Bubble coral (Plerogyra sinuosa) also known as grape coral, pearl coral or bladder coral, is as unusual as corals get. They are in the main image was spotted in Chole Bay in the Mafia Island Marine Park in Tanzania. It was about a meter in circumference, the largest piece of Bubble coral we have ever seen.
They have large fleshy polyps that when inflated resemble bubbles or grapes hence the name. The polyps inflate during the day and deflate at night when the coral puts out tentacles to capture food. In Tanzanian waters Bubble Coral is usually a very light brownish yellow color.
In Tanzanian waters , if one carefully inspects them in the wild, one can often find Ghost Gobies hiding on the polyps. They are usually tiny, around 1 cm in length and one has to look carefully for them.
As with many corals, bubble coral is a zooxanthellate and its bubbles contain large numbers of single celled algae with which it has a symbiotic relationship. The algae has a safe stable environment in the coral bubble which expands according to the light. The coral obtains nutrients produced by the algae through photosynthesis. This relationship provides the coral with up to 70 % of its food requirements. The balance is captured by the tentacles that come out at night. These can be seen in the images at the bottom of the post. The bubbles can inflict a painful sting and provide protection for the organism as a whole.
These corals are large polyp stony corals (LPS) and are small underneath the bubbles and have a light skeleton with petal shaped septa. They are found across the Indian Ocean and Red Sea to the mid-Pacific in sheltered reefs at depths from 3 to 35 meters.
They can be kept in an Aquarium and prefer a gentle water flow and medium lighting. Because of the stinging ability they cannot be kept too close to other corals or organisms. A bubble coral can make a very attractive addition to an Aquarium and they have a healthy appetite for small pieces of fish or shrimp.