A mammalian
order, Sirenia comprises two living families: the dugongs, or sea cows, family Dugongidae,
with one species, Dugong dugon; and the manatees, family Trichechidae, with three species
in the genus Trichechus. Steller's sea cow, Hydrodamalis stelleri, was a species of dugong
found in the Arctic in 1741 by the explorer Vitus Bering but exterminated by 1768 through
overhunting. Once widely distributed, sirenians are now confined to tropical coastal
waters and estuaries (marine and freshwater).
Sirenians
are fishlike in shape, and the body is almost devoid of hair. The forelimbs are flippers,
and the tail is a horizontally flattened fin; hind limbs and dorsal fins are absent. The
eyes are small, the ears are not external, and the nostrils can be closed by sphincters.
Sirenians breathe by frequent visits to the surface. They graze on aquatic vegetation.
Dugongs grow up to 4m (13 ft) in length and weigh almost 900 kg (2,000 lb). The manatees
reach 4.5m (15 ft) in length and weigh up to 1,600 kg (3,500 lb). They are less adapted to
aquatic life than the dugong, and they have some body hair and have nails at the tips of
their flippers; the nails aid in food gathering. The shape of the tail is also different.
Dugongs and manatees mate in February and March in shallow water, the females lying on
their backs. The gestation period lasts about a year. The young are suckled from two
mammary glands placed high on the torso. This practice may have given rise to the siren
and mermaid myths. Both dugongs and manatees are greatly reduced in number, with some
species nearing extinction.