There are currently some 58 recognized species of seagrasses, in 12 genera. In contrast to marine algae, seagrasses are vascular marine plants with the same basic structure as many terrestrial plants; in addition to having separate roots and leaves, they produce flowers, fruit and seeds. Although small flowers may be found on some species in winter or early spring, most tropical species spread largely through vegetative propagation by the growth and branching of rhizomes. Those species that do flower depend upon the water to carry the pollen from the male to the separate female flowers. The resulting fruit are often carried some distance from the parent plant before the seeds are released. To grow, seagrasses need nutrients and good light. Therefore, they are commonly found in shallow, clean waters, and often associated with mangroves. They thrive in waters where there is shelter from wave action and strong currents, which could create turbulent, muddy water. Although normally found in shallow water they can grow at depths of 32m and have been found in clear water at 68m.
Seagrass beds provide habitat for a wide variety of marine organisms, many of which are commercially important. While birds, dugongs and turtles consume the plants directly, seagrass beds serve important roles as nurseries and essential habitat for juveniles of many other species. The beds have a relatively high rate of primary production; some are capable of binding 1,000g of carbon per square metre per year; or three to five times the near-shore average for phytoplankton. One explanation for this efficiency is the advantage conferred by their dense root system. Anaerobic bacteria in subtidal mud have been shown to bind dissolved nitrogen into nitrates easily available to these plants, driving this detritus-based food web. This dense root system additionally serves to stabilize the sediment, reducing potential sediment erosion in shallow waters.
It is believed that the origins of seagrasses are in terrestrial plants, which returned to the sea. This appears to have occurred around the ancient Tethys Sea, bounded by Africa, Gondwanaland and Asia, around 100 million years ago. Currently, seagrass beds are found on the coasts of North America, Eastern Asia, temperate Australia, and South Africa. However, the recent encroachment of humans in these regions increasingly threatens these habitats, with poorly understood consequences to the biological diversity of seagrass beds or other closely associated habitats.