Sirenia

Sirenia Diversity

Dugong skeleton displayed at Philippine National Museum

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Sirenia, commonly known as sea cows, is an order of fully aquatic, herbivorous mammals that inhabit swamps, rivers, estuaries, marine wetlands, and coastal marine waters. The Sirenia currently include four living species: the West Indian manatee, the Amazonian manatee, the West African manatee, and the Dugong. The order also includes the Steller's sea cow (Hydrodamalis gigas), which was hunted to extinction within 27 years of its discovery in the 18th century.

Description

Sirenians are large, typically slow-moving animals with bodies that taper to a flat, paddle-shaped tail. They have two forelimbs, or flippers, with each species having a distinct number of nails on their flippers. Their skin is thick and wrinkled, often hosting commensal algae, which can give them a greenish appearance. Sirenians have a sparse distribution of hair and a large, mustache-like vibrissae on their upper lip, which is sensitive and aids in their feeding process.

Habitat and Distribution

Sirenians are found in parts of the Atlantic Ocean and the Indo-Pacific. The West Indian manatee inhabits the Caribbean Sea and the southeastern United States, the Amazonian manatee is found in the Amazon Basin, the West African manatee ranges along the west coast of Africa, and the Dugong is distributed in the warm coastal waters from East Africa to Australia, including the Red Sea, Indian Ocean, and Pacific Ocean.

Diet

Sirenians are herbivorous, primarily feeding on a wide variety of aquatic plants. Their diet includes sea grasses in marine environments and freshwater vegetation in rivers and estuaries. They have a low metabolism and can consume up to 8% of their body weight in vegetation daily.

Reproduction

Sirenians have a long gestation period, typically about 12 to 14 months, and usually give birth to a single calf, although twins are rare but not unheard of. The mother and calf share a strong bond, with the calf dependent on its mother for milk and protection for up to two years.

Conservation

All species of Sirenia are listed under the IUCN Red List of Threatened Species, with statuses ranging from Vulnerable to Critically Endangered. Major threats to their survival include habitat destruction, boat collisions, entanglement in fishing gear, and hunting. Conservation efforts are in place globally, including protected areas, boat speed regulations, and rehabilitation programs for injured individuals.

Evolution and Fossil Record

The Sirenia, along with the elephants and hyraxes, are believed to have descended from a common ancestor that lived about 50 million years ago. The fossil record of Sirenia is rich, with several extinct genera and species that provide insight into their evolutionary history. The Steller's sea cow, the largest sirenian to have lived, is a notable example of an extinct species within this order.

See Also

• Marine mammal
• Aquatic adaptation
• Herbivore

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