Oh… they are just one of the most awesome and fascinating ecosystems in the world! Coral reefs are “made of” living creatures called corals that come in many different shapes and sizes. They are sessile, which means, that they need something solid to sit on while they do their thing. Usually, these cozy and solid places are skeletons of their already dead neighbors (or other solid structures like rocks, concrete etc.)
And what is the big deal with coral reefs?
Besides these ecosystems being completely awesome to look at they also hide other impressive stuff. Scientists estimate that the first reefs appeared about 240 million years ago, but today’s reefs have probably started growing “only” about 50 million years ago. The biggest coral reef of today’s time is the Great barrier reef (Australia). It is estimated to started growing about 20.000 years ago and today, it covers an area about 344 km2. It composes of about 2900 individual reefs and about 900 islands and can be seen from space. Fascinating, huh?
Yeah, yeah, that’s amazing but I still don’t get the point, why they get so much attention… well in a way I did. You see, the reefs have been around for a really long time and with this, they provided the feeding, living and nursing areas for many organisms. Scientists estimate, that currently, every fourth marine species is dependent on a coral reef at least at some point in its life. This is due to their “sticking around” for so long and giving the organisms enough time to adapt and find a special niche in this ecosystem. This is also the reason why we define corals as keystone species and ecosystem engineers.
To get a bit better idea of the importance of coral reefs, I feel obligated to provide some numbers you can relate to. Currently, there are about 212. 000 (+/- 29%) described marine species, and scientists annually add about 1000-1500! However, there are many speculations as to how many marine species still need to be found and described. The estimations are, that there are between 320.000 and 1,5 million (!) marine species out there free swimming hidden from our eyes!
What are corals?
Corals are actually very interesting animals (even though they don’t have fur, eyes or legs). They are one of the most long-living marine creatures and can reach up to 4000 years (deep-sea corals). Until now, we know of about 600 species of corals. The corals mostly grow very slowly (about 2,5 cm a year) but there are also some species that do it faster (for example the fastest growing coral can grow to 15 cm a year). They have the structure of a polyp, that is attached to the hard surface on one side and possesses the tentacles on the other (mouth). These tentacles contain stinging cells (nematocysts) with which the animal catches the close by swimming plankton. Because of their ability to sting we put them in the same phylum as jellyfish (Cnidaria). Inside the polyp there is also a digestive and reproductive system, that takes cares of the needs for continuing the family tree.
We divide them into two groups: the soft and the hard. Soft corals (look like this)are usually found in deeper water (to about 6000 meters of depth) and can live to up to 4000 years. On the other hand, the hard corals (that look like this) are the ones that we think of when we hear the word coral. The name hard corals come from the polyps, whose outer parts are made from calcium carbonate and makes them hard. Majority of them are found in shallow (to up to 50m) and warm waters (between 22-29 °C) around the equator but they can also be found in colder waters and at greater depths (map shown here).
The hard corals, living in shallow waters, have developed an interesting symbiosis with single-cell algae, called zooxanthellae (the brown stuff) (dinoflagellates from genus Symiodinium). The algae get a free spot on the coral and harness the energy with the process called photosynthesis. The extra energy they get is then being delivered to the coral and in return, the coral is providing the nutrients to the algae. This mutualistic benefit goes so far, that when a coral loses the algae (due to high water temperatures) many times or for an extended period of time, it leads to its slow death. The process, in which the coral loses their beloved partner is called bleaching.
The life cycles
Corals are also impressive to look at from the reproductive point of view. The coral can be a male, a female, or both and because of this, they are capable of reproducing either asexually or sexually. Asexual reproduction comes in handy, hen it is not your time of the year (or the lunar cycle) and you just want to increase the size of your colony, or after something bad (like a bad storm, or you are hit by fishing equipment) happens. There are two methods of asexual reproduction: budding (dividing of the polyp when it reaches a certain size) and fragmentation (after a disturbance). The downside to this is, that all of the organisms you make have the same genetic material (which is really bad for continuing your bloodline and adaptation to changes).
On the other hand, this is exactly what you get when you reproduce sexually. In the time of spawning, most corals release enormous numbers of eggs and sperm into the water (and in some species the fertilization happens inside of the body). All reproductive cells are brought to the surface (because of their light weight) where the fertilization process happens. As you all know, the ocean is really big, so in order for a success, the event is carefully scheduled and the amounts of these free-flowing genetic material is really, really big. After the fertilization, a free-swimming larva develops (that also presents a good snack for many organisms) that swims down to the sea floor after a few days in order to find a place to call home. The small polyps are preyed upon by some fish (parrot fish, butterfly fish), sea slugs (from group Nudibranchs) and sea stars (crown of thorns) species and need to compete and fight off the algae and seagrasses for their survival.
Problems of the coral reefs
Most of them are related to human activity in one way or another. We speak about overfishing and presence of the invasive species, that drastically change the relationships on the reef. Pollution, warming of oceans and changing of the chemistry drastically change the living conditions and decrease the overall ecosystem health. The rising water temperature leads to bleaching of the corals and the rising acidity dissolves the carbonate structures quickly, which makes it impossible for these organisms to survive. Another negative thing are the unaware tourists that break the corals just for fun or in order to bring them home as souvenirs. All of these changes influence the health and future of the coral ecosystems, one of the most fascinating and beautiful ecosystems in the world.
To fight this negative scenario there are many projects that work on saving the coral reefs. Scientists are doing their best to help the coral reefs. They collect sperm and eggs of corals on spawning occasions and fertilize them in the lab in order to enable higher survival rates. There are also companies like Calera that helps young corals find a suitable living habitat (eco-friendly concrete blocks). But of course, these projects don’t reach the desired potential, if the normal people (like you and me) are not educated about these problems and do not know the importance of a healthy ecosystem.
What do you think of coral reefs? Have you ever see them with your own eyes? Did you know all these fascinating facts about them and were aware of how important they are for the health of our planet? Let me know your answers in the comments below, I would be very happy to read them.