Seagrass

Lore & Background

Three independent seagrass lineages (Hydrocharitaceae, Cymodoceaceae complex, and Zosteraceae) evolved from a single lineage of monocotyledonous flowering plants. 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. 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. 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. The cell walls of seagrasses contain combinations of features known from both angiosperm land plants and marine macroalgae together with new structural elements.

Reader's Guide

Seagrasses profoundly influence the physical, chemical, and biological environments of coastal waters. Though they provide invaluable ecosystem services by acting as breeding and nursery grounds 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. 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. Seagrasses form important coastal ecosystems, and their ability to cope with environmental perturbations depends, to some extent, on genetic variability obtained through sexual recruitment.

Did You Know?

• Seagrasses are the only embryophytes (land plants) that grow in marine environments.
• At least one seagrass species, Thalassia testudinum, uses crustaceans and worm larvae as pollinators instead of relying solely on water currents.
• The cell walls of seagrasses contain sulfated polysaccharides, a feature common in macroalgae but unusual for flowering plants.