Seagrasses are the only embryophytes (commonly referred to as "land 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.
Overview 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:
It lives in an estuarine or in the marine environment, and nowhere else. The pollination takes place underwater with specialized pollen. The seeds which are dispersed by both biotic and abiotic agents are produced underwater. The seagrass species have specialized leaves with a reduced cuticle, an epidermis which lacks stomata and is the main photosynthetic tissue. The rhizome or underground stem is important in anchoring. The roots can live in an anoxic environment and depend on oxygen transport from the leaves and rhizomes but are also important in the nutrient transfer processes. 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.
Evolution
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. Terrestrial plants evolved perhaps as early as 450 million years ago from a group of green algae. Seagrasses then evolved from terrestrial plants which migrated back into the ocean. Between about 70 million and 100 million years ago, three independent seagrass lineages (Hydrocharitaceae, Cymodoceaceae complex, and Zosteraceae) evolved from a single lineage of the monocotyledonous flowering plants. Other plants that colonised the sea, such as salt marsh plants, mangroves, and marine algae, have more diverse evolutionary lineages. In spite of their low species diversity, seagrasses have succeeded in colonising the continental shelves of all continents except Antarctica. Recent sequencing of the genomes of Zostera marina and Zostera muelleri has given a better understanding of angiosperm adaptation to the sea. During the evolutionary step back to the ocean, different genes have been lost (e.g., stomatal genes) or have been reduced (e.g., genes involved in the synthesis of terpenoids) and others have been regained, such as in genes involved in sulfation. Genome information has shown further that adaptation to the marine habitat was accomplished by radical changes in cell wall composition. However the cell walls of seagrasses are not well understood. In addition to the ancestral traits of land plants one would expect habitat-driven adaptation process to the new environment characterized by multiple abiotic (high amounts of salt) and biotic (different seagrass grazers and bacterial colonization) stressors. The cell walls of seagrasses seem intricate combinations of features known from both angiosperm land plants and marine macroalgae with new structural elements.
Taxonomy Today, seagrasses are a polyphyletic group of marine angiosperms with around 60 species in five families (Zosteraceae, Hydrocharitaceae, Posidoniaceae, Cymodoceaceae, and Ruppiaceae), which belong to the order Alismatales according to the Angiosperm Phylogeny Group IV System. The genus Ruppia, which occurs in brackish water, is not regarded as a "real" seagrass by all authors and has been shifted to the Cymodoceaceae by some authors. The APG IV system and The Plant List Webpage do not share this family assignment.
Cell walls
… excerpt ends here. Continue reading the full article.
