Seagrasses are common on coral reefs. Most inhabit the sandy bottoms of lagoons, and some grow in sand at the base of the outer reef crest. One species in Australia, Thallosodendron ciliatum grows on bare rock surface (eg. on dead coral and exposed limestone). Seagrasses are not generally able to withstand high wave-action and therefore are more common on sheltered parts of reefs. Because they are vigorously grazed by many fish, dugongs and turtles, they may not be very conspicuous even where they are abundant and actively growing.

• To be a plant in the group Angiospermae.
• To have monocotyledons.
• To have underwater pollen and underwater pollination (the only flowering plants to have this).

Strap weed, silky weed, eel grass.

Seagrasses have roots (rhizomes), shoots and leaves. The specialised roots (rhizomes) creep along the bottom, and produce upright shoots. The leaves are generally long and strap shaped, but are paddle-shaped in the genus Halophila. They are green in colour except when covered with other algae and invertebrates that grow on their leaves (epiphytes).

Seagrasses are found in temperate and tropical regions of the world but not in boreal waters. They are attached to rocky or sandy bottoms by rhizomes (see above) and roots. The rhizomes are generally below the surface in soft bottoms (sand or silt). On coral reefs they are generally found on floor of lagoons.

Seagrasses are photosynthetic and autotrophic. Slight increases in the levels of nutrients in the water can increase the rate of growth of seagrasses, but excess nutrients favour the growth of epiphytes which decrease the amount of light available for photosynthesis, and hence decreases the growth rate of the seagrass.

Seagrasses are the staple diet of dugong and turtles, are also grazed by fish. Even if they are grazed to the bottom, seagrass can regrow from buried rhizomes. Seagrasses are very important members at the base of the food chain. Not only do they serve an important role as primary producers, but they support an amazing array of invertebrate life and have been shown to be crucial to the life cycle of many species, in particular fish and prawns. They have another important role in being a source of detritus which forms the basis of yet another food chain.

Seagrass flowers are pollinated underwater by pollen which is carried in water currents. Flowers may be monoecious (male and female elements together) or dioecious (separate male and female flowers). Seeds are produced after flowering and may float considerable distances before they sink and germinate.

Seagrasses are important nursery grounds for many pelagic fish at one stage or another of their juvenile development. As many as 70% of pelagic fish which are important as commercial fish stocks spend at least a part of their juvenile life in seagrass beds. Many other animals breed in seagrass beds, and they are home to crabs, prawns, worms, fish, other algae, sponges and other invertebrates.

Because seagrasses grow in the same sheltered harbours and estuaries favoured by people, they are greatly affected by direct and indirect human activities. Many seagrass beds have disappeared over the last thirty years, and much of the loss has been attributed to dredging, changes in the amount of sediment in the water and water pollution, in particular eutrophication.