- Fish
- Pterocaesio chrysozonaScientific Name
- Least ConcernIUCN Threatened Status
Bubble Coral
Bubble coral is one of the most distinctive and visually striking corals on Koh Tao’s reefs. During the day its large, water-filled bubbles expand to maximise light exposure — at night they retract to reveal long sweeper tentacles capable of stinging nearby corals.
Least Concern
Overview
Bubble coral is hard to miss once you know what you’re looking for. During daylight hours the colony expands its transparent, grape-like vesicles — water-filled extensions of the tissue that maximise the surface area exposed to light. The effect is a cluster of shiny bubbles that catches the light beautifully and looks unlike anything else on the reef.
The bubbles serve a practical purpose. Each one contains zooxanthellae — the symbiotic algae that provide the coral with most of its energy through photosynthesis. By expanding the vesicles during the day, the coral increases the surface area available for light absorption. At night, the vesicles retract and long, stinging sweeper tentacles emerge instead. These tentacles are used both to capture plankton and to wage chemical warfare on neighbouring corals, keeping competitors from encroaching on the colony’s space.
Bubble coral typically grows in sheltered spots — under overhangs, in crevices, or on the shaded faces of reef structures where it’s protected from strong surge. You’re most likely to find it at depth rather than in the shallows, though it does occur at sites like Hin Pee Wee and No Name Pinnacles in the 8–15 metre range.
The species is generally considered hardy and less susceptible to bleaching than many other coral species, though it’s not immune. Well-inflated, evenly-distributed bubbles are a sign of a healthy colony. Deflated or unevenly expanded vesicles can indicate stress.
Identification
During the day, Bubble Coral extends rows of pale, water-filled vesicles that give the colony its name — each one smooth, translucent, and roughly spherical, ranging from pea-sized to grape-sized depending on the colony. The overall effect is unmistakable and unlike any other coral on Koh Tao's reefs.
Logan Brown
Conservation Notes
The bubbles are fragile and will burst if touched. Never handle or prod bubble coral — the vesicles are extensions of living tissue and rupturing them causes direct harm to the animal. Give the colony a wide berth, particularly if your buoyancy isn't fully controlled. The sweeper tentacles that emerge at night can also sting — avoid contact during night dives.
Bubble Coral Facts
Where to find them
Look for bubble coral in sheltered spots — under ledges, in crevices, or on the shaded underside of reef structures. At Twins, check the shaded faces of the pinnacles at around 10–14 metres. At Japanese Gardens, look in the reef crevices on the deeper sections of the site. If you find a colony fully expanded in good light, slow down and look closely — the translucent quality of the bubbles in sunlight is one of those small reef details that sticks with you.
Frequently Asked Questions
Most colonies around Koh Tao are between 30 and 60cm across. Larger colonies up to a metre across exist but are less common. Growth is slow, so a large colony represents many years of undisturbed development.
During the day the colony expands large water-filled vesicles to maximise light exposure for its symbiotic algae. At night those vesicles retract and long sweeper tentacles emerge to feed on plankton and defend the colony’s territory. It’s effectively a different-looking animal between day and night dives.
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