The order Sirenia encompasses large, plant-eating marine mammals, often affectionately known as “sea cows.” Their existence has long captivated human imagination, inspiring ancient myths of sirens and mermaids due to their graceful movements and occasional surface appearances. These mammals are fully adapted to aquatic life, inhabiting warm regions across the globe.
Living and Extinct Species
The Sirenian order includes four living species, divided into two families: manatees and dugongs. The three manatee species are the West Indian, West African, and Amazonian manatees. Manatees possess a rounded, paddle-shaped tail, while the dugong has a notched, fluke-like tail, similar to a whale’s.
Beyond these living species, the order also includes the extinct Steller’s sea cow. This gigantic sirenian was first described in 1741 after its discovery in the Bering Sea. Intense overhunting led to its complete disappearance by 1768, less than three decades later.
Unique Biological Characteristics
Sirenians have specialized anatomical features for their aquatic existence. Their bodies are fusiform, or torpedo-shaped, which helps reduce drag as they move through water. They lack a dorsal fin, and their forelimbs have evolved into paddle-like flippers, primarily used for steering. These adaptations allow for efficient movement. Sirenians are generally slow-moving, typically cruising at about 8 kilometers per hour, but can reach 24 kilometers per hour in short bursts.
Sirenians exhibit pachyostosis, a condition where their ribs and other long bones are unusually dense and solid, containing minimal bone marrow. This increased bone density acts as a natural ballast, counteracting the buoyancy provided by their blubber and thick skin, helping them remain submerged while feeding. Their muscular, prehensile upper lip is highly effective at grasping and manipulating aquatic plants. They share an evolutionary lineage with elephants, diverging from a common terrestrial ancestor around 50 million years ago.
Global Habitats and Distribution
Sirenians are found predominantly in warm, shallow, and calm waters, preferring tropical and subtropical regions. Their habitats include coastal areas, estuaries, rivers, and marine wetlands. Water temperature is an influential factor; they prefer waters above 18°C and may migrate to warmer refuges when temperatures drop.
Manatees are distributed across the Americas and Africa. West Indian manatees inhabit coastal waters, lagoons, and rivers of the Caribbean, Gulf of Mexico, and southeastern United States, extending to northern South America. The West African manatee occupies inland mangroves and coastal flats along the West African coast, from Senegal to Angola. The Amazonian manatee lives exclusively in the freshwater environments of the Amazon River Basin. In contrast, dugongs are found in coastal waters throughout the Indo-Pacific region, from East Africa to Australia and the Pacific Islands, and are primarily marine.
Conservation and Human Impact
All four living sirenian species are currently listed on the International Union for Conservation of Nature (IUCN) Red List of Threatened Species. The dugong and Amazonian manatee are classified as Vulnerable, while the West Indian and West African manatees are also listed as Vulnerable or Threatened. Their slow movement and low reproductive rates make them susceptible to human-related disturbances. Females typically give birth to a single calf after a gestation period of 12 to 14 months, and calves remain dependent on their mothers for an extended period.
Collisions with watercraft are a major threat, often causing severe injuries or fatalities due to their slow speed and presence in shallow, busy waterways. Habitat degradation also poses a substantial risk, particularly the loss of seagrass beds and other aquatic vegetation caused by coastal development, dredging, and pollution. Entanglement in fishing gear, such as gillnets, presents another serious danger, often leading to drowning. Hunting for meat, oil, and other products continues in some regions, despite legal protections.