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Why Does My Coral Deflate at Night?

In spite of its rocklike appearance, coral is actually an aquatic animal, scientifically categorized in the class Anthozoa. Thousands of coral species are distributed in marine waters worldwide, all of which have various characteristics. Depending on the species, coral deflates or retracts for different reasons, for example, poor environmental quality and illness. However, coral structures and regular nocturnal activity are major reasons for coral deflating at night.

Structure
- In zoology, a polyp is an animal form whose body is an elongated, saclike structure. An individual coral is a polyp that sits in a calcium-carbonate "cup" termed a calyx. Its mouth opening is situated at the top of the sac and is fringed with tentacles.
Diet
- Many species of coral consume organic byproducts that zooxanthellae algae produce through photosynthesis -- as the zooxanthellae lives symbiotically inside coral. However, in spite of that food source, most coral also feeds off of plankton that it captures with its tentacles. It consumes plankton primarily at night. Coral can appear to deflate at night because of its nighttime feeding habits.
Feeding Habits
- Ordinarily, many types of coral appear to shrink in the day. The reason is that when they feed at night, they extend their tentacles to capture the plankton. When the day comes, and their nighttime feeding is over, their tentacles retract, giving a more shrunken or deflated appearance to the coral overall. However, some species of coral have structures called vesticles that inflate to look like bubbles while their tentacles are retracted. The vesticles inflate during daytime hours to expose the zooxanthellae algae living inside them to more light for photosynthesis. While inflated, these vesticles are generally the size of grapes. At night, the vesticles deflate, allowing the coral to extend its tentacles for plankton feeding.
Cynarina and Plerogyra
- Many of the types of coral that appear with bubblelike vesticles are members of the genera Cynarina and Plerogyra. The only two species of coral within the genus Cynarina are Cynarina lacrimalis and Cynarina lacrymalis. Common names for these types of coral include cat's eye coral, doughnut coral, solitary cup coral, teary star coral, meat coral and owl's eye coral. The genus Plerogyra comprises 10 species and subspecies. Two common species are Plerogyra sinousa and Plerogyra lichtensteini, which have common names such as bubble coral, octobubble coral, pointed bladder coral, grape coral, pearl coral, octopus coral and pearl bubble coral.

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