With grape-sized, sac-like membranes, bubble corals look soft, squishy and egg-like from the outside. But underneath, Plerogyra sinuosa and its like are hard, stony corals. Although they’re popular with home aquarium enthusiasts, it’s somewhat of a find to spot bubble coral on the reef.
WHEN YOU COME ACROSS CLUSTERS OF BUBBLE CORAL, at first glance you might take them for large fish eggs, or perhaps errant egg clutches of some animal, say a cephalopod. Once you realize they’re corals, you might suspect they’re some soft coral variant.
In reality, these Indo-Pacific denizens are conventional stony corals with outsized vesicles – soft, fleshy membranes that they inflate through the day to soak in solar energy for photosynthetic purposes.
At night, they pull the vesicles in, reveal their hard calcareous skeletons and extend the usual coral-polyp tentacles that work to capture plankton and other small prey from the passing currents.
BUBBLE CORALS ON THE REEF ARE A LITTLE SHY
Relatively uncommon, these corals are found in a range from the waters off Madagascar in the Indian Ocean, through the Red Sea, east to the South Pacific.
Although they receive the bulk of their nutrition from the photosynthesis services of embedded zooxanthellae algae, they’re somewhat fragile, even by coral terms. They’re vulnerable, apparently, even to injury from strong currents.
As a result, they tend to gravitate towards crevices and other protected areas of the reef, avoiding serious currents but off-setting their preference for lower-light habitation with their zoox-laden, inflated vesicle trick.
ALWAYS LOOK FOR STUFF
For more information about stony corals: What Lies Beneath – Reef-Building Coral Polyps
For more information about the different types of corals: Stony, Soft or Gorgonian, They’re All Coral Polyps
ARMED AND DANGEROUS
Like most reef-building corals, Plerogyra species receive the great bulk of their nutrition through the photosynthetic action of embedded zooxanthellae algae. But, like most, they earn about 20 percent of it by extending their stinging tentacles into the currents, paralyzing passing plankton and hauling it in.
Attuned to capturing microscopic prey, the stings of most small-polyp corals are so mild that we human divers would be unlikely to feel them.
However, bubble corals are large polyp stony corals, with fleshier polyps, longer tentacles and an attitude. Well-armed, they can be aggressive towards competition, sweeping their powerful tentacles around them to kill off other organisms that might impinge on their territory. Home aquarium sources note that it’s wise to keep them separated from other tank dwellers.
In light of this, perhaps it’s a good thing that’s it’s not common to find bubble corals on the reef. In its descriptions for Plerogyra sinuosa and Plerogyra linchtenstini, the reference Coral Reef Animals of the Indo-Pacific notes: “This species is capable of stinging the unsuspecting diver.” It doesn’t generally say that for other species.
The Encyclopedia of Life lists seven species of bubble corals, but confirmation of some of them appears to be not final. Other sources suggest fewer. The “bubbles” are somewhat similar but the underlying structures are sowhat different. Here are three prominent varieties:
“Bubble Coral” (Plerogyra sinuosa)
Just plain “bubble coral,” Plerogyra sinuosa, is one of those species in nature in which their common name can also be the term for the broad group encompassing other species. Specimens of Plerogyra sinuosa can range from small clusters to large colonies several feet across. The species’ underlying stony architecture tends to meander around in a maze-like pattern, with, perhaps, exaggerated coralites. The grape-size membranes inspire other street names, including grape coral and bladder coral.
“Branching” Bubble Coral
The membranes of Plerogyra simplex, pretty much resemble those of P. sinuosa. But look underneath and you’ll see why it includes “branching” in its name. While each of P. simplex’s tubular branches support many vesicles, they all build out from a single base. They can develop into large colonies.
Pearl Bubble Coral
The only species in the genus Physogyra linchtenstini features smaller (pearl-shaped) bubbles and skeletons of thick plates.
DISCUS, CAULIFLOWER & ROUNDED BUBBLEGUM CORALS
A truly striking bubble coral species is Plerogyra discus. I can’t find photos with creative commons licenses but a site called reeflex.net features stunning photos of P. discus and several other species.
PRINCIPAL SOURCES: Coral Reef Animals of the Indo-Pacific, Terrence Gosliner, David Behrens, Gary Williams; Bubble Coral – How To Identify Four Species, reefdivers.io; BUBBLE CORAL – FACTS AND PHOTOGRAPHS, seaunseen.com; Plerogyra sinuosa, et.al., Corals of the World; Plerogyra sinuosa (Bubble coral), et.al., Encyclopedia of Life; Plerogyra discus – Bubble coral, et.al., reflex.net; Plerogyra sinuosa, et.al., Wikipedia.com.