It has evolved no defenses because it has no natural enemies, and eats only aquatic plants. In fact, it is the only completely herbivorous marine mammal in the entire world. Manatees prefer shallow water, between 6 and 7 feet deep, where they can find their favorite foods. The nutritious roots of water plants, especially the water hyacinth, are a manatee favorite. Manatees are, in fact, so effective at eating their way through weed-choked canals, that they have actually been employed in some countries to clear canals for people.
Manatees are what are called hind-gut animals.
Unlike the cow, which does most of the digestion in the stomach, manatees are more like horses, having a long colon for digestion. It takes 148 hours for food to pass through the manatee digestive tract. The manatee needs to drink water in addition to eating. When in fresh water, this is not a problem, and the animal drinks frequently. However, when the manatee is in the ocean or brackish coastal areas, it cannot get fresh water to drink.