Shark Files: Sluggish Life Drives Nurse Shark's Success

The creature earns the honor of shark with the lowest metabolism.

Being sluggish works for slugs, but it turns out a species of shark makes the low-energy lifestyle work, too. The lowest, in fact.

Researchers from Mote Marine Laboratory have found that nurse sharks register the lowest average metabolic rate among sharks. The scientists say information about shark metabolism matters because of the predators' suspected large role in ecosystem health.

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"If we know about a shark's metabolism - their basic energy needs - then we can start to estimate their energy use in the wild to better understand their impact on the ecosystem," said Mote's Nick Whitney, manager of the site's behavioral ecology and physiology program, in a release.

"Sharks are often the top predators in the food web, consuming a lot of calories from animals on lower levels," he explained. "As such, they often have a larger impact on the balance of the ecosystem than other species. To better understand the ecosystems that we want to preserve, we need to better understand sharks."

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The scientists monitored nurse sharks in a special tank that allowed for the volume of oxygen to be measured. As the sharks breathed, their oxygen consumption level was used to determine their energy needs both at rest and while swimming.

"A slug's life for me!" the nurse sharks' numbers might well have said, once the metabolic numbers were crunched. When compared with a hard-swimming food devourer like the mako shark, for example, the nurses registered just 18 percent of the mako's average metabolic rate.

"Overall, nurse sharks have a very low metabolic rate. They don't move much, and when they do move, it's a lot of work," Whitney said. "With this low metabolism, they probably don't need to consume a lot of calories in order to maintain themselves. So their impact on the ecosystem could be less than you'd expect from other large predators. If they had a higher metabolic rate, like a mako shark, you'd expect their impact to be greater."

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Despite all the talk of a lazy lifestyle, however, nurse sharks may have the last laugh. The reason has to do with their special anatomy. When they're hanging out on the seafloor, taking it easy, nurse sharks are able pump water through their mouths and out over their gills, allowing them to remain stationary instead of having to swim constantly, as other shark species do.

And that low-key approach seems to help keep the species abundant ("nurse sharks are often one of the most prevalent sharks in tropical and sub-tropical ecosystems," the scientists wrote).

"With their low metabolic rate, nurse sharks are pretty lazy," Whitney said, "but the interesting thing is that this can be a very successful strategy. Nurse shark populations are doing very well, compared with many other shark species. Their low-energy strategy is not the only factor, but it is part of their success."

The Mote Marine Laboratory study has appeared online and in April will be published in the Journal of Experimental Marine Biology and Ecology.

Great white sharks are the biggest predatory fish in the world. And despite their mass, they can travel at ridiculous speeds, at over 35 miles per hour, to track their prey. Marine biologist Joe Butler traveled with two friends off Hans Bay, South Africa, in hopes of seeing some great whites. Which they did. See more of Butler's story on a

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on the Seeker Network.

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"In order to bring them in closer, to give everyone a good look, the crew would employ a tuna head on the end of a long rope and drag it out of the way before the shark had a chance to grab it," Butler said.

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This amazing photo, taken from inside the cage, shows the shark grabbing the bait before anyone had a chance to react. "There's actually quite a sobering moment when you realize that proverbially you're the fish out of water, this is their home, and you’re not actually supposed to be there," Butler said.

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"I think a lot people have this image in their head of them being sort of an idealistic predator, but in reality these animals are still quite vulnerable. However, seeing them in their natural environment is something I would recommend to anyone in a heartbeat." Above, Butler (left), prepares to cage dive with his two classmates.

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