Coral: The animal
The Coral Organism
For a long time, scientists were unsure of what corals really were calling them "plantanimals". They have now determined that corals are actually tiny animals belonging to the Phylum Cnidaria. Corals are anthozoans (members of the class Anthozoa) and are sessile, polyp-shaped organisms.
Living coral polyps live in a cup-like skeleton that is made up of calcium carbonate that the organisms continually lays down. The coral builds up the skeleton beneath them so the animal will grow upward and outward. In polyps, the mouth is in the center of a ring of tentacles on the "top" side of the organism. The tentacles contain stinging cells used to capture food and for defense. They have an incomplete gastrovascular cavity which means the mouth is where food is consumed and wastes are expelled.
*"Fun" Fact: Most hard corals (pillar coral being one exception) shun the light of day and feed only at night. When feeding, the polyps open and extend their tentacles into the water column trying to capture tiny planktonic organisms.*
For a long time, scientists were unsure of what corals really were calling them "plantanimals". They have now determined that corals are actually tiny animals belonging to the Phylum Cnidaria. Corals are anthozoans (members of the class Anthozoa) and are sessile, polyp-shaped organisms.
Living coral polyps live in a cup-like skeleton that is made up of calcium carbonate that the organisms continually lays down. The coral builds up the skeleton beneath them so the animal will grow upward and outward. In polyps, the mouth is in the center of a ring of tentacles on the "top" side of the organism. The tentacles contain stinging cells used to capture food and for defense. They have an incomplete gastrovascular cavity which means the mouth is where food is consumed and wastes are expelled.
*"Fun" Fact: Most hard corals (pillar coral being one exception) shun the light of day and feed only at night. When feeding, the polyps open and extend their tentacles into the water column trying to capture tiny planktonic organisms.*
Almost all reef corals have a symbiotic relationship (mutually beneficial) with an algae called zooxanthellae (zoh-zan-thell-ee). Corals are carnivorous animals; however, they cannot capture enough food on their own. This plant-like algae lives inside the polyps and undergoes photosynthesis, producing food which is shared with the coral. In return, the algae has a safe place to live and use the coral's waste to help grow.
Coral Growth
Even with the help of zooxanthellae the process of producing your own skeleton is slow. In ideal conditions, corals can increase anywhere from 0.5 to 2 cm per year. Under favorable conditions (lots of light, warm water, moderate waves), corals could grow as much as 4.5 cm in one year. Growth can be seen in coral growth rings (much like trees). |
Coral Reproduction
One of the biggest challenges for corals is reproducing. Since they are attached to the sea floor they must figure out a way to mate. Corals can reproduce asexually and sexually. Asexually, corals can form buds or clones of themselves--but this limits genetic diversity and makes them vulnerable to disease and genetic defects.
Most coral species will sexually reproduce by spawning: the release of eggs and sperm into the water. When the egg and sperm meet in the water, a planula (a form of larvae) forms. Coral spawning appears to be linked to the lunar cycle. In recent years, scientists have learned how corals will release hormones called pheromones that "tell" neighbors it's time to spawn. While not completely understood, it is well-documented that many of the dominant corals in Florida, the Bahamas and the Caribbean spawn during the 8th night after the August full moon.
A few species are brooders: this is where only the sperm is released into the water and females will take it in and fertilize internally. This planula is then released through the mouth of the female coral and drifts/crawls away to colonize elsewhere.
One of the biggest challenges for corals is reproducing. Since they are attached to the sea floor they must figure out a way to mate. Corals can reproduce asexually and sexually. Asexually, corals can form buds or clones of themselves--but this limits genetic diversity and makes them vulnerable to disease and genetic defects.
Most coral species will sexually reproduce by spawning: the release of eggs and sperm into the water. When the egg and sperm meet in the water, a planula (a form of larvae) forms. Coral spawning appears to be linked to the lunar cycle. In recent years, scientists have learned how corals will release hormones called pheromones that "tell" neighbors it's time to spawn. While not completely understood, it is well-documented that many of the dominant corals in Florida, the Bahamas and the Caribbean spawn during the 8th night after the August full moon.
A few species are brooders: this is where only the sperm is released into the water and females will take it in and fertilize internally. This planula is then released through the mouth of the female coral and drifts/crawls away to colonize elsewhere.