Coral part 2

Photo taken by Dr. Alexander Mustard. You can find more of his amazing photos on his website

Last time, I mentioned all the factors that determine a coral’s success in an area, and most of those factors relate to hard coral or reef building coral. Soft coral are a bit different from their constructive cousins; for instance, they don’t all rely on zooxanthellae, and so most of this information pertains to hard corals—but don’t worry, I’ll get to soft corals eventually!

For hard coral, the factors that affect their success in an area include water temperature, salinity, depth, water circulation, and clarity.

Most of these factors aren’t hard to explain because we can relate to them. For instance, we don’t like to be too warm or too cold. It’s the same with coral: they don’t like their water to be too warm or too cold, so their ideal range is between 85°F‒70°F. This is not a very strict range because there are reefs and species found outside these temperatures, but for the most part this is important for reef-building coral.

Hard corals also don’t like their water to be too fresh or too salty. They prefer their water to have a salinity level between 30 and 40 parts per thousand (ppt).

Next, we’ll talk about water circulation. This one is really important because coral can’t move elsewhere once the food is gone, they’re sedentary. They’re not like humans, who can get off the couch and grab food when they’re hungry. Coral pretty much have to be in an area that always has food—so it’s like if your cousin decided to live in the supermarket for the rest of their life. Coral feed on zooplankton, which for now let’s describe as plankton-size animals, and that means that the coral’s food isn’t regularly replenishing. Instead, coral rely on water circulation, either through water currents or wave actions, to stir up the water and bring nutrients and food to them. Without good circulation, coral can easily starve, even the hard coral that obtain most of their energy from zooxanthellae.

The final two factors determining coral survivability are especially important to hard coral because of their partnership with zooxanthellae, and these factors are related to sunlight: water clarity and depth. Water clarity is defined by how clear the water is. If the water is really cloudy and full of particles, then the zooxanthellae may not be able to receive enough light for photosynthesis. Depth has similar effects: sunlight can only penetrate so far into the ocean before the only visible light comes from bioluminescent creatures. If the coral is anchored too far from the surface, then the zooxanthellae may not get enough light and the pair can die.

There are coral that can survive outside of these factors; for instance, they have discovered coral in the deep ocean at depths of 6,000 m (20,000 ft)—that’s almost four miles below the ocean’s surface. Deep-sea coral are fascinating because they vary from their shallow-water relatives, but I’ll get to those later.

If the hard coral’s habitat changes beyond these set factors, then it can have devastating impacts on the organism. If the conditions don’t return to the normal, then the coral will expel their zooxanthellae in a process called Coral Bleaching. Some research has suggested that the coral can reacquire their zooxanthellae or even acquire new zooxanthellae. However, there are still a lot of unknowns in regards to Coral Bleaching and coral health, which is why continued research is so important!

If the coral die then the reef dies, and from there it’s a domino effect that will very quickly impacts us. Without the reefs the world fish populations will dramatically decrease. Less fish means less food for the animals that live in the ocean, and less fish for us as well. If we compete with creatures like dolphins and whales for food, then those populations of animals will dramatically decrease as well. All in all, coral reefs are the rainforests of the sea in terms of biodiversity, habitat, and oxygen production.

It’s not all desolate and hopeless. There is research being done through the field of aquaculture in trying to grow coral ourselves to help save the reefs, most of which are being done at aquariums and universities. Researchers are trying to figure out ways to preserve the reefs that we have now, and to protect them from further climate-change-inflicted damage.

However, a grim-looking future faces us if we can’t curb the effects of climate change on our oceans, and that’s why YOU are so important in this struggle too. Everything you can do, big or small, to saving energy and reducing plastic use to conservation research programs will help lessen the impact of climate change and preserve our reefs for future generations.


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