How do pufferfish inflate?

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Pufferfish inflate to prevent predators eating them, but how do they do it and why don't their ribs break when they do so?

Pufferfish inflate by rapidly pumping water into their stomach. Studies of the Long-spine porcupinefish (Diodon holacanthus) show it has evolved a stretchy stomach which has lost its digestive function to allow it to puff up.

The stomach, which has a special lining, is large and folds back on itself within an enlarged cavity in the peritoneum — a membrane within the abdominal cavity.

When the puffer is threatened, the stomach expands into the peritoneal space and the stomach unfolds to fill gaps beneath the head, dorsal, anal fin and caudal peduncle. The fish balloons and the spines that lie on the surface of its skin stick out, making it a highly unattractive meal!

These fish also lack some ribs and have no pelvis, allowing them to become ball-shaped without breaking any bones. Their skin is also adapted for stretching and the dermis layer contains lots of collagen fibres that allow it to expand by 40%. When the fish expands, these become hard and the fish becomes a stiff, tight sphere.

Although puffers have evolved to suck in water, if lifted out they can sometimes suck in air. They sometimes have difficulties expelling this from their stomach, so take extra care when catching them.