Science Questions

Why do Portuguese Man o’ War beach themselves?

Sun, 8th Feb 2009

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Merin asked:

Why do Portuguese Man o’ War, those are jellyfish, beach themselves?


Helen - It’s a great question.  Portuguese man o’ war, a type of jellyfish but actually they’re lots of different individual animals that live together.  They’re not really considered as being a single organisms which  is rather intriguing.  They can’t actually swim.  Other jellyfish have a way of pulsing their tentacles and moving so they’re basically at the mercy of wind and currents and what’s going on around them.  They have this lovely blue and pink tinged pneumatophore full of gas that floats them on the surface.  That catches the wind if the wind is blowing in that direction. The really cunning thing about these guys is that, like humans, Portuguese man o’ war can be left-handed or right-handed.  About half of them in the population has this pneumatophore that’s shaped towards the left and half that’s shaped towards the right which means they will go in different directions if the wind blows in a certain direction.  I think that’s very cool indeed.  You can imagine why that might be a good thing.  It means that half the population will get swept up on a beach if the wind’s going in a certain direction.  Some of them won’t.  They won’t all necessarily land up on a beach ‘cos that’s not great.  They usually get trodden on and will desiccate very quickly and dry out.  It’s all very much at the mercy of the elements.

Chris - I saw them in Australia, some of them doing exactly as you said. They were getting blown up on the beach because they have this sail on the top which pushes them along.


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Meryn asked the Naked Scientists: Hello there naked scientists Why do blue bottles (Portugese Man-o-War jellyfish) beach themselves? regards Tait Mc Jarrow ( I am 9 years old) What do you think? Meryn , Fri, 16th Jan 2009'_War

So I don't think it is a matter of them beaching themselves but rather they don't have a choice. But that is just my guess, it could be totally wrong  Chemistry4me, Sat, 17th Jan 2009

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