Octopus Coral, Galaxea fascicularis
Octopus Coral, (Galaxea fascicularis), also known as Fluorescence Grass Coral, Galaxy Coral. Star Coral, Crystal Coral, Brittle Coral and Starburst Coral, is a species of colonial stony coral commonly found on reef slopes in the Indian and Pacific ocean regions, as well as the Red Sea and the Gulf of Aden. It is most commonly found where wave action is weak, usually at a depth of 79 inches to 49 feet below sea level. It is a common coral species among reef aquarium enthusiasts.
This coral usually forms small colonies. The early stage of the coral colony is formed of low domes, but as the colonies grow, they become more irregular with massive columns that can reach 16 feet across. The corals are the calcareous (calcium carbonate) skeletons of polyps. The variability seen in shape is partially caused by the activities of horse mussels which bore into the skeletal framework.
Individual polyps are embedded in circular, tube-shaped corallites less than a half-inch across, made of limy material extruded by the polyps. Large numbers of ridge-like septae (wall structures) line the corallites and radiate from the center. The polyps feed only in the daytime, during which time the tentacles are extended, hiding the skeleton from sight.
The typical coloration of this coral ranges from green and gray to reddish-brown. The tentacles are often contrast in coloration to the coral, and are usually tipped with white. A few of the tentacles are modified into sweeper tentacles which extend further than the rest, upwards of 12 inches in length. These sweepers are used to deter other organisms from settling close by.
The size and proximity of the corallites varies depending on the amount of light incident on the coral, even over different areas of the same colony. In brighter areas, small, closely packed corallites maximize the photosynthetic potential of the symbiotic algae contained within the corallites. In less well-lit areas, there are larger corallites and polyps with longer tentacles with greater food capturing ability.
The Octopus Coral gets its food from two sources. The polyps contain symbiotic algae called zooxanthallae which can usually obtain sufficient energy from sunlight for the coral’s needs. The polyps can also extend their tentacles to catch and ingest organic particles, sediment, zooplankton, bacteria and other organic matter. This supplies the rest of the coral’s needs.
This species reproduces asexually by budding. It can also reproduce sexually, with both sperm and eggs being released into the water table in synchronized spawning events, with fertilization taking place externally. The larvae that develop from the eggs drift as part of the zooplankton. Those that survive settle on the seabed, undergo metamorphosis and develop into a polyp. Over time, the polyp grows a skeleton that forms the basis of the coral, and grows into a new colony.
Like with most corals that contain zooxanthallae, the Octopus Coral is likely in danger due to climate change. As the warming waters kill off the micro-algae, it causes bleaching of the coral. Without the zooxanthallae, the coral fails to thrive and the polyps may die. However, compared to other reef corals, this species is more resistant to bleaching. This particular coral takes eight years to reach maturity, making it difficult to assess its status except over long time scales. Other threats include trawling, reef destruction, pollution and tourism.
This species is a popular coral in the reef aquarium trade. It is widely available, but due to its fragile skeleton, care in handling is needed. Tank temperature needs to be maintained to between 74 and 81 degrees F. In needs a well-lit area with low to moderate water flow. It should not be placed within 12 inches of any other coral or sessile (base-attached) invertebrate because it will use its sweeper tentacles to sting intruders and defend its territory.
Feeding can be done using the larvae of brine shrimp. These are commercially available for live feeding in the form of cysts which hatch within 24 hours, or as dead, pasteurized product known as “Instant baby brine shrimp” (IBBS) with a long shelf life. IBBS can be enriched with probiotics or antibiotics to treat coral disease.