Fish are the largest group of vertebrates and live in fresh water or the sea. Most fish are oviparous and lay eggs, though some keep their eggs in their mouths until hatching and sharks are viviparous, giving birth to live young. Fish can be carnivores, herbivores or omnivores and breathe through their gills. They move using fins and a tail and have scales covering their skin for protection. Fish live either alone, in pairs, or in groups like shoals, packs, or schools.