can raise its temperature to startling levels and
give off light (New Scientist, 21 November 1998, p 27).
Further experiments suggest that sonoluminescence is occurring in the water
jets fired by the shrimp. "The preliminary results hint at light emission,
but more work is in progress," Lohse reports.
He estimates that the temperature inside the collapsing bubbles is around
5000 °C. "It's a cold plasma," he says. These extreme conditions only last
for around 200 picoseconds, however, and are most common 3 millimetres from
the tip of the shrimp's claw, so it seems unlikely that the animals are
the balls of plasma as weapons.
>From New Scientist magazine, 14 October 2000.
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