Porpoises (also called mereswine) are small cetaceans of the family Phocoenidae; they are related to whales and dolphins. They are distinct from dolphins, although the word 'porpoise' has been used to refer to any small dolphin, especially by sailors and fishermen. The most obvious visible difference between the two groups is that porpoises have shorter beaks and flattened, spade-shaped teeth distinct from the conical teeth of dolphins. The name derives from French pourpois, originally from Medieval Latin porpaloose (porcus pig + piscus fish). Porpoises, divided into six species, live in all oceans, and mostly near the shore. Freshwater populations of the finless porpoise also exist. Probably the best known species is the harbour porpoise, which can be found across the Northern Hemisphere. Like all toothed whales, porpoises are predators, using sounds (echolocation in sonar form) to locate prey and to coordinate with others.