Seagrasses
Definition
Seagrasses are flowering plants that grow fully submerged in shallow coastal waters such as bays and lagoons. Unlike seaweeds, they are true plants with tiny flowers and strap-like or oval leaves, adapted to live entirely underwater.
Evolutionary Background
- Seagrasses are believed to have evolved from terrestrial (land) plants that returned to the ocean around 70 to 100 million years ago. While green algae gave rise to early land plants about 850 million years ago, seagrasses are a unique example of plants that recolonised marine environments after adapting to life on land.
Photosynthesis and Food Production
- Just like land plants, seagrasses rely on sunlight to perform photosynthesis, during which they produce their own food and release oxygen into the water. This makes them vital for marine life and water quality.
Reproduction Methods
Seagrasses reproduce both sexually and asexually.
- Sexual reproduction occurs through a process called submarine pollination, where pollen from male flowers is carried by water currents to female flowers, enabling fertilization underwater.
- Asexual reproduction takes place when the plant grows new shoots from its rhizomes (horizontal underground stems). This allows seagrasses to quickly regenerate after damage caused by grazing animals or storms.
Photosynthesis and Food Production
Asexual Reproduction Definition Asexual reproduction is a type of reproduction in which a single parent gives rise to new offspring without the involvement of gametes (sperm or egg). The resulting offspring are genetically and physically identical to the parent, often referred to as clones.
Key Features of Asexual Reproduction
Types of Asexual Reproduction Several forms of asexual reproduction are observed in nature, depending on the organism:
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Species and Diversity
Seagrasses belong to the plant order Alismatales and include around 60 known species spread across four families. Some commonly found species include:
- Sea Cow Grass (Cymodocea serrulata)
- Thready Seagrass (Cymodocea rotundata)
- Needle Seagrass (Syringodium isoetifolium)
- Flat-tipped Seagrass (Halodule uninervis)
Habitat and Distribution
- Seagrasses can grow on various types of sea floors, from mud to rock, but they thrive best on muddy and sandy substrata. In India, seagrass beds are found all along the coast, especially in Tamil Nadu’s Palk Strait and Gulf of Mannar, where they form dense underwater meadows.
Ecological Importance
Seagrasses are called “ecosystem engineers” because of the many benefits they provide to marine ecosystems. They are also known as the “lungs of the sea” due to their role in releasing oxygen during photosynthesis.
Key Ecological Roles:
- Carbon Sequestration: Although they cover just 0.1% of the ocean floor, they account for 11% of the organic carbon buried in the sea. They absorb 83 million tonnes of CO₂ per year and can store carbon 35 times faster than tropical rainforests.
- Water Purification: Seagrasses trap fine sediments and suspended particles, improving water clarity.
- Nutrient Filtration: They absorb excess nutrients (like nitrogen and phosphorus) before these reach sensitive ecosystems such as coral reefs.
- Erosion Control: Their deep root systems help stabilize the sea floor, reducing coastal erosion.
- Food and Habitat: Seagrasses provide food and shelter for a wide range of marine species including fishes, shrimps, octopuses, oysters, and crabs.
- Support for Endangered Species: Animals like the dugong (sea cow) and green turtle feed directly on seagrass, while bottle-nosed dolphins prey on fish and other organisms living in seagrass beds.
- Detritus and Nutrient Recycling: When seagrasses die, their decomposed parts become detritus, which feeds species like sea cucumbers, worms, and crabs. Their decay also releases nutrients that are reabsorbed by phytoplankton and other marine plants.
- Safe Zones: They offer protection to juvenile fishes and invertebrates from predators and strong water currents, and serve as nursery habitats for commercially important species like squids and cuttlefish.
Threats to Seagrass Ecosystems
Despite their importance, seagrass meadows are disappearing rapidly, with global losses estimated at 2–5% per year. Around 30,000 square kilometers of seagrass have already been lost in recent decades.
Causes of Decline:
- Natural disturbances: such as grazing by marine animals, storm damage, ice-scouring, and drying (desiccation).
- Human activities: like coastal construction, overfishing, pollution, eutrophication (excess nutrients), and mechanical damage from boat anchors and fishing gear.
