Describing Coral Colony Shape
Learning to identify corals not only improves your diving experience, but will allow you to understand more about reef ecology. Learning coral species is very difficult, and takes years of learning and practice. Corals have many common names, which vary greatly depending on the region, because of this, coral growth forms are a more generally accepted identification method.
Basics of Coral Anatomy
Corals are colonial animals made up of many small individuals called polyps. Each polyp resembles a jellyfish turned upside down, and sites in it’s own cup, called a calice or coralite. On hard corals the polyps are always visible to the naked eye. The top of the polyp consists of a mouth surrounded by tentacles, that are covered in stinging cells used to capture prey. Mostly corals extend their tentacles at night to feed on zooplankton, which when captured, is pulled into the mouth of the corals. The coral does not have a digestive system, but just a digestive gut, so it will feed, digest, then excrete every 24 hours or so. The skin of the coral is clear, but inside the tissue of the coral is full of unicellular algae called zooxanthallae. This unicellular algae are actually a plant, but live inside the coral to take advantage of the constant light levels and supply of nutrients inside the coral body. In return, the zooxanthallae provide energy to the coral in the form of sugars and carbohydrates which they create using photosynthesis.
The coral animal, along with its algae, grows on a skeleton of coral created from calcium in the sea water. A single coral species can grow in a wide variety of shapes and sizes depending on the environmental conditions (light, currents, sedimentation, etc). Assessing the shape of the coral gives us information on the conditions present on the reef. Below you will find a commonly accepted list of coral growth forms, which describes the shape of the overall colony.
Growing in a ‘dendritic’, or tree like, shape. Long, tapered branches with regular splitting. This is one of the fastest growing forms, as all of the polyps on a branch contribute to skeletal growth in a single direction. Generally these corals grow fast when conditions are good, but are easily susceptible to mortality due to temperature increases, algae overgrowth, or predation.
On Koh Tao they can be found in high abundance in shallow, flat reef zones such as Ao Leuk, Chalok Ban Kao, and Hin Wong, and the bays around Koh NagnYuan.
Commonly known as “crazy’ branching. Irregular, dense, bushy like corals. These corals require strong sunlight and currents in order to grow. They provide very diverse and complex habitats for juvenile reef fish and invertebrates. Are also a fast growing coral form, but tends to be susceptible to predation and temperature induced bleaching.
On Koh Tao these corals are very common in most island
Small, un-splitting branches which resemble fingers(‘digits’). Often provide important nursery areas for juvenile reef fishes. Coral extends the colony size through adding more branches, generally height of the colony does not change.
On Koh Tao, they are found all around the island, although they are not as common as Branching or Corymbose corals.
Thin, plate-like corals which grow along or over the existing substrate (they do not extend vertically). These corals tend to be very effective at out-competing or overgrowing existing living substrate such as sponges and other corals. On Koh Tao, they are highly susceptible to predation by both the Crown Of Thorns Starfish and Drupella snails.
Thin, plate like corals which grow in 3-D Shapes, often resembling plants (foliage). These corals tend to grow in areas with high levels of sunlight.
In some shallow reef areas of Koh Tao, a specific genus of Foliose coral, Pavona, is taking over the entire reefs. This may be due to the ability of this coral to deal with high levels of sedimentation, giving it the ability to out compete all other corals.
Thin, plate-like corals which grow in thin, horizontal sheets resembling shelves. These corals grow in areas with low wave action and clean water. These corals will not survive well in areas with high structural threats such as anchors, snorkelers, etc.
Dense, spherical or hemispherical corals. These corals are very slow growing, but tend to be very resilient. They are a major contributer to the long-term, solid structure of the reefs, and live to be tens of thousands of years old.
On Koh Tao, the largest and most prolific massive corals are that of the species Diploastrea Helopora, which can grow larger than 3 meters in diameter.
Dense, slow growing corals which are not hemispherical, but have a more irregular or columnar shape. These corals can also grow to be very large and old, and are a major pat of the strength of the reef to impacts and disturbances.
On Koh Tao, the most common and abundant species of Submassive coral is of the Porites Genus. This resilient coral grows from about 2m all the way to 20 meters depth, and is found at every reef and dive site.
Free living (non-attached corals). These are generally known as ‘mushroom’ corals, and are a very resilient coral which can grow in areas with dynamic substrates (rubble, sand, etc). Often these corals are a leader in the recovery of an ecosystem, known as a successional species, similar to grass after a forest fire. Where you find large patches of Solitary corals is often a coral reef recovering from some type of disturbance.
On Koh Tao, large fields of mushroom corals can be found around Chalok Ban Kao, Shark Bay, and Koh NagYuan.
Large, table shaped corals which create very diverse habitat for both small fishes, and large fishes underneath. They are generally made up of corals of the branching, corymbose, or digitate shapes, which grow horizontally at one depth, instead of increasing the height.
To see more pictures of corals, and to even try to identify some for yourself, visit our Koh Tao coral image gallery
If you are interested in learning more about coral identification, monitoring, and health assessments please check out our Ecological Monitoring Program Certifications available through both PADI and SSI.