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.