Swell Shark

January 10, 2016
By Damond Benningfield


Swell sharks live in coastal Pacific water. Credit: National Oceanic and Atmospheric Administration

The oceans are full of surprises. Listen to this description, for example, and see if you can guess what type of fish it is. An adult is about three feet long, fairly shy, and not a very good swimmer. It has a wide mouth full of small teeth, but it lies in ambush and waits for food to swim right on in. And when it’s in trouble, it puffs up like a balloon.

Got it? The answer, of course, is a shark — a swell shark. It’s found in coastal Pacific waters from central California to southern Mexico, and perhaps off Chile as well.

The swell shark lives in regions where the bottom is rocky and covered with algae. It hides in caves or crevices during the day. At night, it opens its mouth, and either sucks in nearby prey or patiently waits for it to swim in or be carried in by the currents. Scientists recently discovered that the swell shark glows in the dark. The green glow may help it find mates.

This odd little shark’s most interesting trait, though, is its defense mechanism. When it’s threatened by a predator — another type of shark, or a sea lion or other mammal — it scrams for the nearest rocky crevice. Once there, it reaches around and grabs its tail with its mouth. It then sucks in a lot of water, causing it to swell to twice its normal size. The shark is so tightly wedged into the crack that it’s hard for a predator to pull it out. When the danger has passed, the shark expels the water with a sound like a barking dog — another surprising fact about this “puffy” little shark.