Coral is the name of invertebrates’ animals from Cnidaria phylum . They are ancient animal related to jellyfishes and anemones. Their hard skeleton is shaped by calcium carbonate, which is produced by polyps that are individual coral. We find them in colonies with many individual polyps that are all identical according to species. Polyps are cylindrical in shape and elongated at the axis of the vase-shaped body. They also have tentacles surrounded a central mouth opening. An exoskeleton is excreted near the base and throughout generations, the colony creates a large skeleton giving protection and force. Concerning their reproduction, this is an asexual reproduction and for their food, polyps may eat microscopic zooplankton and also small fishes. Neverthless, how can they eat fishes? This is simple, polyps have tentacles that are going out from their mouth and thanks to their nematocysts, which are small venomous cells, polyps kill small fishes by injecting venom inside of their prey. And then, tentacles are contracting to bring the prey into the stomach. After digestion, the mouth is opened again for another cycle. This is with polyps that corals are feeding at night when their tentacles extending.
Corals are sessile animals in the Anthozoa class. This is to say they are immobile animals in the same place while their whole life. In their life’s cycle, they do not have a medusa stage, which means that the most corals are colonial and the initial polyp is going to bud in order to produce another ones and thanks to that the colony is going to develop from the first one. Ultimately, it will give coral that we observe.
Biodiversity of coral reefs
Coral reefs are home to 25% of all marine life and are the basis of other ecosystems formation. Furthermore, thousands of animals and vegetal are associated with corals such as parrotfishes that are shaping great sand stretches conducting to the formation of the sea floor, especially where there is a mangrove formation and also coastal forests.
Tropical and subtropical oceans are where corals are found until 45 meters deep.
The four biggest coral reefs in the world are situated in Australia, in New Caledonia, in Belize, and in Florida. The most ancient coral reef is in Kimbe Bay in Papua New Guinea with a concentration of 60% of coral species. Indonesia, the Philippines, Maldives or Papua New Guinea hold an important part of world corals.
For their survival, coral reefs need sunlight, clear water, warm water temperature (20-32°C), clean water and saltwater. Sunlight is especially for algae that are in symbiosis with corals that are completely depending on those vegetables, which need sunlight to do photosynthesis. Clearwater is also essential for corals in order to let sunlight through and going directly to algae. It is evident that warm water temperature can be dangerous for corals but between 20-32°C, it is a good temperature so they can live. Furthermore, clean water is important knowing that corals are sensitive to pollution and sediments. Indeed, sediments can obstruct sunlight’s passing and algae cannot do photosynthesis anymore. Saltwater is an essential factor for corals surviving but require a certain balance in the ratio of salt to water.
Coral reefs are an ecosystem, which is vital for a lot of marine species such as fishes but they also protect coastlines from many diverse effects such as wave action and tropical storms. Furthermore, coral reefs provide habitats and shelter for marine organisms where they can lay and live. From a scientific point of view, coral reefs are the source of nitrogen and essential nutrients and also assist in carbon and nitrogen fixing. They help with nutrient recycling as well. For those many reasons, this is why a large number of marine species are living on reefs.
Importance of coral reefs for humans
Coral reefs are also very important for humans. Indeed, they provide fisheries and so food for people, they bring tourism that creates jobs and bring incomes, and they allow advances in the medical field. On the natural side, coral reefs are protecting coastal littoral and may avoid disasters.
Main threats to coral reefs
They are a number of factors threating corals. Most of them are natural and other ones are caused by humans. Corals have survived 10’000 of years of natural change, but many of them are going to die because they are not going to survive to the human activity.
- Climate change is a huge problem that cannot be missed. Corals must live in a 20-32°C-water but not too high. If the water temperature is too high, bleaching’s process is going to be harmful to them.
- Destructive fishing practices must abolish such as cyanide fishing, whose, the goal is to squirt sodium cyanide into the water in order to catch easily fishes without killing them. Dynamite fishing is another technique for which dynamite or other explosives products are used in order to scoop dead fishes up easily. The explosives are destroying, at the same time, underwater environment. Bottom trawling, ghost fishing are other destructive fishing practices.
