Sirenians: Gentle Seagrass Grazers

Sirenians (Sirenia), also known as sea cows, are a group of mammals that includes dugongs and manatees. There are four species of sirenians alive today, three species of manatees and one species of dugong. A fifth species of sirenian, the Stellar's sea cow, became extinct in the 18^th century due to over-hunting by humans. The Stellar's sea cow was the largest member of the sirenians and was once abundant throughout the North Pacific.

Identifying a Sirenian

Sirenians are large, slow-moving, aquatic mammals that live in shallow marine and freshwater habitats in tropical and subtropical regions. Their preferred habitats include swamps, estuaries, marine wetlands, and coastal waters. Sirenians are well-adapted for an aquatic lifestyle, with an elongated, torpedo-shaped body, two paddle-like front flippers, and a broad, flat tail. In manatees, the tail is spoon-shaped and in the dugong, the tail is V-shaped.

Sirenians have, over the course of their evolution, all but lost their hind limbs. Their hind limbs are vestigial and are tiny bones embedded in their body wall. Their skin is gray-brown. Adult sirenians grow to lengths of between 2.8 and 3.5 meters and weights of between 400 and 1,500 kg.

All sirenians are herbivores. Their diet varies from species to species but includes a variety of aquatic vegetation such as seagrass, algae, mangrove leaves, and palm fruit that falls into the water. Manatees have evolved a unique tooth arrangement due to their diet (which involves the grinding of a lot of coarse vegetation). They only have molars that are replaced continuously. New teeth grown in at the back of the jaw and older teeth move forward until they reach the front of the jaw where they fall out. Dugongs have a slightly different arrangement of teeth in the jaw but like manatees, teeth are continuously replaced throughout their life. Male dugongs develop tusks when they reach maturity.

The first sirenians evolved about 50 million years ago, during the Middle Eocene Epoch. Ancient sirenians are thought to have originated in the New World. As many as 50 species of fossil sirenians have been identified. The closest living relative to sirenians are the elephants.

The primary predators of sirenians are humans. Hunting has played a major role in the decline of many populations (and in the extinction of the Stellar's sea cow). But human activity such as fishing, and habitat destruction can also indirectly threaten sirenian populations. Other predators of sirenians include crocodiles, tiger sharks, killer whales, and jaguars.

Key Characteristics

The key characteristics of sirenians include:

• large aquatic herbivores
• streamlined body, no dorsal fin
• two front flippers and no hind legs
• flat, paddle-shaped tail
• continuous tooth growth and replacement of molars

Classification

Sirenians are classified within the following taxonomic hierarchy:

Animals > Chordates > Vertebrates > Tetrapods > Amniotes > Mammals > Sirenians

Sirenians are divided into the following taxonomic groups:

• Dugongs (Dugongidae) - There is one species of dugong alive today. The dugong (Dugong dugong) inhabits coastal marine waters of the western Pacific and Indian Oceans. The dugong has a V-shaped (fluked) tail and males grow tusks.
• Manatees (Trichechidae) - There are three species of manatees alive today. Members of this group are usually solitary animals (except for mothers with their young). Manatees prefer freshwater aquatic habitats and coastal saltwater marshes. Their distribution includes the Caribbean Sea, the Gulf of Mexico, the Amazon Basin, and parts of West Africa such as the Senegal River, Kwanza River, and Niger River.