Fish
Swell Shark
Cephaloscyllium ventriosum · also called swell shark
The swell shark is a small, sluggish catshark of Pacific kelp forests with two party tricks. Threatened, it bends into a U, bites its own tail, and gulps water until it doubles in size, jamming itself into a rock crevice where nothing can pull it out. And its skin glows green under blue light — in a pattern that other swell sharks can see and we cannot.
Overview
Swell sharks live on rocky reefs and in kelp forests of the eastern Pacific, from California to the Gulf of California, with a separate population off Chile. They grow to about 1 m, stout and soft-bodied, mottled yellow-brown with dark saddles, and a broad flat head with cat-like eyes. They hunt fish and crustaceans at night, often by lying still with the mouth open.
Inflating
Grabbed or cornered, a swell shark curls into a U, grips its tail, and swallows seawater into a very stretchy stomach, roughly doubling its girth. Wedged into a crevice, it becomes impossible to extract. One that has puffed up with air out of the water releases it with a bark as it deflates.
Glowing
Its skin absorbs blue ocean light and re-emits it as green, producing a pattern that stands out to other swell sharks — whose eyes are tuned to green — but is invisible to humans and to most predators and prey.
Mermaid’s purse eggs
The female lays amber, purse-shaped egg cases with long curling tendrils that wrap around kelp to anchor them. The pup hatches after 9–12 months.
Status
The swell shark is Least Concern, common, hardy, and harmless to people.
Swell Shark: common questions
How and why does a swell shark puff itself up?
When a swell shark is grabbed or cornered, it curls its body into a U, takes its own tail in its mouth to make a closed loop, and rapidly swallows seawater into a specialised, highly stretchy stomach. This can roughly double the shark's circumference in a few seconds. If the shark has retreated into a crack in the reef, the sudden increase in size wedges it in place so a predator such as a larger shark or a seal cannot drag it out. In the open it simply makes the shark a much more awkward, rounder mouthful. Once the threat passes, the shark releases the water and returns to normal size.
Does the swell shark really glow?
It is biofluorescent, which is not the same as bioluminescent. It does not make its own light. Instead, molecules in its skin absorb the blue light that penetrates deep seawater and re-emit that energy as green light, creating a glowing green-and-dark pattern across the body. Special cameras with blue lights and yellow filters reveal it clearly. Research suggests the pattern is meaningful to the sharks themselves, because their eyes are especially sensitive to green wavelengths, so a swell shark probably sees other swell sharks glowing against a dim blue background even though the effect is invisible to human divers.
Is a swell shark dangerous to people?
Not at all. It is small, slow, and completely uninterested in anything as large as a human. It has small teeth suited to little fish and invertebrates, and its main defensive move is to inflate and hide rather than to bite. Divers in Californian kelp forests encounter resting swell sharks regularly and can approach them closely. The only way to get bitten would be to handle one roughly, and even then the bite would be minor.
What are the egg cases with tendrils found in kelp?
They are swell shark egg cases, sometimes called mermaid's purses. Each is a flattened, amber-coloured capsule a few centimetres long, with long, coiling tendrils sprouting from the corners. After laying, the female's movements and the current wrap these tendrils tightly around kelp stipes, sea fans, or other structure, anchoring the egg where it will not be swept away or easily found. The single embryo inside develops over about nine months to a year, feeding on its yolk, and eventually cuts its way out as a fully formed miniature swell shark.