The marine algas Udotea (top) and Halimeda can both be occasionally found on the reef. Udotea is only lightly calicified, but the calcium carbonate disks of Halimeda are in some areas a major component of the reef substrate.

      On the reef, however, despite its teeming life, plants seem absent. In fact, plants are the primary producers on the reef, just like every other environment. Most of the algae found on the reef grows as a short "turf," a fine carpet of hairs that is a mix of dozens or hundreds of species of brown, red and green algaes. While diving, look closely at an area of bare coral rock and you will probably see a fine carpet of "hairs" growing on it.

      The algal turf grows at a prodigious rate, but a herd of grazers—tangs, parrotfish, damselfish, sea urchins, snails and many others—keeps it clipped short. If an area of reef were caged off to prevent the entry of herbivores, the turf would quickly sprout into a thicket. The farmerfish damsel (Stegastes lividus) does just this, by force of personality keeping out all intruders from his own luxurious green patch of hair algae.

      Some reef algaes, the so-called coralline algaes, are calcified, providing them with protection both from grazers and physical damage by surge. These appear as small pink "trees," or flat, encrusting pink or lavender growths on old chunks of coral. Some of the coralline algaes grow in areas of very high wave action, indeed preferring areas that are too turbulent for even corals to survive.

      On reefs facing the open ocean, it is a ridge of coralline red algae that receives the full force of the crashing ocean waves, dissipates their energy, and allows less robust organisms including corals to thrive. Other varieties of coralline algae grow deep on the reef, below the level at which reef-building corals can survive, where they contribute significantly to reef growth and sand production.

      One recognizable green macro-alga that can sometimes be seen on shallower reefs is Halimeda, a heavily calcified alga made up of chains of green disks, each the size of a small button. These disks are calcium carbonate, like coral, and in some areas Halimeda rubble is a major component of the reef substrate.

      Sometimes an inshore reef will merge with shallow beds of turtle grass, one of the very few true marine plants. These grassy beds provide an environment for seahorses, pipefish, damselfish, wrasses, and the young of some reef fishes, including butterfly-fish, as well as small crustaceans, mollusks and worms. The sea grass also provides forage for the rare dugong (Dugong dugon), or sea cow, which ranges across Indonesian waters.

      Plankton

      The diver will rarely see plankton, and if he or she does, it will usually be apparent as a cloudiness of the water, or an irritating backscatter in photographs. But plankton is an important link in the reef food chain. Reef areas rich in plankton will be characterized by an abundance of filter-feeders, animals that have evolved methods of sifting or snaring plankton from the current—including soft corals, mussels and oysters, anemones, crinoids, gorgonians and sponges.

      Plankton consists of both "plants"—phytoplankton—and "animals"—zooplankton, and the larger zooplankters are predatory on the diatoms and algae of the phytoplankton. The plankton also contains some temporary members, the meroplankton, which consists of the larval stages of fish and invertebrates. As these grow, they settle out of the plankton stream to become part of the swimming nekton (fish, jellyfish) or the crawling or fixed benthos, or bottom dwellers (sea urchins, gorgonians).