Seagrass: Evolution, Reproduction, Significance and Species
As per the researchers, underwater seagrass in coastal areas appears to trap bits of plastic in natural bundles of fibre called "Neptune Balls". They reported in the journal Scientific Reports that without humans' help or no help from humans, the swaying plants anchored to shallow seabeds and may collect about 900 million plastic items in the Mediterranean alone every year.
According to the lead author Anna Sanchez-Vidal, a marine biologist at the University of Barcelona "We show that plastic debris in the seafloor can be trapped in seagrass remains, eventually leaving the marine environment through beaching."
She further added that this accidental cleanup "represents a continuous purge of plastic debris out of the sea."
All along with the coastal areas of India, seagrasses occur. They are abundant in the Palk Strait and Gulf of Mannar in Tamil Nadu.
Now, let us study Seagrass in detail.
Three groups of flowering plants (angiosperms) colonised the oceans, since the time of dinosaurs and they are known as 'seagrass'. Do you know that they are the only flowering plants that can live underwater? They grow within the sediment on the seafloor with erect, elongate leaves, and a buried root-like structure (rhizome).
Seagrass has roots, stems, and leaves and can produce flowers and fruits. Closely, they are related to land plants and probably evolved from land-living angiosperms millions of years ago. Seagrasses closest relatives on land are the monocots that are grasses, lilies, and palms.
Where are these grasses found?
They are found along the coast, in clear, shallow waters through which light falls, penetrates and the process of photosynthesis takes place. Some species of seagrass occupy the intertidal zone which is the area between the highest tide and the lowest tide line from where the sea retreats at low tide to expose the seabed.
Some species including Halodule wrightii and Cymodocea rotundata grow close to the mangrove belt and can survive exposure to heat and dryness. Genus Thalassia of seagrasses is found on the stable substrates in the intertidal.
Seagrasses other species including Halophila and Halodule species and Enhalus acoroides (tape grass) are mostly found submerged underwater in the sub-tidal zone. In European waters, Zostera or eelgrass occurs in estuarine areas.
Under suitable conditions, seagrasses form dense underwater 'meadows' and some of them are so large that can be seen from space.
As they depend on light for photosynthesis so most commonly they are found in shallow depths where light levels are high. Several seagrass species live in depths of 3 to 9 feet, but the deepest growing seagrass has been found at depths of 190 feet.
They grow in salty and brackish waters across the world, typically along gently sloping, protected coastlines. Various coastal regions are dominated by one or few seagrass species, regions in the tropical waters of the Indian and western Pacific oceans. They have the highest seagrass diversity. It is said that in the world Antarctica is the only continent without seagrasses. Hence, seagrasses are found across the world, from the tropics to the Arctic.
Evolution of Seagrass
As per some researchers, about 850 million years ago, terrestrial plants evolved from a group of green algae and seagrasses evolved from terrestrial plants that recolonised the ocean about 70-100 million years ago.
How Seagrass produce food?
As discussed above that like other terrestrial plants, seagrasses also require sunlight for photosynthesis so that they can prepare their own food and release oxygen.
How Seagrasses reproduce?
Through sexual or asexual methods, seagrass can reproduce.
Sexual reproduction: In this reproduction from the flower of the male plant, the pollen is transferred to the ovary of the female flower. This is known as submarine pollination. Various species undergo this process and complete their life cycle underwater.
Asexual Reproduction: In this reproduction, seagrasses branch off at their rhizomes or modified subterranean plant stem that sends out roots and shoots from its nodes. Due to this, they can recover after being cut by grazers or disturbed by storms.
Various species of Seagrasses
There are around 60 species that belong to nearly four families in the order Alismatales. Some examoles of seagrasses are Sea Cow Grass (Cymodocea serrulata), Thready Seagrass (Cymodocea rotundata), Needle Seagrass (Syringodium isoetifolium), Flat-tipped Seagrass (Halodule uninervis), Spoon Seagrass (Halophila ovalis) and Ribbon Grass (Enhalus acoroides), etc.
Significance of seagrasses
- They are considered 'Ecosystem Engineers' because they are known for providing various ecosystem services. They are also called 'the lungs of the sea' because they release oxygen into the water through the photosynthesis process.
- They help to maintain the quality of water by trapping fine sediments and suspended particles in the water column and increases the clarity of the water.
They prevent soil erosion due to the extensive vertical and horizontal root systems of seagrasses.
- They sequester up to 11% of the organic carbon that’s buried within the ocean and that they occupy about 0.1% of the ocean floor and absorb around 83 million tonnes of carbon from the atmosphere annually.
- Provide food as well as habitat for fishes, octopuses, shrimp, blue crabs, oysters, etc. Endangered marine organisms like dugong (Sea Cow), green turtle, etc. graze directly on seagrass leaves.
- They protect juvenile and small adult fish from large predators and also protect worms, crab, starfishes, sea cucumbers, etc. from strong currents.
- For important commercial marine like squids, etc. they provide ideal nursery sites.
Beds of seagrass are facing decline across the world. As per researchers, around 30,000 square kilometres of seagrass has been lost during recent decades. Some sort of natural disturbances also they face like grazing, storms, ice-scouring, and desiccation.
Note: Ice-scouring is abrasion and erosion of seabeds by glaciers and desiccation is extreme dryness.
Human disturbances also pose threat including eutrophication, overfishing, release of nutrients, mechanical destruction of habitat, coastal engineering construction, pollution, etc
Therefore, we can say that if habitats of seagrass are lost then the marine organisms that are dependent on them will also not able to survive properly and may also face extinction. It will also result in the loss of marine ecosystem productivity. At the global level protection and restoration should be attempted as it can play an important role in mitigating climate change. It is necessary to take measures to conserve seagrasses and their habitats.