Although corals get much of theirnutrition from their zooxanthellae, mosteat when they get the chance. They prey voraciously on zooplankton. The billionsof coral polyps on a reef, along with allthe other hungry reef organisms, are very efficient at removing zooplanktonbrought in by currents. Indeed, the reef has been called a “wall of mouths.”Coral polyps catch zooplankton withtheir tentacles or in sheets of mucus thatthey secrete on the colony surface. Tiny,hair-like cilia gather the mucus intothreads and pass them along to themouth. Some corals hardly use their ten-tacles and rely on the mucus method. Afew corals have even lost their tentaclesaltogether.Corals have still other ways of feed-ing themselves. There are a number of long, coiled tubes called
mesenterial fila-ments
attached to the wall of the gut(Fig. 14.1). The mesenterial filaments se-crete digestive enzymes. The polyp canextrude the filaments through the mouthor body wall to digest and absorb foodparticles outside the body. Corals also usethe mesenterial filaments to digest or-ganic matter from the sediments. In ad-dition, corals are among the relatively few animals that can absorb
dissolved or-ganic matter (DOM)
(see “The TrophicPyramid,” p.223).
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Corals nourish themselves in a remarkable number of ways. Zooxanthellae are the most important source of nutrition. Corals can alsocapture zooplankton with tentacles or mucus nets, digest organic material outside the body with mesenterial filaments, or absorb dissolved organic matter (DOM) from the water.
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Other ReefBuilders
Although they are the chief architects,corals cannot build a reef alone. Many other organisms help make a coral reef. The most important of these are algae, which are essential to reef growth. Infact, some marine biologists think thatcoral reefs should be called “algal reefs”or, to be fair to both, “biotic reefs.” Onereason for this is that zooxanthellae, which are algae, are essential to thegrowth of corals. There are other algae,however, that also have key roles inbuilding the reef. Like corals,
corallinered algae
produce a “skeleton” of calciumcarbonate. Encrusting coralline algae(
Porolithon, Lithothamnion
) grow in rock-hard sheets over the surface of the reef. They deposit considerable amounts of calcium carbonate, sometimes more thancorals, and thus contribute to reef growth. Coralline algae are more impor-tant on Pacific reefs than Atlantic ones.Encrusting coralline algae not only help build the reef, they also help keep itfrom washing away. The stony pavementformed by these algae is tough enough to withstand waves that would smash eventhe most rugged corals. The algae form adistinct ridge on the outer edge of many reefs, especially in the Pacific. This
algalridge
absorbs the force of the waves andprevents erosion from destroying the reef.Encrusting algae do yet another jobthat is vital to reef growth. Coral skele-tons and fragments create an open net- work, full of spaces, that traps coarsecarbonate sediments (Fig. 14.8). Sedi-ment, especially fine sediment, damagescorals when it settles directly on them,but the buildup of coarse sediment in thereef framework is an essential part of reef growth. The structure of a reef is formedas much by the accumulation of calcare-ous sediment as by the growth of corals.Encrusting algae grow over the sedimentas it builds up, cementing the sediment inplace. Thus, encrusting coralline algae arethe glue that holds the reef together.Some invertebrates, notably sponges and
bryozoans,
also form encrusting growthsthat help bind the sediments