If threatened, the swell shark bends its body into a sharp U-shape, grasps its caudal fin in its mouth and swallows a large quantity of sea water, which makes it swell to twice its normal size. This behavior makes it difficult for a predator to bite or evict a swell shark from its rocky crevice.
Brown blotches and white spots decorate a swell shark's yellow-brown body. By day, this small, harmless and well-camouflaged shark hides in rocky crevices. By night, a swell shark feeds. It actively sucks in some fishes; it captures others by resting open-mouthed and letting prey wander in or be carried in by currents.
People don't catch swell sharks for food, but the sharks are caught accidentally as bycatch in commercial lobster and crab traps, gillnets and trawl nets. Because sharks take five to 20 years to mature and have few young, accidental catches like these threaten shark populations around the world.
Sharks are cartilaginous fish; their skeletons have no bones. Cartilage is less dense and more elastic than bone, but this is not disadvantageous. Sharks need less energy to keep from sinking and they have increased maneuverability.
Swell sharks lay rubbery egg cases with wiry tendrils at the corners. The tendrils catch on rocks and seaweed, anchoring the egg cases and preventing them from being washed to shore. Depending on water temperature, the eggs hatch in nine to 12 months. The newborn have two rows of enlarged denticles down their back that catch on the egg case and aid the shark in pushing itself into its new life in the sea. Ancient legends named the empty egg cases that wash to shore "mermaids' purses."