Tube coral is a type of asymbiotic stony coral. The term "tube coral" includes several species in the genus Tubastraea, which differs greatly from many other stony coral genera. Found around the coast of Fiji, the Solomon islands, and Indonesia, this coral is also popular in home reef aquariums.
Also commonly called sun coral, sunflower coral, or cup coral, tube coral differs from most coral in many significant ways. Unlike most coral, whose small polyps secrete stony skeletons that are joined together, tube coral has large polyps which overshadow the secreted skeletons. Also, instead of being found on the sea floor, tube coral can most often be found in underwater caves, suspended from the ceiling.
Most coral live in symbiosis with microalgae called zooxanthellae, which provide the coral with nutrients via photosynthesis. Tube coral lacks zooxanthellae and so has to provide its own sustenance. Because of this independence, tube coral is considered asymbiotic. Since it does not need to rely on photosynthesis for its food, it has no light requirements and is often found where there is little light.
Tube corals capture their prey. Their primary food source is zooplankton, tiny organisms which are carried along by the ocean's current. For this reason, tube corals always live in areas that have a strong current.
Like all coral polyps, tube coral consists of a tube-like structure attached to a surface on one end and topped with a mouth surrounded by tentacles at the other end. When the coral is not feeding, the mouth is closed and the tentacles are drawn in. The tentacles extend and the coral resembles an anemone when feeding. Tube corals are usually shades of orange, but one species Tubastraea micranthus is black.
Popular in home aquariums, tube coral can usually be purchased in lengths of 2–5 inches (5-13 cm). This coral can be kept in well lit or dark places, since it neither needs light nor is harmed by it. Tube coral is considered moderately challenging for the home aquarium owner, however, since its feeding requirements are unusual. Additionally, this coral normally feeds at night, though it can be trained to feed during daylight hours, so its striking tentacles can only be seen after dark.
Captive coral must be placed in an aquarium set up to provide at least moderate currents. The coral is then fed from an eyedropper each evening. Brine shrimp or plankton are common foods for these corals. Black tube coral is rare in captivity because of the strong current and increased food availability it requires to feed properly.