Coral colonies vary greatly in shape and form, depending upon both the growth pattern and the arrangement of polyps within the colony. Colonies enlarge by asexual reproduction (budding off new polyps) or by sexual reproduction. A larval polyp, called a planula, settles and begins to secrete its calcium carbonate skeleton. All individual polyps become connected by a sheet of body wall, which includes an extension of the gastrovascular cavity and layers of the gastrodermis and epidermis. The resulting colony comprises many interconnected polyps that reproduce asexually as growth continues. Individual colonies derived from a single planula may comprise thousands of polyps and are often large and conspicuous.
Nutrition
Corals feed primarily on animal matter extending from protective skeletons, usually at
night, to feed on plankton and other microorganisms. They capture food with nematocysts,
which are specialized poison cells released from cells on their tentacles. Cilia around
the mouth can aid in catching suspended food as well. Many reef-building corals are found
in association with small, round alga cells called zooxanthellae. Many tropical corals
obtain energy from algae that live within their tissue and give the endodermal cells a
brown or yellow color. These algae photosynthesize and transfer energy-containing
compounds to their coral host, and also remove excretory waste. The relationship of such
zooxanthellae with reef corals is thought to be the limiting factor in the location of
coral reefs: photosynthesis in zooxanthellae requires light levels found only in shallow
ocean waters.