light from the collapsing cavitation bubble

A month or so ago I met a great bloke called Andy who undertook a diving course with Calypso a couple of years ago.  Now he's back for more.

Andy and I completed our Rescue Diver course together and became pretty close, having to blow on each other foreheads for an afternoon and then share the experience of blowing on our instructors forehead a day later.  Andy also shared his air with me which is neither a metaphor nor a lifestyle choice.

As it happens Andy is from just across the water from where my sister lives on the West coast of Scotland;  I know, it's a small world.

Get to the point of all this finger tapping now, I hear you cry!

Well, Andy is studying Marine Biology and on his first day of this visit to Calypso Andy told us of the wonder of Snapping Shrimps.

The animal has one claw bigger than the other and snaps the big claw shut to make a noise in order to stun its prey.  Within the claw there is a plunger which as the claw snaps, the plunger moves out at incredible speed to create a cavitation bubble.

What this means is that the plunger pushes water out of the space within the claw faster than surrounding water can fill the void.  Being a fan of all things Star Trek and Dr Who I liked this bit most of all in that it reminded me of a black hole (but probably is nothing like it!).

The pressure is strong enough to kill small fish.

The snap can also produce light from the collapsing cavitation bubble. As it collapses, the cavitation bubble reaches temperatures equal to the surface temperature of the sun (estimated to be around 5,500 °C). The light is not visible to the naked eye.

All of this happens in less than a millisecond.  All that evolution for a spot of lunch!  What makes this even more incredible is that the sound is so loud that navel maps routinely detail beds of these creatures and when necessary commanders will lower their vessels down close to the colonies, in order to interrupt enemy sonar detection equipment, useless against the background noise of the snapping shrimp.

This last point was confirmed by Navy Mark who came and did his PADI Open Water with us.  He told me that rather than hide because of the noise, Russian subs would operate their radios at a Hz the same as the shrimps.  This was not by design but purely by chance.  When discovered this acoustic signature became known by western radio operators as the key to detection, either that or it turned out to be a bed of shrimp.

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