Seagrass

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, Majidae 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.

Seagrasses are a paraphyletic group of marine angiosperms which evolved in parallel three to four times from land plants back to the sea. The following characteristics can be used to define a seagrass species:

Seagrasses profoundly influence the physical, chemical, and biological environments of coastal waters. Though seagrasses provide invaluable ecosystem services by acting as breeding and nursery ground for a variety of organisms and promote commercial fisheries, many aspects of their physiology are not well investigated. There are 26 species of seagrasses in North American coastal waters. Several studies have indicated that seagrass habitat is declining worldwide. Ten seagrass species are at elevated risk of extinction (14% of all seagrass species) with three species qualifying as endangered. Seagrass loss and degradation of seagrass biodiversity will have serious repercussions for marine biodiversity and the human population that depends upon the resources and ecosystem services that seagrasses provide.

Seagrasses form important coastal ecosystems. The worldwide endangering of these sea meadows, which provide food and habitat for many marine species, prompts the need for protection and understanding of these valuable resources.

Around 140 million years ago, seagrasses evolved from early monocots which succeeded in conquering the marine environment. Monocots are grass and grass-like flowering plants (angiosperms), the seeds of which typically contain only one embryonic leaf or cotyledon.