Sea grasses are seed-bearing, flowering, rooted plants, which grow submerged, exclusively in marine coastal waters and coastal wetlands. There are only about 60 species of sea grasses. The actual number species is being reviewed currently. Like grasses in terrestrial habitats, they form meadows on the bed of coastal seas. They are dependent on light penetration for photosynthesis; therefore they generally grow only in clear, shallow waters, in estuaries and coastal seas. They can not survive out of waters, therefore they often grow where is shelters from a sand bar or coral reefs.
Sea grasses should not be confused with seaweeds. Multi-cellular green and brown algae – also found in marine environment- are usually called seaweeds or kelp. Although they can photosynthesis and make their own food, seaweeds lack complex specialized cellular tissues for transport of food and water that are found in sea grasses. They also lack flowers and fruits –structures that are specialized for reproduction- as well as roots.
WHERE ARE SEA GRASSES ARE FOUND IN THE WORLD
Sea grasses are found in clear, shallow coastal water of every continent expect Antarctica. In tropical oceans, they are nearly always found near mangrove and coral reefs. In temperate areas, they are found near salt marshes and kelp forests. They survive in inter-tidal areas in sites sheltered from wave action or where water is trapped at low tide. About half the world’s sea grass species are tropical, while the other half is temperate. The Indo-Pacific region is the center of sea grass diversity in the world, with 24 species and large underwater meadows.
SPECIAL ADAPTATIONS OF SEAGRASSES
Sea grasses are the only flowering plants that have adapted to a completely submerged life in the sea. Because of this, they have some special adaptations.
1.Adaptations to withstand wave energy
Underground plant parts such as rhizomes (an underground stem that store food and grows horizontally) and roots are extensive and close to each other, providing anchorage in many places to these plants. In addition, leaves of sea grasses are usually flat, ribbon-shaped and flexible; the stems are also flexible (as seen in Enhalus, Thalasia, and Cymodocea spp.). Because of this, the force of waves does not break the leaves. Instead, the leaves bend with the water.
2.Adaptations to growing when completely submerged
Sub-marine pollination.
All except one group of sea grasses are pollinated in sub-marine conditions. Male and female parts of flowers extend well above the petals and so allow foe easy movement of pollen in the waters. Pollen grains are oval-shaped or joined together to become elongated and so move easily in the waters (as seen in Halophila spp.). Further, as flowers are either male or female, cross pollination is favoured. Some species have single sex plants, increasing this cross pollination. Sea grasses also reproduce asexually. They produce new shoots from underground roots and rhizomes as they spread across the sea floor. This produces new units. Remarkably, sea grasses can grow asexually for thousands of years, extending over many hectares.
Air-filled tissues.
Sea grasses have extensive air0filled tissues called lacunae, which contain oxygen that is needed for the life activities of these submerged plants. These lacunae also help in flotation of the leaves. The leaves have very thin cuticle or outer layer. Gas exchange with the environment is facilitated by this thin cuticle and by the air-filled tissues.
Salt tolerance
Compared to land plants, sea grasses can tolerate high salt concentrations found in oceans –up to 36 parts per thousand.
Growth
Sea grasses grow underwater rooted to the sea floor. The underground rhizomes and roots are in black sediments with no oxygen and therefore, need for their survival, sugars and oxygen produced during photosynthesis in the above-ground plants. Thus, sea grasses need more sunlight then alga, which do not have underground parts. This limits the depth to which sea grasses can grow. The need more than 10% of the light falling at the water surface compared to algae, which only 1%
WHAT IS THE IMPORTANCE OF SEAGRASSES MEADOWS
Sea grasses meadows form ecologically and economically important habitats because they provide many ecosystem services.
1.Provisioning services
Many edible fish are found living in sea grasses beds. Sea grasses meadows, like mangroves, are nursery areas for many finfish and shellfish. Because of this, they are critical in coastal fisheries.
2.Regulating services
Sea grass meadows act as filters for coastal waters. The leafy canopy of sea grasses slow down water currents, trapping particles, nutrients organic matter and pollutants washed from inland waters to coastal seas. Because of this, sea grasses act as a filter of coastal waters, clearing and cleaning waters.
Sea grasses meadows stabilize the floor of coastal seas. The underground stems of sea grasses prevent the sediments trapped by leaves from being resuspended, thereby stabilizing the sea beds and preventing sand from being washed away and churned up by wave action.
3.Supporting services
Although they are relatively few species of sea grasses, they can house hundreds of other species –microorganisms, algae, invertebrates and vertebrates. Because of their three-dimensional structure in the water, sea grass meadows provide protection for juvenile fish and many marine larvae. It is estimated that a single acre of sea grass may support as many as 40.000 fishes and 50 million small invertebrates. Sea grasses meadows are the feeding sites of juvenile Horseshoes crabs –which belong to an ancient evolutionary lineage. Globally threatened vertebrate species such as marine turtles and marine mammals (e.g. Dugongs) are also found in these ecosystems in the tropics.
Sea grasses are primary producers. A single acre of sea-grass is estimated to produce over nine tones of leaves per year, providing a vast amount of food for many fauna.
Sea-grasses enrich nutrients in coastal waters. Sea-grassses take up nutrients from the soils and nutrients and carbons from the water column and sediments to use for maintaining their very high productivity. When numerous plants and animals found in this three dimensional habitat –as well as sea grasses- die, the released nutrients enter marine system as carbon and other nutrients.
Sea grasses meadows are carbon sinks. Sea grasses absorb carbon dioxide from the oceans when they photosynthesis. Like forests on land, they fuction, therefore, as carbon sinks, removing carbon dioxide from the seas.
4.Culture services
Recreational services. Beaches are popular for recreation and many people enjoy walking on the beach and paddling on beach fronts. Sea grasses meadows, lika coral reefs, attract divers and snorkellers.
Traditional services. In many countries, traditional fishing practices are supported by sea grasses meadows.
5.other services
biological sentinels. Sea grasses have been called ‘biological sentinels’ or ‘coastal canaries’. Like canaries that were taken into coal mines to test the quality of the air, sea grasses respond to changes in the quality of water, indicating deterioration of the environment by degrading and declining before dying. These changes are visible very quickly so that it is possible to take management actions.
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