What Do Dugongs Eat? Seagrass Grazers of the Indo-Pacific

Which seagrasses do dugongs prefer?

Seagrasses are not seaweeds. They are true flowering plants with roots, leaves and underground stems called rhizomes, whose ancestors moved from land back into the sea. About 60 species grow in shallow, sheltered coastal waters worldwide, and the dugong feeds on many of the species found in its range.

It is a picky eater, though. Dugongs prefer small, delicate seagrasses of the genera Halophila and Halodule, which are low in fiber and rich in nitrogen. The rhizomes of these plants also store starch, an easily digested source of energy. Tougher, fibrous seagrasses are eaten less often. Surprisingly, dugongs often feed in sparse meadows of these tender plants rather than in the lushest beds, because quality matters more to them than sheer quantity.

How do dugongs feed?

A dugong’s head is built for grazing on the bottom. Its snout bends sharply downward and ends in a broad, horseshoe-shaped muzzle covered in thick, sensitive bristles. Because its eyesight is poor, it finds food mainly by touch and probably by smell.

To feed, the dugong presses its muscular upper lip into the sand and plows forward, pulling up whole seagrass plants including the roots and rhizomes. It often shakes the plants to remove sand before swallowing them, and it may gather a small pile before eating. Where the plants cannot be uprooted, it crops just the leaves. A grazing dugong sometimes supports itself on its front flippers, almost walking along the seabed.

Each feeding session leaves a winding furrow of bare sand, called a feeding trail, about as wide as the dugong’s muzzle. Seen from the air at low tide, a busy meadow can be crisscrossed with these trails. Dugongs have few teeth; much of the grinding is done by rough, horny pads on the roof of the mouth and lower jaw.

How much does a dugong eat in a day?

Published estimates vary, but an adult dugong is thought to eat roughly 25–40 kg (55–88 lb) of wet seagrass a day. A captive dugong was recorded eating about 25 kg (55 lb) a day, around 8 percent of its body weight. Because seagrass is mostly water, the amount of dry food is much smaller.

Digestion is slow and thorough. Like horses, dugongs ferment plant material in a long hindgut, and a study of two captive dugongs found that food took about six to seven days to pass through the gut, much longer than in most other plant-eating mammals and similar to manatees. This long journey lets them extract as much nutrition as possible from a low-fiber diet.

Dugongs feed both by day and at night, often timing their meals to the tides. Where the tidal range is large, they move onto shallow banks as the water rises and retreat to deeper channels as it falls.

Do dugongs ever eat animals?

Sometimes. Seagrass is the bulk of the diet everywhere, and dugongs eat seaweeds and other algae when seagrass runs short. In the cooler, subtropical waters of Moreton Bay in Queensland and parts of Western Australia, studies have also found sea squirts, worms and shellfish in their stomachs, and some dugongs there seem to seek out these animals deliberately, perhaps for extra protein. In tropical waters, there is little evidence that dugongs eat animals at all.

Gardeners of the seagrass meadows

Dugongs do not simply eat seagrass; they shape it. When they plow up a meadow, the fast-growing, nutritious species they prefer, such as Halophila and Halodule, are often the first to regrow in the bare patches. Slower, tougher species that would otherwise take over are held back.

Scientists call this cultivation grazing. By returning to the same meadows again and again, dugongs keep them in a young, tender state that suits them best. Their digging also stirs up the sediment and recycles nutrients, so a healthy dugong population is part of a healthy seagrass ecosystem.

Seagrass meadows of Australia and the Red Sea

The world’s largest dugong populations live where seagrass is most extensive. Australia has vast meadows in Shark Bay in Western Australia, the Torres Strait, the Gulf of Carpentaria and inshore waters of the Great Barrier Reef, where tens of thousands of dugongs feed. The Arabian Gulf has the second-largest population.

In the Red Sea, dugongs graze the seagrass beds of sheltered bays on both the African and Arabian coasts. Bays near Marsa Alam in Egypt are among the best places in the world to watch a dugong feed, often accompanied by small yellow-and-black golden trevally that are thought to snap up small animals stirred up by its digging.

Dugongs share these meadows with other grazers, including green sea turtles, which also eat seagrass as adults; see What do sea turtles eat?

Why seagrass loss threatens dugongs

Because they depend on a single type of food, dugongs are highly vulnerable when seagrass disappears. In 1992, two floods and a cyclone destroyed about 1,000 km² (390 sq mi) of seagrass in Hervey Bay, Queensland. Many dugongs left the area, and dozens were later found dead, some apparently from starvation. Similar losses followed major floods and cyclones in Queensland in 2011, and a marine heatwave in 2010–2011 killed large areas of seagrass in Shark Bay.

Seagrass meadows are among the most threatened habitats on Earth. Scientists estimate that close to a third of the world’s known seagrass area has been lost since records began in the late 1800s, through dredging, coastal construction, pollution, muddy runoff that blocks sunlight, boat damage and rising sea temperatures. Damaged meadows can take a decade or more to recover.

When food runs short, female dugongs may delay breeding or stop breeding for years, slowing the recovery of an animal that already reproduces very slowly. That is why protecting seagrass is central to dugong conservation, as the manatee vs dugong comparison also shows: the dugong, unlike the manatee, has almost no other food to fall back on.