There are about 50-100 common green algae on coral reefs. One group the from the Siphonales (Halimeda) are calcified. Halimeda species have a series of discs joined at the inner and outer edge. Small crystals of calcium carbonate are deposited all over the discs. Many small grains of sand are formed when the plants die and this may be the major type of sand found on many coral cays. Sand particles are also produced from grinding down coral skeletons, but these are generally much coarser. Another group of the Siphonales are the Caulerpas. These plants grow by means of a rhizome-like shoot which creeps across the bottom, putting up erect green fleshy branches regularly. Caulerpas are harvested and eaten by many ethnic groups around the Pacific. The green algae are common members of the microscopic layer of slime found over all dead limestone surfaces on coral reefs. This is known as the Epilithic Algal Community (EAC) and is the major source of food for most herbivorous fish and many other smaller animals on reefs.

• To have chlorophyll a and b.
• To be photosynthetic.
• To have no specialised supportive cells during sexual reproduction.

Sea weed, turtle weed

Green algae are plant-like organisms with a very great size range. They may have Filamentous or fleshy bodies, which usually look green.

Green algae are found in temperate and tropical waters. They are attached to bottom, and are often most conspicuous in the intertidal region of rocky coasts and in tide pools. In the tropics they are found in all zones of coral reefs, both exposed and cryptic.

All green algae are photosynthetic (ie. autotrophic), they get all their organic carbon (energy) from photosynthesis.

Herbivorous animals such as fish, crustaceans, worms and gastropods all eat green algae. They form a very important level at the base of the food chain. The EAC (above) may generate up to 60% of all the food generated on a coral reef.

Green algae are generally fast growing and sturdy. They reproduce both asexually (by division of cells) and sexually.

Green algae are solitary, but many plants can often be seen in habitats that suit their needs: for example on (or even in) rocks, sand, shells and other sediments, on other plants and animals, and free-living in the water column.