Mushroom coral (Fungia scutaria) is a peculiarity as corals go for several reasons, they do not have a stable place of abode, they are non colonial in that each specimen is a single coral polyp and they do not live permanently alongside other polyps.
Most corals are stable in the sense that they are firmly attached to the reef whereas a mushroom coral is only attached to the reef in its infancy. In their infancy they are called acanthocauli and are attached to a substrate and the disc shape that develops, combined with the stalk that attaches to the substrate gives them their name of mushroom corals. As the coral grows, so the stalk breaks off and the adult moves on. In the image below a very young acanthocauli can be seen in the centre of the image.
In the images young below juvenile mushroom corals still attached to the substrate are pictured.
Each polyp is approximately 2 cm in diameter and can be seen in detail below. These were photographed at night on a shallow reef.
Mushroom coral are important in reef development as they move down the reef onto more inhospitable sections and when they die they provide a substrate for less sensitive corals to adhere to and provide a base for the expansion of the reef.
When alongside other corals they secrete a mucus which prevents them from being over grown by the other corals. As they are moved by wave action they are often found in holes on the reef and in areas where they have been washed by the waves.
They are oval in shape growing up to 17 centimeters long and have a raised centre giving them a dome shape with a slit like mouth in the upper centre. They have raised ribs extending outwards from the mouth. Between the ribs are short, tapered and widely spaced tentacles that are usually only extended at night, with the mouth extending across the center. The tentacles can be yellow, blue or brown in color. Here is a video where you can see the tentacles in action:
Mushroom coral is a hermaphrodite and can reproduce both by sexual reproduction and asexually. During sexual reproduction eggs and sperms are released into the water through the mouth which mix and fertilize with the fertilized egg dropping onto the substrate, the polyp develops from there growing its calcareous skeleton. Asexually the mushroom coral can reproduce from damaged adult tissue, the result are young acanthocauli.
Mushroom corals are able to withstand harsh conditions and inhabit areas that other corals cannot. They can cope with being immersed in sediment for quite some time and even immersion in fresh water for short periods. Here is a video of a mushroom coral excavating itself from sediment:
Mushroom corals are listed on Appendix II of the Convention on International Trade in Endangered Species (CITES), indicating that trade in this species should be carefully regulated. Because they are easily collected they will likely remain under threat particularly in third world environments where they are harvested for building purposes.