Shark Myths—Sharks Eat Anything, Eat Continuously and Must Swim to Breath

Blue Shark

Blue Shark

Sharks will eat anything.
Wrong. Sharks are pretty discriminate eaters. Even if we didn’t know this from examining the stomach contents of dissected sharks, it’s obvious from the wide array of adaptations found in different shark species. As I said in my shark diversity posts, each shark has evolved to fit a specific diet. Unless they’re injured or starving, there’s little motivation for a shark to waste energy pursuing something that isn’t their normal prey. Although tiger sharks have been nicknamed the “garbage cans of the sea” because a few odd things (license plates, shoes, glass bottles, ect…) have been found in their stomachs, these finds are actually quite rare.

Sharks eat continuously.
Not true. Really, sharks don’t eat much. Depending on the shark, they only need to consume between 0.2 and 3% of their body weight each day to survive. (Compare this to dolphins and some bony fish who, respectively, eat 4 to 12% and 30% of their body weight per day.) Many sharks don’t even feed on a daily basis, and may go days, weeks or even months without eating. They’re able forgo food for so long because they have a low metabolism, a slow digestive rate and don’t expend much energy while not actively hunting. Plus, their stomachs can hold 10% of their body weight, so one bigger meal can last them a long time. If food is unavailable for long enough, they can subsist on the fat reserves in their large livers for a while.

All sharks must swim to breath.
Nope. Not all of them anyway. Pelagic sharks do use ram ventilation to breath; they must swim perpetually to keep water flowing through their mouths and over their gills. But most other sharks are able to lie still on the sea floor and use their mouths to pump water over their gills. Some even have spiracles, holes behind their eyes that allow them to pump water to their gill chambers while their mouths are closed.

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