Castro−Huber: Marine Biology, Fourth EditionIII. Structure and Function of Marine Ecosystems14. Coral Reefs
© The McGraw−Hill Companies, 2003
ReefCorals
Corals are
cnidarians.
Nearly all are inthe class Anthozoa, making them closely related to sea anemones. Unlike many other cnidarians, they lack a
medusa
stageand live only as
polyps.
In reef-buildingcorals, which are sometimes called
her-matypic
corals, the polyps produce cal-cium carbonate skeletons. Billions of these tiny skeletons build a massive reef. There are a few kinds of reef-buildingcorals that are not anthozoans. Fire corals(
Millepora
), for example, are hydrozoans,more closely related to jellyfishes than seaanemones. They do have a medusa stageduring their life cycle, but like other reef-building corals they grow on the reef incolonies of polyps that have limestoneskeletons. They are called “fire corals” be-cause their powerful nematocysts cause aburning sensation if you touch them.Not all corals help build reefs. Nearly all
soft corals
(order Alcyonacea) lack ahard skeleton and, although they are abun-dant on coral reefs (see Fig.14.28), they don’t help build them.
Black corals
(order Antipatharia) and
gorgonians
(orderGorgonacea), including
sea fans
(seeFig.7.9) and
sea whips,
are hard but theirskeletons are made mostly of protein andcontribute little to reef formation. Thesenon-reef-building, or
ahermatypic,
coralsare common in other habitats as well ascoral reef.
Precious corals
live mostly indeep-water habitats rather than coral reefs,though they do have a calcareous skeleton.Precious corals are gorgonians.
•
Reef-building corals are the primary architects of coral reefs.
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The Coral Polyp
You have to look closely to see thelittle polyps that build coral reefs. Coralpolyps are not only small but deceptively simple in appearance. They look muchlike little sea anemones, consisting of anupright cylinder of tissue with a ring of tentacles on top (Figs. 14.1 and 14.2
b
).Like anemones and other cnidarians, they use their nematocyst-armed tentacles tocapture food, especially zooplankton. Thetentacles surround the mouth, the only opening to the sac-like gut.Most reef-building corals are coloniesof many polyps, all connected by a thinsheet of tissue. The colony starts when aplanktonic coral larva, called a
planula,
settles on a hard surface