Scleractinian corals, also known as stony corals — or just hard corals — are the primary reef builders in the oceans. Their polyps secrete calcium carbonate to form a skeleton. A minority of species live as single polyps, but most stony coral species are colonial, and the structures they build ‘grow’ over time. They form a myriad of shapes: mounds, branches, fingers, plates, and encrustations.
In several previous posts I discussed and displayed photos of a number of so-called soft corals, all of which are octocorals, i.e., their polyps have eight tentacles. Stony corals are hexacorals: their polyps have tentacles in multiple of six. Most seem to feed at night, so you are not likely to see the tentacles extended during daylight hours.
The coral in the first photo (top right) forms a rounded hemispherical head that is grooved in a meandering, maze-like pattern. Corals that form these kinds of colonies often are referred to as ‘brain corals’ because the patterns of the ridges and grooves are reminiscent of the sulci of a brain. The one pictured is Diploria labyrinthiformis, a species common throughout the Caribbean Sea.
Below is another example of a ‘brain coral’ — this one from the Red Sea. This species (Platygyra daedalea) forms massive colonies, sometimes more than a meter in diameter. Most brain corals belong to the family Faviidae.
I mentioned above that some hard corals live as single corallites. Members of the family Fungiidae fit that description. Known by the common name Mushroom Corals, these are free-living, i.e., they are not attached to the substrate. The photo below is a macro image of Fungia scutaria, an Indo-Pacific species. This is the most common mushroom coral found in Hawaii, where this one was photographed. The overall shape of the corallite usually is oval, with septae radiating from a central mouth, as shown in the photo. The corallite can attain a width of 15 cm to 18 cm (6 in to 7 in).
While hard corals all have calcareous skeletons by definition, the rest of their anatomy consists mostly of soft parts. Some of those can obscure the skeleton to the point that it is hard to tell, just by looking, that some species are indeed hard corals. A good example is Bubble Coral (Plerogyra sp.), shown in the next photo, below. The stalked corallites of this genus form rounded colonies. The skeleton of the colony is obscured during daylight hours by bubble-shaped vesicles. When this coral feeds, the vesicles deflate, and the tentacles emerge.
By the way, in this macro image you can see clearly that this coral is infested with little pancake-shaped critters. They are Waminoa flatworms. The photo was taken at Bunaken Island, Indonesia where much of the bubble coral seemed to be sporting Waminoa.
Corals of the genus Goniopora seem to feed mostly during daylight hours. The polyps have 24 tentacles arranged in a way that makes them look like flowers. The macro photo below shows the clustered polyps of a Goniopora species from the Red Sea.
A favorite subject with underwater photographers are the colorful members of the family Dendrophylliidae. They grow tubular skeletons topped by a cup-shaped calyx, so they are known by the common names Tube Corals or Cup Corals. Another common name is Cave Coral, a name that refers to this family’s habit of growing on the walls and roofs of underwater caves, and underneath ledges. Some members of the family are colonial, while others are solitary. All of them have gloriously colored tentacles, usually in shades of red, orange, or bright yellow.
I will close with two photos of a colonial Dendrophyllid species from Hawaii, Tubastrea coccinea. Both photos are of the same colony, located inside a small underwater cavelet, but taken at two different times. The first image was captured during daylight hours and the tentacles are retracted into the calyces.
The final photo is a macro image of a polyp from the same colony as above, but this one was taken during a night dive. When their beautifully colored tentacles are extended for feeding, these corals are a sight to behold.
I hope the readers of Photo Synthesis have enjoyed my underwater photos as much as I have enjoyed presenting them during the past month.