Why do pufferfish puff?

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 A perfect place for your Fighter to rest his little fins — the Betta Bed Leaf Hammock.
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Review: Betta Bed Leaf Hammock
21 November 2017
 Just look at that little face... No wonder then, that so many fishkeepers find these little puffers so hard to resist.
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Join the puffer fish fan club!
28 September 2017
 Special care needs to be taken when catching Pictus catfish and other species with spines.
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Travels with your fish
03 August 2017

The simple answer - defence - as this superb National Geographic video shows.

It starts by giving a great insight into the natural habitat of Tetraodon mbu, the world's largest freshwater pufferfish.

Juveniles are shown inhabiting a shallow river, which drains into Lake Tanganyika. Their diet is crabs but they must also make sure that they don't end up on the menu of a rather ravenous otter.

Check out the video to see what happens...

Brilliant!