Bubble coral is the name given to several stony coral species belonging to the genus Plerogyra and Physogyra. These corals get their name from the large vesicles that give the colonies the bubble or grape appearance.
These corals form inverted cone-shaped colonies in the seabed. The corals have branches that extend and reveal vesicles that are filled with water. These vesicles are not true bubbles, but modified tentacles. Some bubble coral species have each polyp on individual branches while others share branches. The vesicles range in size, from a few millimeters to 2.5 centimeters. The vesicles are spheroid and usually green to brown in color. They inflate during the day and deflate during the night, so depending on the available light. When deflated, the skeleton and corallites of the corals can be found as well as the tentacles. Sometimes sweeper tentacles can be observed, these having the role to “sweep” competing coral colonies.
Many species can be found throughout the Indo-Pacific region. Some species inhabit the Red Sea, others extend half-way on the Eastern African coast, while some can be found on the North, East and West Australian coast, New Caledonia, Samoa, Ryukyu Islands and the Maldives. These species normally prefer areas with milder currents and tidal waves. They can be commonly found in protected areas such as crevices or caves, despite the low amount of light there. Some species can even be found at depth of over 40 meters, while others can thrive in shallow brighter and somewhat turbid waters.
Bubble coral species belong to two genus: Plerogyra and Physogyra. Common species include Physogyra lichtensteini, Plerogyra sinuosa, P. flexuosa and P. simplex. While the appearance is similar between the species, Physogyra has small vesicles which can be just a few millimeters in diameter, while Plerogyra species have vesicles of up to 2.5 centimeters wide.
Depending on the location of the colony, these corals might obtain their food from the symbiotic zooxanthellae or by catching zooplankton organisms. The vesicles are actually modified tentacles which hold water as well as higher concentrations of zooxanthellae than most other coral species. These high concentrations help the corals gain enough nutrients from the small dinoflellate even in low light conditions. The vesicles inflate during the day, enlarging the exposed area while they deflate during the night to uncover the skeleton as well as the tentacles. The tentacles are used to capture organisms swimming too close to the colony.
These corals have male and female colonies and can reproduce sexually and asexually. During spawning season which can occur as early as June and as late as November, the polyps release large quantities of gametes into the water. There, fertilized eggs become planulae, which are swimming larvae. Once they settle onto the substrate, they start secreting a calcium carbonate exoskeleton and produce another colony. Asexual reproduction is performed through budding, where a new portion grows at the edges of the colony, develops, falls off and then forms another bubble coral colony.