Cup coral [77k]

This sand-dwelling Cup coral, about one inch (2.5 cm) across, has nearly transparent tentacles and a bright reddish-orange oral disc. This genus is solitary, not colonial, meaning its polyps are not connected to any other corals. This type of coral is termed ahermatypic, which means it does not contribute significantly to the "building" of the reef over successive generations. Both hermatypic and ahermatypic corals use calcium and bicarbonate ions dissolved in water to build a calcium carbonate "skeleton" structure, which forms the framework of a coral reef. In a way that is not entirely understood, zooxanthellae hosted within some corals help enhance the skeleton-building process.  Ahermatypic corals are more likely to be found in caves, caverns, or deep water where light is relatively dim, since they generally do not host zooxanthellae.

Sea anemones (order
Actiniaria) and corals are closely related and similar in structure, but sea anemones do not produce a calcium carbonate (limestone) skeleton. They are similar in appearance, but in this case the "cup" formed on the lower half of the column is formed from calcium carbonate, a characteristic of corals. Also similar are sand-dwelling tube anemones (order Ceriantharia), which commonly use mucus, sand and other reef detritus to form protective tubes.  Tube anemones have tentacles around the mouth as well as around the oral disc margin. Though anemones often live together in clusters, they do not share tissue. Corals form colonies-- the corals are connected at their bases by a lateral sheet of tissue.

A small goby is oriented head-down on the near side of the coral in the center of the photo, apparently taking advantage of the protection offered by the nearby tentacles.

Identification: family Dendrophylliidae, genus Balanophyllia(?) or Paracyathus(?)


related links:
marine biology resource links from "Marine Biology, Third Edition (Castro/Huber)" textbook page
more about "cup corals" from WetWebMedia (aquarist site)

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