Galaxea
Colonial stony corals with sweeper tentacles and varied colony forms.
Galaxea is a genus of colonial stony corals belonging to the family Euphylliidae. They are commonly called crystal, galaxy, starburst, or tooth coral. These corals are plentiful on reefs across the Indo-Pacific region and the Red Sea, typically living in water shallower than 20 metres and preferring murky environments. They are occasionally kept in reef aquariums.
The shape of Galaxea colonies varies by species. *G. fascicularis* grows as long, thin columns and ranks among the largest known corals. *G. paucisepta* and *G. longisepta* form flat plates, *G. acrhelia* is tree-like, and other species develop domes or rounded mounds. Their colours are mostly olive or brown, often with a purple tint. The corallites housing individual polyps are small and tightly packed, with raised edges or even stalks. Numerous fine septa line the corallite edges, arranged in whorls and protruding as sharp ridges. The polyps host symbiotic protists called zooxanthellae and grow in shallow water to maximize sunlight absorption. They often feed during the day, extending yellowish or greenish tentacles, frequently with white tips. Specialized sweeper tentacles—long defensive organs tipped with powerful cnidocytes—discourage other corals from settling nearby.
The World Register of Marine Species lists these species: *Galaxea acrhelia*, *G. alta*, *G. astreata*, *G. cryptoramosa*, *G. fascicularis*, *G. horrescens*, *G. lauensis* †, *G. longisepta*, *G. pauciradiata*, and *G. paucisepta*.
Quick Facts
- Taxon
- Galaxea
Facts from the source article.
Lore & Background
The colonies of Galaxea have various forms according to species. G. fascicularis forms long, thin columns and is one of the largest corals known. G. paucisepta and G. longisepta form flat plates, G. acrhelia is arborescent and other species form domes and rounded mounds. Their colours are mostly olives or browns but they are often tinged with purple. The corallites in which the individual polyps sit are small and crowded and have raised edges or may even be stalked. There are a large number of fine septa on the edge of the corallites, arranged in whorls and protruding as sharp ridges.
The polyps contain symbiotic protists called zooxanthellae and grow in shallow water to maximise the uptake of sunlight. They often feed in the day, extending yellowish or greenish, often white tipped tentacles. They have certain specialised sweeper tentacles, long defensive organs, tipped with powerful cnidocytes which discourage other corals from living close by.
Reader's Guide
Galaxea is notable for its diverse colony forms, ranging from long thin columns in G. fascicularis to flat plates in G. paucisepta and G. longisepta, arborescent growth in G. acrhelia, and domes or rounded mounds in other species. Their colours are mostly olives or browns, often tinged with purple. The corallites are small and crowded with raised or stalked edges, and numerous fine septa arranged in whorls. The polyps contain zooxanthellae and grow in shallow, turbid waters to maximise sunlight. They feed during the day, extending yellowish or greenish tentacles often with white tips. A key defensive adaptation is the presence of specialised sweeper tentacles—long organs tipped with powerful cnidocytes that deter other corals from settling nearby. This aggressive trait, combined with their abundance on Indo-Pacific and Red Sea reefs, makes them a significant component of turbid reef ecosystems. They are also sometimes kept in reef aquaria, though their sweeper tentacles require consideration in mixed tanks.
Did You Know?
- Galaxea is also known by common names crystal, galaxy, starburst and tooth coral.
- The polyps have specialised sweeper tentacles tipped with powerful cnidocytes that discourage other corals.
- They are found in water less than 20 metres deep and favour turbid sites.
Frequently Asked Questions
What is Galaxea?
Galaxea is a genus of colonial stony corals in the family Euphylliidae, found throughout the Indo-Pacific and the Red Sea. In the hobby they go by several nicknames, including crystal, galaxy, starburst, and tooth coral.
What depth and water conditions do Galaxea prefer in the wild?
They typically live on reefs shallower than 20 metres (about 66 ft) across the Indo-Pacific and Red Sea. They seem to favour turbid, murkier water over the crystal-clear conditions many other reef corals demand.
Can I keep Galaxea in a home reef aquarium?
They are occasionally maintained in reef tanks, though they aren't as ubiquitous in the hobby as some other LPS. Their natural tendency to thrive in murkier water can make them a bit more forgiving for aquarists than corals that need pristine parameters.
How is Galaxea different from other euphyllids?
While they share sweeper tentacles with other members of Euphylliidae, Galaxea stands out for the dramatic range of colony architectures within a single genus—ranging from towering columns to broad flat plates to branching, tree-like structures.
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