Though coral may look like a rock, it is actually made of organisms that form a colony, and these organisms are called coral polyps. Polyps take millions of years to expand and become a coral reef and start when a polyp attaches itself to a rock and divides. They grow in a huge group because they are colonial organisms and depend on each other to survive.
Above: the Great Barrier Reef, the largest coral reef in the world
Coral polyps are usually less than half an inch in diameter. They are invertebrates and are related to jellyfish and anemone. All polyps have a protective limestone base, which serves as their anchor to a piece of rock or a nearby polyp. They also are shaped like a sac, with the “sac” being 2 layers of cells and the inside being the inside of the polyp. Corals are connected to each other with a jelly-like tissue which allows them to share nutrients. Tentacles bud off from the top of the polyp and they are used to catch prey, including zooplankton
below: basic coral anatomy
and small fish. Interestingly, they are able to sting the prey because they carry venom. In addition, they open up during the night and are retracted during the day time.
Coral get their bright colors from a certain type of algae called zooxanthellae. The zooxanthellae are the hosts of coral polyps. Without the zooxanthellae, corals would appear to be transparent. Coral polyps also rely on zooxanthellae for food: they are able to steal the nutrients the zooxanthellae produce from photosynthesis. In return, the zooxanthellae are able to use many of the coral’s waste products, forming a symbiotic relationship.
There are two types of coral polyps: soft and hard (also called stony). Soft coral has no solid exoskeleton, though it is reinforced by small pieces of calcium carbonate. They do not form reefs, although they are present in coral reefs. Most soft corals also look quite jelly-like and, not surprisingly, soft. Hard coral has a hard solid exoskeleton made completely out of calcium carbonate. They are extremely important for reef building.
How coral polyps reproduce
How do coral reproduce if they are stationary organisms? They have two main ways of reproducing: creating a clone of themselves (asexual reproduction) or finding another coral and combining their genetic information to make a new coral (sexual reproduction).
Clones of the parent polyp are created when a new polyp buds off, or when the parent polyp is split in half and it is able to grow. This is similar to plants that are able to continue growing when you cut a branch off, stick it in the ground, and water it.
Sexual reproduction requires both male and female reproductive organs , and even though polyps are unable to move, they are able to reproduce by two methods called broadcast spawning and brooding. Corals can contain both male and female parts, like some flowers. Broadcast spawning is when all the coral in an area release their gametes in a synchronized time and those gametes are able to meet each other to produce a new polyp. The timing of this is very hard and depends on environmental cues. When broadcast spawning happens, the water around the area is murky due to how many gametes are dissolving in the water. 75% of all corals do broadcast spawning.
Above: corals using broadcast spawning. The water around is murky due to the released larvae/ gametes.
The other 25% of corals use another method called brooding, when coral polyps release their sperm and rely on them to land on carriers.
The importance of corals
Coral reefs harbor 25% of all the marine life on the planet. They create a very diverse and colorful ecosystem. But what makes corals so special?
Corals are able to protect marine life and the shoreline from rough waves. They serve as a buffer and are able to reduce the energy of incoming waves by about 70-95%. This way, they prevent damage to important producers in the ecosystem, such as seagrass and seaweed. Once those important producers are able to survive, more animals will be drawn in.
In addition, coral polyps are able to harbor bacteria that fixate nitrogen gas. Nitrogen gas (N2) has no nutritional value, but it can be fixed into molecules that seagrass are able to use as nutrients, such as NH3 and NH4. After being fixed, these molecules can be used by seagrass and can enter fishes’ bodies, continuing the nitrogen cycle that other animals depend on.