his article gives a brief description of the structural similarities and differences between seagrasses and vascular plants found on land.
Although seagrasses live in marine waters, they evolved millions of years ago from land plants and have many of the same morphological features, such as leaves, roots, flowers, seeds, and conducting tissues. Similar to terrestrial vegetation, seagrasses use the process of photosynthesis to manufacture their own food and produce oxygen through structures called chloroplasts. Land plants have chloroplasts in both the stems and leaves, but in seagrasses, chloroplasts are found only in the leaves. Since seagrasses don’t need to overcome the force of gravity, they don’t have the strong supportive stems and trunks found in land plants; instead, they depend on the natural buoyancy of the water to provide support. This allows seagrass blades to remain flexible to bend and move with the force of currents and waves. To protect leaves from this high-energy environment, however, seagrasses have developed tube-like structures, called sheaths, which extend down through the vertical rhizome. This structure, not found in land plants, protects the meristem and any newly formed leaves, which extend up through the sheath of previously formed leaves.
Seagrasses are the only flowering plants which grow in marine environments. There are about 60 species of fully marine seagrasses which belong to four families (Posidoniaceae, Zosteraceae, Hydrocharitaceae and Cymodoceaceae), all in the order Alismatales (in the clade of monocotyledons). Seagrasses evolved from terrestrial plants which recolonised the ocean 70 to 100 million years ago.
The name seagrass stems from the many species with long and narrow leaves, which grow by rhizome extension and often spread across large “meadows” resembling grassland; many species superficially resemble terrestrial grasses of the family Poaceae.
Like all autotrophic plants, seagrasses photosynthesize, in the submerged photic zone, and most occur in shallow and sheltered coastal waters anchored in sand or mud bottoms. Most species undergo submarine pollination and complete their life cycle underwater. While it was previously believed this pollination was carried out without pollinators and purely by sea current drift, this has been shown to be false for at least one species, Thalassia testudinum, which carries out a mixed biotic-abiotic strategy. Crustaceans (such as crabs, Majid zoae, Thalassinidea zoea) and syllid polychaete worm larvae have both been found with pollen grains, the plant producing nutritious mucigenous clumps of pollen to attract and stick to them instead of nectar as terrestrial flowers do.
Seagrasses form dense underwater seagrass meadows which are among the most productive ecosystems in the world. They function as important carbon sinks and provide habitats and food for a diversity of marine life comparable to that of coral reefs.