A crescent-shaped tail fluke, with a concave trailing margin, is characteristic of the dugong family. Dugongs have seven cervical vertebrae. Manatees, belonging to the family Trichechidae, are easily distinguished from the family Dugongidae by their flat oval tail fluke. They have only six cervical vertebrae.
Steller’s Sea Cow, Hydromalis stelleri, now extinct, was large for a recent sirenian, growing to a length of 9 m. This species has been included in the Dugongidae. The bodies of Steller’s Sea Cows were covered with a thick skin said to resemble the bark of trees and inhabited by large numbers of small crustaceans. Whilst these sea cows wallowed in the shallows, gulls consumed the parasites. They were discovered at Bering and Copper islands in the Bering Sea by a German -- Georg Wilhelm Steller, a ship’s doctor. Steller, who named the species in 1774, studied them when his vessel was shipwrecked in 1741. It is estimated that there were at most only 2000 animals in the group when the species was discovered. The animals were fearless, and this behaviour proved to be their downfall. Within 30 years, they were exterminated by whalers and sealers who preferred their meat to fish and the meat of whales and seals. Steller’s Sea Cows were unique among sirenians in that they lived in cold waters and fed on algae rather than on more complex water plants. There have been no confirmed sightings of Steller’s Sea Cows since the late 18th century.
The Dugong Dugong dugon, the sole living member of the family, seems to have had a wider distribution in the past. In recent times it has been restricted to the warm coastal waters of the Indo-Pacific region. Throughout most of its range it is now much depleted in numbers. It exists from the Solomon Islands in the east to the head of the Red Sea in the west and from the Philippine Islands and the Persian Gulf in the north to Brisbane, Perth, and Mozambique in the south. It is entirely marine and rarely even enters estuaries. Manatees, however, seem more adaptable and inhabit the coastal, estuarine, and riverine waters of the tropical and subtropical Atlantic region: one species is from West Africa, another is from the Caribbean, and the third is found in the Amazon Basin.