Coral reefs of Similan

Coral reefs: Biology and growth


Chances are you have heard of coral reefs. If you are a diver then in all likelihood you have dived on a coral reef and like the rest of us have been amazed and stupefied at the sheer size, color and abundant life of these wonders of nature. Before you dismiss reefs as just a fantastic collection of multi-colored rocks and plants, here is one amazing fact. They are neither; in fact, corals are animals that breath, eat, reproduce, and die like any other member of the animal kingdom. To be specific, corals are invertebrates of the family Cnidaria.

Distribution of Coral Reefs:
distribution coral reefs


Cnidarians There are over 9,000 species of Cnidarians that inhabit the aquatic realm. This amazing class of animal is distinguished by a highly specialized hunting system. They have a stinging cell called a nematocyst. This is a barbed harpoon that the animal uses to stun and capture it’s unfortunate prey.

The versatile Cnidarians come in two distinct flavors of body types. The first is the medusa, which is a free swimming creature. The second type is the polyp, which is fixed and does not move. Both types are round and symmetrical, and possess a mouth which is surrounded by tentacles armed with stinging cells. Many Cnidarians live in colonies that is a single organism composed of thousands and sometimes millions of individual polyps.

Hard coral polyps vary in size depending on the type of corals. Solitary coral polyps can reach up to 25 cm, which is truly enormous in the world of polyps. Most polyps that congregate in colonies of thousands if not millions range in diameter from 1 to 3 mm. These tiny little polyps are the foundations of coral reefs.

How are corals classified?

All corals, including the magnificent reef building hard corals, are part of the class Anthozoa. This class of 6,000+ animals includes hard corals, soft corals, gorgonians and even the humble anemones. Within this class there are 2 distinct subdivision of corals.

Hexacorals Hexacorals: as the name would suggest, these 4,000+ species are made up of polyps with a mouth that is surrounded by 6 tentacles or multiples of 6 tentacles. They create a hard limestone skeleton which is the foundation of coral reefs. Types of corals included in this subdivision are table corals, staghorn corals, brain corals, and mushroom corals among others.

Octocorals Octocorals: these corals, like there hexacoral relatives, are composed of polyps with a mouth, but in this case the mouth is fringed by 8 tentacles. Unlike hexacorals, they do not secrete a limestone skeleton and are actually soft throughout their whole body. There skeleton has been so reduced that it is now composed of needle-like structures that are buried deep in the animals’ soft tissue. This subdivision includes gorgonians, sea whips, broccoli corals, and sea fans.

hexacorallia vs octocorallia

Reef Builders

Coral reefs are defined as the limestone structures laid down by hard corals, with a living coral outer layer. Hard corals fall into two distinct categories:
Non-zooxanthellate corals: these types of hard corals lack the symbiotic algae and are not capable of building reefs. They are also more rarely encountered by divers since they thrive in cold and deep waters and derive their energy from the capture of plankton and other suspended particles. They are abundant and at their best at depths below 5,000 meters.

zooxanthellate corals Zooxanthellate corals: these are the primary hard corals responsible for building reefs. They have a truly amazing symbiotic relationship with several species of zooxanthellae algae that enables them to build reefs.

One hell of a partnership

coral polyp diagramPolyps have an amazing symbiotic relationship with zooxanthellae algae, tiny unicellular green algae living inside the polyps’ tissues. The polyp provides the algae with an abundant supply of nutrients and a protective environment, as well as a healthy supply of carbon dioxide from its own respiration. In return, the algae use its ability to photosynthesize to turn carbon dioxide and water into oxygen and carbohydrates using the sun light. The grateful polyps happily use the oxygen to breath and the carbohydrates as the key fuel in their never-ending task of laying down more and more limestone skeleton.

This relationship is so well developed and efficient that the algae provide the polyps with up to 90% of its nutrients. This leaves the polyp the simple task of only hunting for 10 % of its food, so it can focus the rest of its effort on reef building. As a result, these algae are considered absolutely essential to the health and productivity of any coral reef.

Coral sex

coral spawningPolyps are capable of some interesting methods of reproduction. Being hermaphrodites, polyps can reproduce asexually, producing clones of themselves, but they do need to have sex to maintain genetic diversity. As a result, they are also able to reproduce sexually – during carefully coordinated mating sequences, corals release a cloud of eggs and sperm into the water column that mingle and fertilize to produce the next generation of polyps – this is called mass spawning.

With a harpoon for hunting, are corals herbivores?

Corals are blessed with enormous diversity when it comes to their food options, but they are actually carnivores, using their barbed harpoon cells to capture passing plankton and larvae in the current, as well as some bigger prey like small fish.

Which of the Similan dive sites have the most corals?

Richelieu Rock is the most colorful dive site around the area, covered of soft coral, the pinnacle hold a wide variety of marine life.

How old are some of the giant coral pinnacles we see when diving?

This can be somewhat of a misleading assumption, for although coral is very slow-growing, only a few mm or cm per year, the massive corals we see would be extremely ancient. Some reefs we know are actually 5,000 to 10,000-year-old. This paradox is explained by how corals grow. Even on the massive corals that are the size of a truck or small house, not the whole structure is composed of live corals. In fact, the only part that is alive is a thin crust of polyps that inhabits the surface. Everything underneath that is actually dead coral and is merely the surviving limestone skeleton of the animal.

As an animal, how do corals clean and defend themselves?

Well with only one orifice, the mouth of a coral polyp has to become quite multi-functional. The polyp uses its mouth not only to consume food, but also to expel waste products, and even to clean itself. The polyp does this by continuously secreting a layer of mucus. This layer serves to wash away debris, maintain the polyps’ hygiene and repel diseases and particles. This mucus layer is the main reason why as divers we should never ever touch coral. (Not even with one finger to steady ourselves when taking a picture). Touching the coral strips the polyp of its mucus layer and leaves it vulnerable to disease and death.

So how do corals defend themselves?

Polyps do not only use their tentacles as a means of hunting for prey, but these versatile appendages serve as a fearsome defensive weapons too. Each tentacle has an array of stinging nematocysts buried deep within the soft tissue. These cells are capable of delivering a very powerful and painful sting to anything careless enough to approach the polyp. In some cases, the nematocyst is capable of delivering a dose of lethal toxins.