Of course, none of this means that blimpy sharks can't also be terrifying, toothy, speedy predators. Sharks are still extremely efficient swimmers—it's just that bulky, buoyant species are more energy efficient at slower speeds. The blimp-esque bluntnose sixgill shark is roughly 12-foot-long, fat-headed, and so sluggish it's earned the noble nicknamed of "cow shark." They are, however, distinctly un-blimp-like in their capacity to rip an animal to shreds with their six rows of teeth. Even large sharks typically rely on their speed, agility (and, let's be honest, their toothiness) to catch food.