What is seagrass?
- Seagrasses are marine flowering plants, found on all continents except Antarctica.
- They have roots, stems and leaves and produce flowers and fruits.
- They are closely related to land plants, and probably evolved from land-living angiosperms (flowering plants) millions of years ago.
- The closest relatives to seagrass, on land, are the monocots – grasses, lilies and palms.
Where are seagrasses found?
- Seagrasses are found along the coast, in clear, shallow waters that allow light for photosynthesis, to penetrate. Some species occupy the intertidal zone, the area between the highest tide line and lowest tide line, from which the sea retreats at low tide to expose the seabed.
- These species, like Halodule wrightiiand Cymodocea rotundata, grow close to the mangrove belt and can survive exposure to heat and dryness thanks to the high humidity. Seagrasses from the genus Thalassia are found on stable substrates in the intertidal.
- Other species of seagrasses, like some Halophila and Halodule species and Enhalus acoroides(tape grass), are always found submerged underwater in the sub-tidal zone. Zostera or eelgrass occurs in estuarine areas, mostly in European waters.
- When conditions are suitable, seagrasses form dense underwater ‘meadows’ – some of which are large enough to be seen from space.
- Seagrasses lack stomata. Instead, they have a thin cuticle layer which allows gases and nutrients to diffuse directly into the leaves from the surrounding water.
How many seagrass species are known?
- There are 72 different species of seagrasses in the world, belonging to four families. Compared to land plants, which have much higher rates of species diversity, the number is not high.
- The tropical waters of the Indo-Pacific hold the highest diversity of seagrasses in the world, supporting 14 different species. This is probably because seagrasses “evolved first in this part of the world. “Seagrasses in this region have had a longer time to diversify — and because of the excellent growing conditions.
- India, being in the Indo-Pacific region, has high seagrass diversity: 14 species belonging to 7 genera. The Gulf of Mannar and Palk Strait house all the 14 species found in India, while the Lakshadweep and Andaman and Nicobar Islands have 8 and 9 species respectively.