Sharks are often considered the most efficient predators in the sea. They have extremely developed pectoral fins that allow greater stability and maneuverability when catching fast-moving prey. Most sharks swim continuously. Because they lack swim bladders, sharks sink to the bottom when they stop swimming. Sharks breathe by extracting oxygen from seawater as it passes over their gills; if they stop swimming, the water circulation needed for respiration would cease, resulting in suffocation. Other sharks, like the nurse shark and blacktip reef shark, are capable of pumping water over their gills as they rest on the ocean bottom.