Red algae are important members of coral reefs. Red algae are unusual among the algae because they can include in their cell walls calcium carbonate which makes the plants hard and resistant to wear. Red algae that grow this way are referred to as "coralline" algae, because they are hard like corals. In addition to the common upright forms of coralline algae some species can grow in a thin mat over rocks and other hard structures, and are called crustose when growing in this form. The upright and crustose forms of red algae bind and infill coral skeletons to form massive sedimentary structures which are strong enough to resist wave-action and erosion. For this reason it has been suggested that coral reefs should be renamed "Coralgal Reefs". Fleshy red algae are also common on coral reefs.

• To be a plant with chlorophyll a.
• To have red pigments (phycobiliprotein) which make these plants look red or pink.
• No flagellated cells.
• Floridean starch - a special kind of energy storage material in the cells.

Dulse, lava bread, nori

Red algae range from microscopic forms to large fleshy algae. They are usually red but some species living high on the intertidal fringe such as Porphyra umbilicalis (lava bread or nori) may appear to be olive-brown. Some encrusting forms are pink or white.

Red algae are found in all the regions of the world. They are attached to bottom or other hard surfaces, 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 the coral reef. Crustose coralline algae are particularly conspicuous as an algal ridge at the reef front in many Pacific Ocean reefs.

All red algae are photosynthetic and autotrophic - they get all their energy needs from photosynthesis.

Red algae are cropped and grazed by herbivorous animals such as fish, crustaceans, worms, gastropods. They form a very important layer at the base of the food chain. The EAC (see Green Algae) may generate up to 60% of all the food generated on a coral reef.

In general, red algae are slow growing compared to green algae. They reproduce both asexually and sexually. Red algae have some of the most complicated sexual cycles of any organisms, and some stages in their reproductive cycle are completely unknown.

Being members of the benthic community, they share their space with a variety of other plants and invertebrates. The branched structures of fleshy algae provide an ideal living space for many small invertebrates, particularly small, clinging crustaceans such as amphipods. Some of the invertebrates living in red algae take on a red colouration, providing effective camouflage.

Some red algae are eaten by Japanese and other peoples including the Welsh (who eat "lava bread"). Many red algae are important in the production of agars which find a ready use in the food and manufacturing industries. Red algae, like brown algae tend to suffer when conditions become eutrophic and can be replaced by green and blue-green algae.