The dugong (Dugong dugon) is a marine mammal belonging to the order Sirenia, often nicknamed the “sea cow” for its grazing habits. It is the only strictly herbivorous marine mammal, meaning its diet is almost entirely plant-based. Dugongs are highly specialized feeders whose existence is inextricably linked to the health of coastal seagrass meadows across the Indo-West Pacific. Their focus on this submerged vegetation dictates their behavior, anatomy, and distribution, making them seagrass community specialists.
Seagrass: The Staple Diet
Seagrass forms the primary component of the dugong’s diet. These marine mammals consume vast quantities of this underwater flowering plant daily, with a single adult capable of eating up to 40 kilograms of seagrass. They generally prefer fast-growing, pioneer species of seagrass, such as those belonging to the Halophila and Halodule genera. These species are typically lower in fiber and higher in nitrogen, making them easier to digest and more nutritious.
Dugongs often target the energy-rich rhizomes, which are the horizontal stems lying beneath the sediment. These rhizomes contain high concentrations of starch and carbohydrates, providing a more substantial energy source than the leafy blades. This preference for the below-ground parts of the plant often leads them to forage in areas with lower overall seagrass biomass, where the plants are more sparse but the nutritional content is higher.
Supplementary Food Sources
While dugongs are considered specialized herbivores, their diet can include other items when seagrass sources are depleted or unavailable. They are considered facultative omnivores, meaning they opportunistically consume small invertebrates that live within the seagrass beds. These additional food sources are minor components of the diet but can include organisms like sea squirts, mollusks, and crustaceans.
Dugongs may also ingest macroalgae, or seaweed, when seagrass is scarce due to environmental changes or habitat degradation. However, these supplements are generally considered a temporary measure rather than a regular part of their feeding strategy in healthy habitats.
Physical Adaptations for Grazing
The dugong possesses several physical adaptations that facilitate its grazing behavior. Their snout is sharply downturned, which is an adaptation for feeding along the benthic, or bottom, seagrass communities. The large, muscular upper lip forms a highly mobile muzzle covered in rough, bristly hairs that act like tactile sensors, helping the dugong locate food in turbid water.
This muscular muzzle is used like a prehensile tool to grasp and pull up the rhizomes and entire seagrass plants from the sediment. The dugong’s molars are simple and peg-like, but their lower jaw and palate also feature rough, horny plates that aid in grinding the tough, fibrous plant material. As they graze, dugongs often use their forelimbs, or flippers, to “walk” along the seabed, creating distinct feeding trails that resemble miniature plough marks.
Shaping the Ecosystem
The dugong’s method of feeding establishes them as “ecosystem engineers” in their marine environment. By rooting out the rhizomes and creating distinct feeding scars, dugongs alter the composition and structure of the seagrass meadows. This grazing pattern effectively prunes the meadows, preventing the dominance of certain species and promoting the growth of faster-growing, more nutritious seagrass varieties they prefer.
The disturbance and removal of older growth stimulates new growth, which keeps the seagrass community healthy and productive. By maintaining these habitats, dugongs support the many other species that rely on seagrass for shelter and nursery grounds, including fish and invertebrates. Consequently, the presence of a healthy dugong population is often used as an indicator of a well-functioning seagrass ecosystem.