The order Sirenia contains dugongs, manatees and sea cows. There are two families containing four species in this order. There is a fifth extinct species of sirenian that are extinct. The four living species in the order of sirenia are the Amazonian manatee, African manatee, dugong and the West Indian manatee. In order to escape from danger they can swim at speeds of up to 16 miles per hour although normally they swim a great deal slower.
Also known as sea cows sirenians spend all of their lives in the water. Flippers are formed by the forelimbs and the back limbs have been reduced to a vestigial pelvis. The tail of the sirenia order is flattened and enlarged forming a paddle. The streamlined body is almost hairless and the ears do not have a pinnae and they are missing eyelids. The eyes are closed with the mechanism that acts like a shphincter. The sirenia species can weigh almost 2000 kg. A condition known as pachyostosis makes the bones rather dense and the additional mass is what helps them to stay suspended at the water's surface. Valves close the nostrils that are on the snouts top, stiff bristles cover the mobile, large lips.
The smallest of the sirenia order is the dugong which comes from the Malay word duyong. Duyong translates to mermaid or lady of the sea. The majority of dugong can be found in Australian waters. This species has poor eyesight so they rely on their hearing which is rather good. The male of the dugong species has small tusk that are used to challenge others during mating season. Sea grass is fed on in shallow water by this member of the order.
The majority of Amazonian manatees have a bright pink or white patch on the breast. A male that was almost three meters long is the largest that has ever been recorded. The wet season is when the Amazonian manatees do most of their feeding since this when the new vegetation is available to them. From September to March they will stay in deeper portions of lakes and in the main channels of rivers. The absence of available plants might lead to months of fasting.
West Indian manatee is brown or gray in coloration and can weigh as much as 3,000 pounds. They have a rather slow metabolic rate because of the large quantities they have to eat in order to get the required nutrients. There are two species considered by some taxonomies, the Antillean Manatee and Florida Manatee. The West African manatees are very similar to the West Indian manatee although they are the most threatened species.
The sirenia's skull has a large premaxillae that points down while the nasal openings extend backward. They have a broad dentary, there is a semicircular tympanic bone where the ears would be and there is a large petrosal loosely attached to the basicranium. The group known as subungulates is what sirenians belong to. The fossils of the sirenia go back to Eocene.
The steller's sea cow's belonging to the Hydrodamilis genuse make up the fifth and exinct sirenia order. They were related to the dugongs and believed to be the size of an African elephant. The steller sea cow made its home in the Bering sea and ate a diet that consisted exclusively of seaweed. This species became extinct in the middle of the 1700's soon after being discovered.
The rest of the species in the order of sirenia are in danger from habitat loss, hunting and even being hit by boats. This order is a vegetarian eating varieties of marine algae and other plants. The families are social, and found in large numbers socializing with each other.
The order of sirenia makes its home in coastal water and bays in the tropical regions. The torpedo-shaped body of the sirenia is adapted to live in the waters it calls home.