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 * Not to be confused with order Sirenidae (aquatic salamanders)
 * For the Gothic metal band, see Sirenia (band)

Sirenia is an order of fully aquatic, herbivorous mammals that inhabit rivers, estuaries, coastal marine waters, swamps, and marine wetlands. The order evolved during the Eocene epoch, more than 50 million years ago. Elephants are thought to be the closest living relatives of the sirenians.

The term “sea cow” may be used to refer generically to members of the order Sirenia.

There are four surviving species of sirenians: West Indian Manatee (Trichechus manatus), Amazonian Manatee (Trichechus inunguis), and West African Manatee (Trichechus senegalensis) in the family Trichechidae, and Dugong (Dugong dugon) in the family Dugongidae.

Characteristics
Sirenians grow between 2.5-4 meters long and can weigh up to 1500 kg. Steller's sea cow, extinct since 1786, could reach lengths of 8 meters.

Manatees and the Dugong are the only marine mammals classified as herbivores. Unlike the other marine mammals (dolphins, whales, seals, sea lions, sea otters, and walruses), sirenians eat primarily seagrasses and other aquatic vegetation and have an extremely low metabolism and poor tolerance for especially cold water. Sirenians have been observed eating dead animals (sea gulls), but their diet is made up primarily of vegetation. Like dolphins and whales, manatees and the Dugong are totally aquatic mammals that never leave the water — not even to give birth. These animals have been observed eating grass clippings from homes adjacent to water ways, but in this rare occurrence, only the top portion of the sirenia is lifted out of the water. The combination of these factors means that sirenians are restricted to warm shallow coastal waters, estuaries, and rivers, with healthy ecosystems that support large amounts of seagrass and/or other vegetation.

Trichechidae differ from Dugongidae in the shape of the skull and the shape of the tail.

Adaptations for aquatic life
Sirenians, including manatees and the dugong, have major aquatic adaptations: forelimbs have modified into arms used for steering, the tail has modified into a paddle used for propulsion, and the hind limbs (legs) are but two small remnant bones floating deep in the muscle. They appear fat, but are fusiform, hydrodynamic, and highly muscular. Their skulls are highly modified for taking breaths of air at the water's surface and dentition is greatly reduced. The skeletal bones of both the manatee and dugong are very dense which helps to neutralize the buoyancy of their blubber. The manatee appears to have an almost unlimited ability to produce new teeth as the anterior teeth wear down. They have only two teats, located under their forelimbs, similar to elephants.

The lungs of sirenians are unlobed and flattened. In sirenians, the lungs and thoracic diaphragm extend the entire length of the vertebral column. These adaptations help sirenians control their buoyancy and maintain their horizontal position in the water.

Vulnerability
The three manatee species (family Trichechidae) and the dugong (family Dugongidae) are endangered species. All four living species are vulnerable to extinction from habitat loss and other negative impacts related to human population growth and coastal development. Already the Steller's Sea Cow has been hunted into extinction by humans.

Sirenians and Mythology
The name “Sirenia” comes from the siren myth of ancient Greece. It is unclear whether sirenians inspired the siren myths. However, there is a well-documented history of mariners mistaking sirenians for mermaids or sirens. Christopher Columbus, on his first voyage to the Americas, mistook sea cows for sirens. Columbus wrote, “three sirens…came up very high out of the sea: but they were not as beautiful as they are painted, as in some ways they are formed like a man in the face” (CITE NEEDED, The log of Christopher Columbus's first voyage to the New World. Translated in 1903 by John Boyd Thacher, http://www.columbusnavigation.com/diario.shtml).

Classification
† extinct
 * ORDER SIRENIA
 * Genus †Ishatherium
 * †Ishatherium subathuensis
 * Family †Prorastomidae
 * Genus †Pezosiren
 * †Pezosiren portelli
 * Genus †Prorastomus
 * †Prorastomus sirenoides
 * Family †Protosirenidae
 * Genus †Protosiren
 * †Protosiren minima
 * †Protosiren sattaensis
 * †Protosiren fraasi
 * †Protosiren smithae
 * Family Dugongidae
 * Genus †Sirenotherium
 * †Sirenotherium pirabense
 * Subfamily Dugonginae
 * Genus Dugong
 * Dugong dugon, Dugong
 * Subfamily Hydrodamalinae
 * Genus †Dusisiren
 * †Dusisiren dewana
 * †Dusisiren jordani
 * †Dusisiren takasatensis
 * Genus †Hydrodamalis
 * †Hydrodamalis cuestae
 * †Hydrodamalis gigas, Steller's Sea Cow
 * Family Trichechidae
 * Subfamily †Miosireninae
 * Genus †Anomotherium
 * †Anomotherium langewieschei
 * Genus †Miosiren
 * †Miosiren canhami
 * †Miosiren kocki
 * Subfamily Trichechinae
 * Genus †Potamosiren
 * †Potamosiren magdalenensis
 * Genus Trichechus
 * ''Trichechus manatus, West Indian Manatee
 * ''Trichechus manatus manatus, Antillean Manatee
 * ''Trichechus manatus latirostris, Florida Manatee
 * ''Trichechus senegalensis, African Manatee
 * ''Trichechus inunguis, Amazonian Manatee
 * ''Trichechus bernhardi, Dwarf Manatee
 * Genus †Ribodon
 * †Ribodon limbatus