Manatees are large, fully aquatic, herbivorous mammals that inhabit warm, shallow waters, earning them the nickname “sea cows.” The question of whether they live in freshwater or saltwater is complicated because their habitat flexibility is unique among marine mammals. The answer depends on which of the three species is being observed. Their ability to survive in various salinity levels showcases a remarkable biological adaptation.
The Straight Answer: Manatees are Euryhaline
The most commonly known species, the West Indian Manatee (Trichechus manatus), is classified as euryhaline. This means it can tolerate and thrive across a wide range of water salinities, moving freely between the ocean, brackish estuaries, and inland rivers. This flexibility allows them to exploit food sources in coastal marine environments while still accessing the freshwater sources they require.
West Indian Manatees are frequently found in saltwater bays and shallow coastal areas where they graze on seagrass beds. However, they must regularly seek out freshwater to drink. Studies suggest they are susceptible to dehydration if they remain in hyper-saline conditions for extended periods without a source of fresh water. They migrate up rivers, seek out freshwater springs, or gather around drainage pipes to access potable water.
Manatees also use freshwater environments for thermal refuge during the winter months. Because they have a low metabolic rate and minimal insulating fat, they cannot tolerate water temperatures below 20°C (68°F) for long periods, which can lead to “cold stress syndrome.” In the United States, they aggregate in natural warm springs and warm-water discharges from power plants, many of which are located in freshwater or brackish zones. This need for warmth drives their seasonal movements.
Species Differences and Geographic Distribution
The habitat preference of manatees is strongly tied to their species, as the three recognized species exhibit different levels of salinity tolerance. The Amazonian Manatee (Trichechus inunguis) is the only species exclusively restricted to freshwater habitats. This species inhabits the vast river systems and associated drainage areas of the Amazon River Basin, including seasonally flooded forests.
The Amazonian Manatee has evolved specific physiological adaptations for a life spent entirely in a low-salinity environment. Unlike its relatives, this species cannot survive in the ocean or even in brackish estuaries. Its ecology, from its diet of aquatic plants to its internal biology, binds it to the freshwater environment of the Amazon.
In contrast, the West Indian Manatee (Trichechus manatus) and the African Manatee (Trichechus senegalensis) are the euryhaline species. The West Indian Manatee has a broad range spanning from the southeastern United States, through the Caribbean, and along the northern coast of South America. The African Manatee shares this adaptive flexibility, inhabiting coastal areas, estuaries, and slow-moving rivers along the western coast of Africa. Both species regularly venture far upriver, demonstrating their comfort in freshwater, but they also feed and travel extensively in the marine coastal zone.
The Biology of Salt Tolerance
The ability of the euryhaline manatees to move between freshwater and marine environments is facilitated by a sophisticated biological process called osmoregulation. This is the mechanism by which an animal maintains the proper concentration of water and salt in its body fluids, regardless of the external environment. Manatees possess specialized kidneys that are highly efficient at filtering the blood.
When a manatee is in a marine environment, it inevitably ingests some saltwater while grazing on seagrasses, which raises the salt concentration in its body. To counteract this, their kidneys conserve water by producing highly concentrated urine. This function is supported by endocrine adjustments, such as increased levels of the hormone vasopressin, which acts as an antidiuretic to reduce water loss.
Conversely, when the manatee is in a freshwater river, it actively drinks large quantities of water, and its body must work to retain necessary salts. The kidneys produce a much more dilute urine, and hormone levels shift, with an increase in aldosterone to help conserve sodium. This internal physiological machinery allows the West Indian and African manatees to be aquatic nomads.