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• Seagrasses are flowering plants that grow submerged in shallow marine waters like bays and lagoons. In fact, they are the only flowering plants which grow in marine environments.
• Their flowers are tiny, leaves are strap-like or oval shaped.
• Evolution :

• Belong to the order Alismatales, having 4 families and 60 species.
• Occur all along the coastal areas of India. Seagrass used to be very abundant in the Palk Strait and the Gulf of Mannar, but now it is endangered. Seagrasses abound in the waters around the islands of Kurusadi, Pullivasal in the Gulf of Mannar.
• Habitat: Found in shallow, salty and brackish waters in many parts of the world, from the tropics to the Arctic Circle, except Antarctica.
• Seagrasses inhabit all types of substrata/layers from mud to rock, but lush green seagrass beds are found extensively in muddy and sandy substratas.

• Seagrasses obtain their food through photosynthesis and release oxygen. As a result, they are called the lungs of the ocean.
• They reproduce through both sexual (submarine pollination) and asexual means (branching of at the rhizomes).
• Decline: Seagrasses are facing decline at the rate of 2-5% annually. Almost 30,000 square km of seagrass has been lost globally in the last decade.
• Causes of decline: eutrophication, mechanical destruction of habitat, overfishing, desiccation (extreme dryness), abrasion and erosion of sea beds by glaciers.

Seagrasses are called ecosystem engineers because:

1. They maintain the water quality by trapping fine sediments and suspended particles, prevent turbidity (which affects the behaviour of marine animals and decreases the recreational quality of coastal zones)
2. They filter nutrients before they can reach sensitive habitats like coral reefs.
3. They provide stability to the sea bottom. Without seagrass, sea bottoms are affected by intense wave action from currents and storms. Roots of seagrass prevent soil erosion and help avoid this.
4. Provide habitat and food to sea creatures. Dugongs feed on seagrass; bottle-nosed dolphins feed on the organisms that live in seagrass meadows.
5. Release nutrients after decomposition. During decomposition of seagrass, food is provided to worms and sea cucumbers.
6. They absorb carbon dioxide (83 million tonnes annually). Seagrass occupies 0.1% of the ocean floor, still it stores about 11% of the organic carbon buried in the ocean. They can capture carbon from the atmosphere about 35% faster than tropical rainforests, making them one of the most productive ecosystems in the world.

#Note1: Rhizomes are subterranean (underground) plant stems that send out roots and shoots from their nodes. Therefore, they can easily recover after being cut by grazers like dugongs (sea cows) and disturbed by storms.

#Note2: Though they appear similar, sea grasses and seaweeds are very different. Seagrasses have leaves, roots, and veins, produce small flowers and seeds, which seaweeds lack.