Seagrass meadows are expansive underwater habitats that play a fundamental role in marine ecosystems. These unique environments are formed by flowering plants, not algae, possessing true roots, stems, and leaves, and producing flowers and seeds. Found in shallow, salty, and brackish coastal waters across every continent except Antarctica, seagrasses thrive where sunlight can penetrate for photosynthesis. These submerged meadows are highly productive ecosystems, laying the foundation for diverse marine life.
Major Herbivores of Seagrass Meadows
Large marine animals are significant grazers of seagrass, consuming substantial amounts of these underwater plants.
Dugongs, often called “sea cows” due to their diet, are primarily herbivorous mammals. They can eat up to 30 to 40 kilograms (66 to 88 pounds) of seagrass daily. They use sensitive bristles on their upper lip to locate and excavate entire seagrass plants, leaving distinct grazing trails. These giants inhabit warm coastal waters across the Indian and western Pacific Oceans.
Manatees are another prominent group of marine mammals that depend almost exclusively on seagrass and other aquatic plants. An average manatee consumes about 10 to 15 percent of its body weight in vegetation daily, amounting to 100 to 200 pounds of seagrass. They use their front flippers to pull plants towards their mouths and possess prehensile lips with seven distinct muscles to tear the vegetation. Manatees are found in both fresh and saltwater environments, primarily in shallow coastal areas where seagrass is abundant.
Green sea turtles are also dedicated herbivores as adults, with seagrass forming a large part of their diet. These turtles possess finely serrated beaks that enable them to effectively tear seagrass leaves. Their diet is so prominent that it contributes to the greenish tint of their body fat, giving them their common name. Green sea turtles are commonly observed grazing in seagrass meadows.
Smaller Grazers and Detritivores
Beyond the large marine mammals and turtles, a variety of smaller organisms also feed on seagrass, either directly or by consuming decaying plant matter.
Sea urchins, which are spiny marine invertebrates, graze on seagrass. They use specialized mouthparts to scrape plant material. Sea urchins also consume algae and detritus present within the meadows.
Certain fish species, such as parrotfish and surgeonfish, contribute to grazing within seagrass meadows, although their primary diet often consists of algae. Parrotfish may occasionally nibble on seagrass blades, often targeting the small organisms and algae (epiphytes) that grow on the seagrass leaves. While seagrass itself makes up a small portion of their diet, their grazing on epiphytes can influence the health of the seagrass plants.
Crabs and other invertebrates also play a role in the seagrass food web, primarily as detritivores. When seagrass leaves die and decompose, they form detritus, a crucial food source for many bottom-dwelling organisms. Various crustaceans, including some burrowing shrimps, actively collect and consume this decaying plant material. Juvenile blue crabs, for instance, utilize seagrass beds for the availability of small crustaceans and mollusks that thrive within these productive habitats.
The Role of Seagrass in Marine Food Webs
Seagrass meadows serve as a foundational element in marine food webs, converting sunlight into organic matter through photosynthesis and providing energy to a wide array of organisms. This primary production supports both herbivore-based food chains, where animals directly consume living seagrass, and detritivore-based food chains, which rely on decaying seagrass. The energy contained within seagrass is transferred through the ecosystem, starting with the grazers mentioned previously.
The complex structure of seagrass beds also provides shelter and nursery grounds for numerous marine species, including juvenile fish, crustaceans, and other invertebrates. These smaller organisms then become food for larger predators, extending the flow of energy through the food web. The consumption of seagrass is important for the overall health and productivity of coastal marine environments.