Swell Shark

Named for its ability to inflate itself with seawater, the Swell Shark (Cephaloscyllium ventriosum) is a fascinating kelp forest inhabitant even before it hatches from its egg case.

Exactly how the little Swell Shark can inflate itself with water was first studied shortly after World War II. It was discovered that — unlike pufferfishes (teleosts of the superfamily Tetraodontoidea), which inflate by pumping water into a special, highly distensible sac along the belly — the Swell Shark simply swallows water and holds it in the cardiac (forward) potion of its stomach. Through this simple mechanism, the Swell Shark can rapidly increase its diameter two or three times.

Underwater observation of Swell Sharks in the wild has revealed that inflation is merely the beginning of their defensive tactic. When threatened, a Swell Shark typically bolts for the nearest convenient rocky crevice and enters it head-first. Once inside, the Swell Shark curls into a U-shape and — with its head and tail innermost — inflates its stomach. As a final touch, the Swell Shark grasps its caudal fin between its teeth, thereby presenting to the hostile outside world a fat, inflated ‘inner tube’ that is wedged into its rocky shelter in a way that is extremely difficult for biting predators to evict.

(Source: ReefQuest Centre for Shark Research)