Manatees eat large volumes of aquatic plants, and seagrass in particular carries abrasive sand and silica that grinds enamel away. Instead of a fixed adult dentition, a manatee produces new molars at the back of each tooth row. Those teeth move forward along the jaw as the ones ahead of them wear down, are shed at the front, and are replaced continuously through life.
This is sometimes described as being the same as an elephant's arrangement. It is not. Elephants replace teeth in a small number of successive sets, each erupting from behind and moving forward, with a fixed lifetime total of six per quadrant. Manatees have no such limit and continue producing new molars indefinitely.
The more pressing problem for manatees is not teeth but food and warmth. Seagrass die-offs in the Indian River Lagoon contributed to an unusual mortality event in Florida beginning in 2020, and dependence on power-station warm-water outfalls creates a hazard whenever those plants are retired.