A Swell Adaptation

Puffer fish have one of the most dramatic defense
mechanisms of any creature in the sea, thanks to
bodies customized for inflating like balloons.
When threatened, puffers puff themselves up by
gulping down water and pumping it into their highly
expandable stomachs, making their bodies difficult
for predators to swallow. One spiny puffer species,
called the balloonfish, swells to about 3 times its
usual volume. Most of that increase is thanks to
the stomach, whose many folds let it expand by
50 to 100 times! At the same time, hundreds of
sharp spines all over its body stand erect. Not an
appetizing prospect for a predator.
There's room for this ballooning because the
puffer's peritoneum - the membrane that lines
the abdominal cavity - also has many folds that
unfurl. Puffer skeletons lack ribs and a pelvis, and
its flexible spine is able to curve over the inflated
stomach. As the body becomes spherical, it engulfs
even the puffer's dorsal and tail fins, depriving
predators of an easy grabbing point.
These modifications seem extreme, but scientists
think the inflating mechanism may have grown out
of less dramatic behaviors seen in puffers' modern
relatives. Ocean sunfish, for example, will cough
to eject inedible particles they ingested. Triggerfish
blow jets of water to disturb prey. Like the inflation
practiced by puffers, these behaviors involve
"pumping" water into the mouth. The muscle
movements in all these behaviors are remarkably
similar - the only difference is that instead of blowing
the water out, puffers force it into their stomachs.
Only a few evolutionary tweaks led from coughing,
to water blowing, to the truly amazing inflation feat
of the modern puffer fish.