- Overfishing is also a threat that causes an effect on coral reef population. Careless tourism such as diving, snorkelling, and fishing are part of a threat as well. Indeed, by touching reefs, collecting corals, and many more, it contributes to the destruction of this ecosystem.
- Pollution is a huge problem caused by industrial activity. Wastes are poisoning reefs by being thrown away directly into oceans. On the other hand, some pollutants are increasing the level of nitrogen in seawater affecting an overgrowth of algae and avoiding sunlight passing through.
- Coral mining for bricks making, briques et cement for new buildings is an issue that is increasing. Sedimentation, which is another issue is caused by construction along coasts.
Coral reefs are not only affected by human activity, but also by natural effects such as the crown of thorns starfish. This animal is part of phylum Echinodermata and is corallivore, which means it is destroying corals by eating polyps. It is covered in long poisonous spines that cause necrosis of tissue and are toxic for many species including humans, which means this species doesn’t have many predators. It lives in tropical zones of Indo-Pacific Ocean in coral ecosystems. They only feed corals, more precisely polyps.
Corals have not only crown of thorns starfish as predators, but they can also die naturally because of the loss of algae. Warm temperatures of oceans cause this problem. Indeed, at high temperatures, algae are expelled from corals and they are white, which is the colour of their skeleton. As we know, high temperatures are caused by climate change and this factor can be problematic in the long term. Bleaching is an undeniable factor but acid seas’ factor is another one. Furthermore, carbon dioxide in the atmosphere is captured by seas and is becoming acid. What is exactly happening? In short words, oceans are absorbing carbon dioxide and dissolving it by forming acids. Corals and molluscs are the first species that suffered from this acidity.
What does exactly mean coral bleaching?
Coral bleaching is a result of warm temperatures of the ocean. Indeed, corals are in symbiosis (= endosymbiosis) with microscopic algae named as zooxanthellae that are living inside their tissues. This relationship between corals and algae is primordial for both of them and for the whole reef’s health. Those algae by living within their tissues give colouration to coral. They provide nutrients thanks to photosynthesis and in return corals give carbon dioxide and ammonium for photosynthesis. Corals use glucose (= sugar) and oxygen produced from algae to produce calcium carbonate for their skeleton. The zooxanthellae provide for up to 90% of the coral’s energy needs thanks to photosynthesis. After expelling, corals begin to starve and if polyps die of starvation, corals will decay. Corals can survive without algae but not for a long time if it persists its chance of survive will diminish. For the survival of corals, zooxanthellae must enter again the tissues of corals polyps and doing again photosynthesis.
As we already know, corals provide food, jobs, income, and protection to humans being all over the world. Nevertheless, coral reefs are endangered with all marine animals that need coral reefs for their reproduction, and essential for living.
Pollution, invasive species, diseases, bleaching, and global climate change are part of the factors that cause their loss. If anything is taken to protect them, we are going to notice a rapid decline and the loss of a significant ecosystem, which will have huge social, economic, and environmental consequences around the world. This is why, this is important to be interested in corals and their issues because it will have a direct impact on the terrestrial and marine world if nothing is undertaken for coral’s survival, which is, above all, animals.
Photosynthesis: This is a process used by plants and other organisms to convert light energy into chemical energy. Carbon dioxide + water are transformed into sugar and oxygen.
Echinodermata: This is a phylum coming from the kingdom of Animalia. In this phylum, you can find sea stars, sea urchins, sand dollars, and sea cucumbers among other species.
Symbiosis: There are two organisms that need to live together in order to receive and bring nutrients to the other one and vice versa.
Zooplankton: This is animal plankton in contrast to phytoplankton that is vegetal plankton. It is going up at night to feed and going down to avoid predators. This is polyps’ food.
Nematocysts: This is an organ that is found inside of cnidocysts that are epidermis cells of Cnidaria. This little organ is able to inject a venom that paralyzes the prey.
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