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Unlock This CourseSeagrasses are remarkable flowering plants that have adapted to live completely underwater in marine environments. Unlike seaweeds (which are algae), seagrasses are true plants belonging to the Plant Kingdom. They're the only flowering plants that can survive and reproduce whilst fully submerged in saltwater.
Found in shallow coastal waters around the world, seagrasses form underwater meadows that are amongst the most productive ecosystems on Earth. These marine gardens support countless species and play crucial roles in ocean health.
Key Definitions:
Seagrasses belong to the Plant Kingdom (Plantae) and are classified as angiosperms (flowering plants). They evolved from land plants that returned to the sea around 100 million years ago, developing special adaptations for marine life.
Seagrasses have evolved unique structural features that allow them to thrive in underwater environments. Understanding these features helps us appreciate how these plants have adapted to marine life.
Like all plants, seagrasses have roots, stems and leaves, but these are specially modified for underwater life. The basic structure includes underground rhizomes, roots for anchoring and blade-like leaves that extend upwards into the water column.
Anchor the plant in sediment and absorb nutrients. Unlike land plants, seagrass roots can also absorb nutrients directly from seawater.
Underground stems that spread horizontally, allowing seagrasses to form large meadows. They store energy and produce new shoots.
Long, narrow leaves that carry out photosynthesis. Their ribbon-like shape reduces water resistance and prevents damage from currents.
There are approximately 60 species of seagrasses worldwide, grouped into several families. Each species has adapted to specific environmental conditions and water depths.
Different seagrass species vary in size, shape and habitat preferences. Some form dense meadows in shallow water, whilst others grow in deeper areas with less light.
Found in temperate waters, eelgrass has long, ribbon-like blades up to 1 metre long. It's one of the most widespread seagrass species and forms extensive meadows in shallow bays and estuaries.
Common in tropical waters, turtle grass has broader, shorter blades and forms dense meadows. It's an important food source for sea turtles and manatees in the Caribbean and Gulf of Mexico.
Posidonia oceanica, known as Neptune grass, forms some of the oldest living organisms on Earth. Some meadows in the Mediterranean are over 100,000 years old! This seagrass has broad, flat blades and forms dense underwater forests that support incredible biodiversity. However, it's under threat from coastal development, pollution and climate change, making its conservation crucial for Mediterranean marine ecosystems.
Seagrasses can reproduce both sexually through flowering and asexually through vegetative growth. This dual reproduction strategy helps them colonise new areas and maintain existing meadows.
Seagrasses produce flowers underwater, which is unique amongst flowering plants. The flowers are usually small and inconspicuous, adapted for underwater pollination.
Flowers develop on special shoots called inflorescences. Male flowers release pollen into the water current.
Pollen travels through water currents to reach female flowers. This is called hydrophilous pollination.
Seeds develop in fruits and are dispersed by water currents, sometimes travelling hundreds of kilometres.
Seagrasses also spread through vegetative growth. Rhizomes extend horizontally and produce new shoots, allowing a single plant to form large clones covering extensive areas.
Living underwater presents unique challenges that seagrasses have overcome through remarkable adaptations.
Seagrasses have developed special features to survive in saltwater environments where light levels are reduced and water movement is constant.
Special salt glands and cellular mechanisms allow seagrasses to process saltwater and maintain proper water balance within their tissues.
Enhanced chlorophyll content and efficient photosynthetic systems allow seagrasses to capture limited underwater light for photosynthesis.
Seagrass meadows are amongst the most productive ecosystems on Earth, supporting diverse marine life and providing essential ecological services.
Seagrass meadows provide shelter, food and nursery areas for countless marine species. They support complex food webs and serve as critical habitat for many commercially important fish species.
Young fish find protection amongst seagrass blades, with many species depending on seagrass meadows for their early life stages.
Green sea turtles and other marine herbivores graze directly on seagrass, making these meadows essential feeding grounds.
Countless invertebrates live on seagrass blades and in the sediment, forming the base of complex food webs.
Shark Bay in Western Australia contains the world's largest seagrass meadow, covering over 4,000 square kilometres. This UNESCO World Heritage site supports dugongs, dolphins, sea turtles and over 230 fish species. The seagrass meadows here are dominated by several species including Amphibolis antarctica and Posidonia australis. Research has shown that these meadows store massive amounts of carbon and are crucial for maintaining water quality in the bay.
Beyond supporting marine life, seagrasses provide crucial environmental services that benefit both marine ecosystems and human communities.
Seagrass meadows are incredibly efficient at capturing and storing carbon dioxide from the atmosphere. They store carbon both in their tissues and in the sediments beneath them, making them important in fighting climate change.
Seagrasses can store up to 35 times more carbon per hectare than tropical rainforests, making them crucial for climate change mitigation.
Seagrass meadows act as natural barriers, reducing wave energy and preventing coastal erosion. Their root systems stabilise sediments and their blades slow water movement.
Despite their importance, seagrass meadows worldwide are under threat from human activities and climate change. Understanding these threats is crucial for conservation efforts.
Seagrasses face multiple threats including coastal development, pollution, boat damage and climate change. These pressures have led to significant losses of seagrass meadows globally.
Nutrient pollution from agriculture and sewage causes algal blooms that block sunlight needed for seagrass photosynthesis.
Boat propellers, anchors and coastal construction can destroy seagrass beds that take years or decades to recover.
Rising sea temperatures, ocean acidification and sea level rise all threaten seagrass survival and distribution.