Comments on: The mantis shrimp has the world's fastest punch

By: Ken Kzak

Ken Kzak — Fri, 07 Aug 2009 06:26:28 +0000

Water is considered non-compressible. This is true when compared to air and even some other liquids, but if you push hard enough, it does compress, but it doesn’t have to like it.
When you compress anything it creates heat. If you compress water enough it will exceed the boiling point, thus gas bubbles form. Since it’s producing light, this is near explosive levels in the confined micro zone between the Mantis shrimps claw and the surface under assault.
This would constitute leading surface cavitation as opposed to trailing surface [negative pressure] cavitation.
I once read where scientists [at least some jolly folks in lab coats] found they could produce light with intersecting sound waves in a small vessel of water. One sound source just pushes the water back and forth, two sources trap a tiny bit of water between them and compress it.
Probably Discover Magazine about 15 years ago.

By: Priam

Priam — Thu, 06 Aug 2009 16:03:28 +0000

Old thread, I guess, but hey.
It looks like the cavitation comes at the point of impact, as seen in the Berkeley videos. Not a fluid dynamics specialist here, but my take is that when the arm strikes the shell, the sudden slam caves the shell rapidly inwards, causing water to rush to fill the impact zone, lowering the pressure and causing the water to go gaseous for a couple microseconds. There’s your cavitation.
It doesn’t happen on the arm’s back-end, though, because, for one, the arm starts its movement more gradually, and two, the rounded shape means only a small amount is susceptible to strong pressure gradients at a time, whereas on impact the full impact zone is implicated in the pressure event.

By: Matt

Matt — Tue, 22 Jul 2008 00:52:21 +0000

It would only make sense if the shrimp could withdraw its leg quickly as well, which I’m assuming it could if the cavitation is indeed behind it like it says. I suppose it does have some muscle to retract it quickly, though not as quickly as it sends it out. I can’t think of a reason for this either.

By: Blind Squirrel FCD

Blind Squirrel FCD — Mon, 21 Jul 2008 18:05:47 +0000

The idea of cavitation on withdrawal of the leg doesn’t make sense to me. Does the shrimp have another set of fantastic muscles to withdraw the leg at light speed? Working in opposition to the deductor muscles? To what purpose?

By: Matt

Matt — Mon, 21 Jul 2008 09:39:34 +0000

That may be Thorfinn, it makes sense to me.

By: Mike Spear

Mike Spear — Mon, 21 Jul 2008 08:08:56 +0000

….should have read the other comments first.

By: Mike Spear

Mike Spear — Mon, 21 Jul 2008 08:07:53 +0000

For anyone interested here's the actual video that Sheila Patek took at UC Berkley that started this whole fiasco: http://ist-socrates.berkeley.edu/%7Epatek/shrimpMechanics/5000fps.mov

By: Thorfinn

Thorfinn — Mon, 21 Jul 2008 06:23:32 +0000

I have the impression that the cavitation occurs post-contact on the withdrawal leg of the strike.

By: Ed Yong

Ed Yong — Sun, 20 Jul 2008 18:13:45 +0000

Folks, I’ve changed the unfortunate USC Berkeley thing and the link to the videos. You may well be right about the cavitation thing but I’m on holiday on a rickety connection so I’ll check when I get back.

By: Ian Findlay

Ian Findlay — Sun, 20 Jul 2008 09:49:41 +0000

I seem to remember a whole series of articles on this published in the 60′s.