Galaxea fascicularis
A colonial stony coral with long sweeper tentacles and variable colony shapes.
Galaxea fascicularis, a colonial stony coral from the family Euphylliidae, goes by common names like octopus coral, fluorescence grass coral, and galaxy coral. It thrives on reef slopes across the Indo-Pacific and is also kept in home reef aquariums.
Young colonies often start as low domes, but as they grow, they become irregular, hummocky, or columnar, sometimes reaching five meters across. The coral's skeleton is made of calcium carbonate, and its shape can be influenced by horse mussels that bore into it. Individual polyps sit in circular, tube-like corallites less than a centimeter wide, lined with many ridge-like septa. Polyps often feed during the day, and when their tentacles are out, the skeleton is hidden. Colors range from green and gray to reddish brown, with tentacles usually a contrasting shade tipped in white. Some tentacles are modified into sweeper tentacles that can stretch up to 30 centimeters to keep other organisms away. The size and spacing of corallites change with light levels: in bright spots, small, packed corallites boost photosynthesis from zooxanthellae; in dimmer areas, larger corallites and longer tentacles improve food capture.
This coral is found from the Red Sea and Gulf of Aden across much of the Indo-Pacific, on reef slopes with weak wave action, between two and fifteen meters deep.
It gets energy two ways: symbiotic zooxanthellae provide most of its needs from sunlight, and polyps also catch organic particles, sediment, zooplankton, bacteria, and dissolved organic matter. It reproduces asexually by budding and sexually by releasing sperm and eggs into the water for external fertilization. The resulting planula larvae drift as plankton; after heavy losses to predators, survivors settle, metamorphose into polyps, and start building new colonies.
Like other corals with zooxanthellae, *G. fascicularis* is stressed by climate change and warmer seas, which can bring stronger storms, more El Niño events, and disease outbreaks. Higher temperatures can kill the algae, causing bleaching and potential polyp death. Still, it is more resistant to bleaching and burial by sediment than many reef corals. It is common but takes about eight years to mature, making long-term status hard to gauge. It is popular in the aquarium trade—Indonesia exported 17,550 live pieces in 2005, with smaller amounts from Fiji and elsewhere.
Quick Facts
- Taxon
- Galaxea fascicularis
Facts from the source article.
Lore & Background
Galaxea fascicularis forms small low domes initially, but as colonies grow they become irregular, massively hummocky or columnar, sometimes reaching 5 meters across. The variability in shape is at least partly caused by the boring activities of horse mussels (Lithophaga spp.). Individual polyps are embedded in circular, tube-shaped corallites lined with numerous ridge-like septa. The coral's color ranges from green and grey to reddish brown, with tentacles often in contrasting colors tipped with white. Some tentacles are modified into sweeper tentacles that can extend to 30 centimeters to deter other organisms from settling nearby.
The size and proximity of corallites vary depending on light exposure: in bright positions, small, closely packed corallites maximize photosynthetic potential of zooxanthellae; in less lit positions, larger corallites and polyps with longer tentacles enhance food capture. The species is found in the Red Sea, Gulf of Aden, and large areas of the Indo-Pacific, on reef slopes with weak wave action, at depths between 2 and 15 meters.
Reader's Guide
Galaxea fascicularis is a common species on Indo-Pacific reef slopes and is popular in the aquarium trade. It obtains food from two sources: symbiotic zooxanthellae that provide energy from sunlight, and heterotrophic feeding on organic particles, sediment, zooplankton, bacteria, and dissolved organic matter. It reproduces asexually by budding and sexually through synchronized spawning, with planula larvae drifting as zooplankton before settling and forming new colonies.
In aquaria, the coral requires a temperature between 23 and 27 °C, well-lit conditions, and low to moderate water flow. It should not be placed within 30 centimeters of other corals or sessile invertebrates due to its sweeper tentacles. Cultivated corals can be propagated by sawing the skeleton and suspending pieces on threads. Feeding can be done with brine shrimp nauplius larvae, available live or as pasteurized 'Instant baby brine shrimp' (IBBS), which can be enriched with probiotics or used to carry antibiotics.
Threats include climate change, warmer waters, storms, El Niño-Southern Oscillation, coral disease, and bleaching. However, this species is relatively resistant to bleaching and sedimentation compared to other reef corals. It takes about 8 years to reach maturity, complicating long-term status assessment. Other threats include trawling, reef destruction, pollution, and tourism.
Did You Know?
- Galaxea fascicularis colonies can reach up to 5 metres (16 ft) across.
- The size and proximity of corallites vary depending on light exposure, even across different areas of the same colony.
- In 2005, Indonesia exported 17,550 live pieces of this species for the aquarium trade.
Frequently Asked Questions
What is Galaxea fascicularis?
It is a colonial stony coral in the Euphylliidae family, widely known by nicknames like octopus coral, fluorescence grass coral, and galaxy coral. Native to reef slopes across the Indo-Pacific, it is also one of the most popular corals kept in home reef aquariums.
What are Galaxea fascicularis's signature 'powers'?
Its most eye-catching trait is a pair of long sweeper tentacles that can stretch out to roughly 30 cm (12 in) to snag plankton and keep neighbouring colonies at bay. It also produces vivid bioluminescent fluorescence, which is the source of its 'galaxy' and 'fluorescence grass' nicknames.
How does Galaxea fascicularis grow over its lifetime?
Young colonies start as small, low domes and gradually shift into irregular, hummocky, or columnar shapes as they reach maturity around the eight-year mark. At full size a single colony can spread up to five metres (16 ft) across, with each polyp seated in a circular corallite under a centimetre wide.
Where does Galaxea fascicularis live in the wild?
It inhabits reef slopes throughout the Indo-Pacific at depths between about 2 and 15 metres (6–49 ft). Over time, boring organisms such as horse mussels can reshape its calcium-carbonate skeleton, giving older colonies a more irregular, pitted appearance.
Why is Galaxea fascicularis a big deal in the aquarium hobby?
It is one of the most heavily exported LPS/euphyllid corals for home reefs; Indonesia alone shipped 17,550 live pieces in 2005. Its manageable size, striking fluorescence, and relatively hardy nature make it a staple for both new and experienced reef keepers.
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