Galaxea astreata
A cosmopolitan euphylliid coral with plate-like corallites and varied growth forms.
Galaxea astreata, a large-polyp stony coral from the family Euphylliidae, is widespread across the Indo-Pacific. It holds the distinction of being the most common coral species within the Xuwen Coral Reef National Nature Reserve. This coral prefers clear, salty water and is typically found in shallow areas, most often around 15 meters deep, though it can live anywhere from 1 to 30 meters. It thrives at water temperatures near 27 ± 0.5 °C.
The coral builds a distinctive skeleton from calcium carbonate. Its individual polyp cups, called corallites, are flat and plate-like, measuring about 3 to 4.5 millimeters across, and are spaced well apart. Colonies can take several forms—irregular and lumpy, column-like, or encrusting over hard surfaces—and may grow to over two meters in length. The polyps themselves can be pink, grey, green, or brown. Like many corals, it hosts symbiotic zooxanthellae algae within its tissues.
For food, *G. astreata* captures zooplankton and drifting coral larvae. Unlike many corals that feed at night to avoid fish predators, this species extends its polyps to feed during the day and pulls them back at night. Its massive growth form makes it especially effective at catching larvae as they sink through the water column.
Reproduction occurs through broadcast spawning: the coral releases eggs and sperm into the water for external fertilization. The resulting planulae (coral larvae) develop within the plankton. Individual colonies can be hermaphroditic, producing both male and female gametes, or gonochoric, producing only one sex.
Human activities—such as overfishing, pollution, eutrophication, and siltation—have caused a rapid decline in *G. astreata* coverage in the Xuwen reserve. While this species is generally more resilient to environmental stress than others (like *Acropora*), it suffered a 100% mortality rate after the 1997–1998 El Niño event. Tissue death began slowly in the first four months following the disturbance. That same bleaching event, which raised ocean temperatures 2.8 °C above average, also reduced coral species richness off Sesoko Island, Japan by 61% and coral cover by 85%. Coral bleaching, driven by the loss of symbiotic zooxanthellae and their pigments, is a global problem linked to climate change and ocean acidification.
Quick Facts
- Taxon
- Galaxea astreata
Facts from the source article.
Lore & Background
Galaxea astreata has a skeleton made of calcium carbonate, with corallites that are very plate-like, well spaced, and about 3–4.5 mm in diameter. Colonies can be sub-massive, columnar, or encrusting, and can grow over two meters long. The polyps are typically pink, grey, green, or brown. It hosts symbiotic zooxanthellae. It is found in the Indo-Pacific and is the most abundant coral species in Xuwen Coral Reef National Nature Reserve. It is acclimatized to water temperatures around 27 ± 0.5 °C, generally prefers clear, salt waters over turbid brackish waters, and is commonly seen at a depth of about 15 meters, ranging from 1 to 30 meters.
Reader's Guide
Galaxea astreata is a broadcast spawner, releasing gametes into the water for external fertilization; it can be either a hermaphrodite or a gonochore. It feeds on zooplankton and coral larvae, expanding its polyps during the day and contracting them at night. It is an effective predator, especially when larvae sink in the water column. The coral coverage rate in Xuwen rapidly decreased due to human activities such as overuse of fisheries, eutrophication, siltation, and pollution. Although G. astreata is more resistant to environmental changes than some other corals (e.g., Acropora spp.), it experienced 100% mortality after the El Niño phenomenon of 1997–1998, with slow tissue death in the first four months. That bleaching event reduced coral species richness off Sesoko Island, Japan by 61% and coral cover by 85%, with temperatures rising 2.8 °C above average. Coral bleaching, linked to global climate change and rising ocean temperatures as well as ocean acidification, causes loss of symbiotic zooxanthellae and pigment.
Did You Know?
- It can grow over two meters long and has a sub-massive, columnar, or encrusting morphology.
- It had a 100% mortality rate after the 1997–1998 El Niño event, despite being more resistant than some other corals.
- It feeds during the day by expanding its polyps and contracts them at night.
Frequently Asked Questions
What is Galaxea astreata and where does it come from?
It is a large-polyp stony coral in the family Euphylliidae, native to the Indo-Pacific. It is so widespread that it ranks as the most commonly recorded coral species in the Xuwen Coral Reef National Nature Reserve.
What growth forms can Galaxea astreata take?
This species is notably versatile, building its calcium-carbonate skeleton as a sub-massive mound, an upright column, or a flat encrusting sheet. Which form it adopts depends largely on local current and light conditions.
What water parameters does Galaxea astreata prefer?
It thrives in clear, saline water at roughly 27 °C (± 0.5 °C). In the wild it is most often encountered near 15 meters depth, though it tolerates a range from 1 to 30 meters.
What do Galaxea astreata's polyp cups look like?
Each polyp sits in a flat, plate-like corallite about 3 to 4.5 mm in diameter. This disc-shaped, shallow cup is a key visual cue that separates it from euphylliids with more rounded or angular corallites.
Why is Galaxea astreata considered ecologically significant?
Its ability to shift between sub-massive, columnar, and encrusting morphologies lets it occupy a wider range of reef microhabitats than many of its euphylliid relatives. That flexibility is reflected in its status as the dominant species at sites like the Xuwen reserve.
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