Dolphins are marine mammals that are closely related to whales and porpoises. There are almost forty species of dolphin in seventeen genera. They are found worldwide, mostly in the shallower seas of the continental shelves, and are carnivores, mostly eating fish and squid. Dolphins are social, living in pods of up to a dozen individuals. They communicate using a variety of clicks, whistles and other vocalizations. They make ultrasonic sounds for echolocation. Dolphins will stay with injured or ill individuals, even helping them to breathe by bringing them to the surface if needed. One common feeding method is herding, where a pod squeezes a fish into a small volume, known as a bait ball. Individual members then take turns plowing through the ball, feeding on the stunned fish. Dolphins occasionally leap above the water surface, sometimes performing acrobatic figures. Dolphins enjoy riding waves and frequently surf coastal swells and the bow waves of boats, at times "leaping" between the dual bow waves of a moving catamaran. Occasionally, they playfully interact with swimmers.