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Dear Dr Leis,

Recently whilst scuba diving a fellow diver grasped a Threebar Porcupinefish and it responded by inflating like a balloon as I'd heard they would. Another diver commented that these fish are only able to do this a limited number of times before death will occur.

Personally I'm a hands-off diver but I'm now curious to know whether there is any truth in this. Are you able to confirm/deny? The only reference I've found on an internet search is that if they inflate whilst at the surface then they may be unable to subsequently expel the water.


Threebar Porcupinefish
A Threebar Porcupinefish at a depth of 10m, Halifax Park, New South Wales, 28 March 2004. Image: David and Leanne Atkinson
© David and Leanne Atkinson

Dear Simon,

The inflation of Porcupinefishes is a defence measure, and as such there is no limit to the number of times an individual can inflate (and deflate). If the fish inflates at the surface, it is likely to ingest air. Air can be difficult to expel, and can lead to death of the fish as the fish floats and cannot leave the surface. However, if the fish inflates under water, it ingests only water and has no problem deflating once the danger has passed.

Perhaps your friend was told the story about a limited number of inflations by someone with good conservation motives, in an effort to influence divers to not hassle fish in order to see them inflate.

I hope this answers your question.

Jeff Leis

Further reading

  1. Brainerd, E.L. 1994. Pufferfish Inflation: Functional Morphology of Postcranial Structures in Diodon holocanthus (Tetraodontiformes). Journal of Morphology 220: 243-261.
  2. Helfman, G.S., Collette, B.B. & D.E. Facey. 1997. The Diversity of Fishes. Blackwell Science. Pp. 528.
  3. Summers, A. & Bensusen, S.J., 2001. A Fish Story. The spiny puffer's means of defense is hard to swallow. Natural History Magazine. American Museum of Natural History. October: 86-87.