Sizes range from about 5 ft. long to over 30 ft., depending on the species. They have some … Of the nearly 40 species of dolphins in the Delphinidae, 6 are commonly called whales, including the … They are social animals at all stages of their life. There are 40 extant species named as dolphins. Dolphins are from 1.5 metres to 4 metres long, but the largest dolphin, the killer whale, can be up to 8 metres long. All dolphins live in groups called "pods". while others can weigh several tons. Most live in salt water oceans, but some live in rivers – there are oceanic dolphins and river dolphins. The term dolphin usually refers to the extant families Delphinidae (the oceanic dolphins), Platanistidae (the Indian river dolphins), Iniidae (the New World river dolphins), and Pontoporiidae (the brackish dolphins), and the extinct Lipotidae (baiji or Chinese river dolphin). Dolphin, any of the toothed whales belonging to the mammal family Delphinidae (oceanic dolphins) as well as the families Platanistidae and Iniidae, the two that contain the river dolphins. Generally, they are among the smaller whales. They have elongated, streamlined bodies with one dorsal fin on their backs, two pectoral fins underneath, and a tail, or caudal, fin. Though each species has its own unique morphology, or shape, most members of this group share a similar form. Dolphins are small-toothed cetaceans easily recognizable by their curved mouths, which give them a permanent “smile.” There are 36 dolphin species, found in every ocean. Dolphin is a common name of aquatic mammals within the infraorder Cetacea. Dolphins are part of the toothed whales. Some species weigh just over 100 lbs.