coral (Anthozoa) It may seem like a simple rock but in reality it is an invertebrate animal and essential in the aquatic world. It is very similar to the anemone. Currently, there are an estimated 70.000 different species of coral that inhabit oceans around the world but are more abundant in the oceans of the southern hermisphere where there are warm and tropical climates.

All these species are divided into two subspecies: Alcyonaria y Zoantharia. Corals of the Alcyonaria subspecies have eight tentacles including soft corals, underwater fans, and sea pens. Those of the subspecies Zoantharia with more than eight tentacles which include all the species that live in coral reefs.

Corals

Coral colony

Features

Coral plays a very important role in creating coral reefs where a great diversity of species of aquatic animals inhabit and protect themselves from other predators. They are also a meeting point for thousands of species, especially fish, to reproduce.

They are not plants, but they are extremely simple animals that lack a backbone, eyes, ears, nose, or limbs. They are symmetrical creatures around a central axis. They are considered sessile since they cannot move from one area to another like other animals to search for food or flee. Therefore, they have tentacles with stinging cells (which sting like a nettle) that are used to feed and protect themselves.

Its life lasts between 3 months to 30 years, depending on the species and its role in the ecosystem. For example, coral species that make up extensive coral reefs such as the Australian live much longer than other solitary coral species.

The size varies between species, the smallest corals can measure between 1 - 3 ml in diameter, while other species can measure between 10 - 25 cm. When they are united in colonies, the corals can form structures up to 4 m in height and grow one meter each year. They can get to "grow" so much that the Great Barrier Reef is visible from space.

The color is due to the zooxanthellae, organisms that live in their tissues and give them different shades of brown, green or orange. If they do not have zooxanthellae, they will look transparent. However some species have various colors such as blue, green, purple, white, orange and yellow.

Hard corals

The body is formed by polyps that are marine invertebrates that in their development take the form of a closed tube at its lower end where it is attached to the rocks or the seabed by means of a peduncle and open at the other end of the mouth surrounded by tentacles . Sometimes it is protected by a hard exoskeleton formed by calcium carbonate to protect the soft polyp.

Soft corals

Soft corals lack an exoskeleton to protect them, but they do have slightly hardened, resistant and elastic tissues. They can branch out, some adopt a figure similar to the horns of a deer.

Habitat

Coral is found in tropical and subtropical waters throughout the entire planet. However, there are also species that inhabit the polar areas. Reef formations are located in the tropical and subtropical areas of the Indo-Pacific and the western Atlantic Ocean.

As all living things depend a lot on sunlight, that is why they live in shallow waters (less than 60 m) and crystalline. However, there are always exceptions with some species of coral that can live at depths of up to 3.000 m that have adapted to their environment.

Reef corals are very close to the surface and prefer areas with strong waves because they carry food and oxygen.

Coral reef

Coral reef

Food

The polyp feeds on a wide variety such as plankton or small fish. They hunt their prey by extending their tentacles to immobilize or kill it using their nematocysts, which are cells that carry venom that is released by a spike as soon as it comes into contact with other organisms. Once the prey has been captured it is moved towards the mouth by its tentacles.

Once the prey has been digested by the stomach, it is reopened, this time expelling the waste through the mouth and returning to the cycle again.

Predators

Coral predators include starfish, sea slugs, and sea snails.

Reproduction

Coral has different ways of reproducing as they can be gonochoristic (unisexual) or hermaphroditic and each of the forms can reproduce both sexually and asexually. Reproduction enables them to settle in new areas and is coordinated by chemical communication.

Sexual

It is the most predominant sexual route hosted by approximately 25% of the corals that are hermatypic (stony corals), forming colonies of only one sex, the rest are hermaphrodite corals.

75% of hermatypic corals spawn by diffusion by releasing gametes (eggs and sperm) into the water to disperse their young. The gametes will fuse to form a larva called planula, which is pink and elliptical in shape. Several thousand larvae are formed to ensure the formation of new colonies.

When it comes to reefs, the corals will start spawning synchronously, that is, all the corals spawn on the same night. This is so that the male and female gametes are more likely to meet. The right time to release the gametes depends on each species and on environmental signals such as the change in temperature, lunar cycle, length of day and chemical signaling. Usually the most suitable signal is the sunset.

Creators

Within the sexual species, are the breeders that are often ahermatypic (species that do not build reefs). Here the sperm are released by the reproductive corals, which sink into the unfertilized egg-bearing corals, the wait can last weeks. After fertilization has been done, the planules that are ready to settle on the sea floor will be released.

Planulas

Planula larvae swim towards the light and sounds emitted by reefs, away from open water, to reach surface waters. While drifting they will grow before descending in search of a suitable hard surface to settle and start a new colony.

The success rate involves many generations, and although millions of gametes are released, very few new colonies are formed. From spawning to settlement it can take three to two months. Once settled, the larva grows to become a polyp and will occasionally become a coral head due to broe and asexual growth.

Polyp

Polyp

Asexual

Within a coral head, polyps that are generically identical reproduce asexually by budding or by division, either longitudinally or transversely.

Outbreak reproduction involves separating a smaller polyp from an adult as the new polyp grows, forming the parts of its body. The sprout can be intertencaular producing polyps of the same size within the ring of tentacles, or extratentacular, from its base, producing a smaller polyp.

In division reproduction the original polyp divides in two and becomes as large as the original. In longitudinal division, the polyp widens and divides its body along its length. The mouth divides and new tentacles form. In the transverse division, the polyp and the exoskeleton are divided transversely into two parts that will have to generate the missing parts separately.

Asexual reproduction favors a high reproductive rate, delayed senescence, and replacement of dead modules, as well as geographic distribution.

Colonial division

The entire colonies can reproduce asexually forming two colonies of the same genotype. Mechanisms to achieve them include fission, salvage, and fragmentation.

  • Fission: It occurs in some corals where the colony divides into two or more colonies during the early stages of development.
  • Rescue: A single polyp leaves the colony to form another at a different site to create a new one.
  • Fragmentation: It happens when the colony has been fragmented during storms or other natural disasters. In these cases the separated polyps can start new colonies.

State of conservation

Studies by various scientists and environmental organizations estimate that by 2030 more than 50% of the Carolinas formations in the world will have disappeared. In this case, not only humans with their fishing and diving are to blame. Natural disasters such as earthquakes or tsunamis also destroy corals by fragmenting the soil where they live. Hundreds of coral reefs were destroyed in the 2004 Indian Ocean earthquake.

The classification of corals varies according to the species, but for example, the staghorn coral (due to its similar shape with the horns of a deer) is currently classified as an endangered species due to its limited decrease in its distribution.

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