Sirenia is the order of aquatic, herbivorous mammals commonly known as sea cows. There are four species from two families in existence today: the dugong, Dugong dugon, from the family Dugongidae, and the West Indian manatee, Trichechus manatus, the West African manatee, Trichechus senegalensis, and the Amazonian manatee, Trichechus inunguis, from the family Trichechidae. A fifth species, the Steller's sea cow, Hydrodamalis gigas, belonging to the family Dugongidae, became extinct in the late 1700's. The West Indian manatee has two subspecies, the Florida subspecies, Trichechus manatus latirostris, and the Antillean subspecies, Trichechus manatus manatus.

Sea cows are the only marine mammals that subsist exclusively on plants. They inhabit warm coastal areas, swamps, wetlands, rivers and estuaries. While their face resembles that of a walrus, they are actually subungulates and are more closely related to elephants. Unlike walruses and seals, they spend their entire lives in the water. Their forelimbs have evolved into fins and are adapted for steering while swimming and for maneuvering across the muddy coastal floor. They have no hind limbs, though they do have vestigial hind limb or pelvic bones. Their tail is a flat fin, used for propulsion. The tail fins of manatees are rounded, while the tail fins of dugongs are fluked, resembling those of a whale or dolphin.

Sirenia have a hairless appearance,* except for the bristles on their long, fleshy lips. Though they appear fat, they are actually highly muscular, and well-adapted for movement in the water. Their nostrils, located at the top of the snout, have valves allowing them to breathe air at the surface without exposing their bodies. They have no pinnae, or external ear flaps, and their eyes are protected by means of a nictitating membrane, and can be closed with a sphincter-like contraction. They have two teats, like elephants, located under their forelimbs.

Sirenians have a low metabolism and can only survive in warm waters with abundant seagrass and other vegetation. Those living near electric power plants often gather there to take advantage of the warm discharge waters. They are very vulnerable to degradation of their habitat, and manatees in particular often fall victim to collisions with boats in the shallow coastal waters where they make their homes.

* Manatees have hairs sparsely distributed over their bodies, which may help them in detecting water movement and currents. These hairs are not readily apparent except on close examination.

- Sirenia information assembled from on-line sources by Kevin Miler for SeaPics.com. Oct. 17, 2006.

Sources:

http://www.bio.davidson.edu/people/midorcas/animalphysiology/websites/2003/Harshaw/page2.htm

http://animaldiversity.ummz.umich.edu/site/accounts/information/Sirenia.html

http://www.thewildones.org/Animals/manatee.html

http://en.wikipedia.org/wiki/Sirenia