While many aquarium owners use light in their reef tanks, a growing number of them are becoming interested in corals that can thrive with little to no light.
As I have received numerous questions on this topic, I have decided to compile all the essential information in one place.
So, without further ado, let’s dive into it.
Can Corals Survive With No Light?
Most corals cannot survive without light because the zooxanthellae that live in their tissues require light to photosynthesize and produce the coral’s nutrients. However, a few types of corals don’t have zooxanthellae and don’t require light for their survival.
You have two considerations to keep in mind where a coral’s survival is concerned:
1. Coral Polyps Are Dependent On Zooxanthellae
It is easy to dismiss corals as rocks, especially when you find a piece on the beach. They have a similar texture.
However, exploring corals underwater will reveal polyps that live in the rock-like structure you call a coral.
Polyps have a tubular body and a mouth ringed with tentacles.
They extract dissolved calcium from seawater to make the hard skeletons you typically see. You will find hundreds of thousands of polyps in a coral colony.[1]
Some, like the hydrocorals, are dangerous. They have stinging cells that generate a burning sensation when you touch them.
Others look like fleshy plants (Octocorals). You also have the branching variety (Antipatharians).
The polyps matter to this discussion because they feed by snatching bits of food using their tentacles.
In other words, even though corals technically photosynthesize to meet their nutritional needs, they can also eat the traditional way.
However, these polyps make up a small portion of the coral’s nutrients, and the polyps themselves get what they need from zooxanthellae.
2. Zooxanthellae Require Light To Photosynthesize
If corals house polyps (creatures that use tentacles to eat), you should classify them as animals. But if that is true, how do they photosynthesize?
Corals have microorganisms known as zooxanthellae that live in their tissues.
Zooxanthellae are the reason why corals require light to grow. The algae use sunlight to make the nutrients corals consume.[2]
Corals cannot survive without zooxanthellae, and zooxanthellae cannot survive without light.
In that regard, you wouldn’t be wrong in saying that corals cannot survive without light.
But that seems contradictory. If coral polyps have tentacles that catch food, then surely, they can survive without zooxanthellae, right? Well, not necessarily.
Zooxanthellae provide a whopping 90 percent of a coral’s nutrients via photosynthesis.[3] The food a polyp’s tentacles catch is supplementary.
You can’t expect that food to sustain a conventional aquarium coral.
What Types Of Corals Can Survive Without Light?
Even though most aquarium corals require light, there are a few types of corals that don’t have zooxanthellae and don’t require light for their survival.
Here are five examples:
- Tubastrea spp. (Sun Coral)
- Rhizotrochus typus (Black Sun Coral)
- Order Antipatharia (Black coral)
- Dendrophylliidae (Dendrophyllia Coral)
- Balanophyllia spp. (Orange Cup Coral)
Be aware that these types of corals, also known as azoxanthalete corals, can react negatively if you accidentally expose them to light:
- Tissue damage: Azooxanthellate corals may not have the necessary pigments for light protection. Therefore, high levels of light are likely to result in tissue damage and even death.
- Reduced feeding: Azooxanthellate corals rely on feeding on planktonic organisms in the water column for their survival. By exposing them to light, you will reduce their feeding response and compromise their ability to catch food.
- Altered behavior: Azooxanthellate corals may respond to light exposure by retracting their polyps and avoiding the light source. This will significantly impair their growth and reproduction abilities.
What Types Of Coral Are Better Suited To Low-Light Conditions?
As you now know, deep-sea corals can survive at depths with little or no light. Therefore, they have the lowest lighting requirements on the market.
However, if you’re specifically searching for low-light corals for a reef tank at home, ask your local retailer for LPS (Large Polyp Stony) corals.
They have low – moderate lighting requirements.[4] In fact, LPS corals have large polyps. Therefore, you should feed them.
And if you’re looking for a new low-light coral for your reef tank, here’s a list of ten common species you can choose from:
| LPS Coral | PAR (Low to High) | Depth In Ocean |
| Acanthastrea | 50-100 | 5-60 ft |
| Blastomussa | 50-150 | 5-60 ft |
| Caulastrea | 50-150 | 5-60 ft |
| Euphyllia | 50-150 | 10-50 ft |
| Favia | 50-150 | 5-60 ft |
| Goniastrea | 150-250 | 10-50 ft |
| Lobophyllia | 150-250 | 10-50 ft |
| Micromussa | 50-150 | 5-60 ft |
| Symphyllia | 50-150 | 10-50 ft |
| Trachyphyllia | 10-50 ft | 150-250 |
What About Shallow And Deep-Sea Corals?
The information above doesn’t necessarily settle this discussion because you have the shallow corals VS deep-sea corals debate to consider:
1. Shallow Corals
Shallow species support the argument that corals cannot survive without light. They live in warm water bodies in tropical regions.
Their beauty and accessibility have made them an immensely popular public attraction.
Shallow corals use the sun to photosynthesize. Climate change concerns scientists because it creates extreme temperatures that bleach these species.
Shallow corals generate a green light to defend against intense sunlight, but this protective mechanism is not always enough.[5]
2. Deep-Sea Corals
Sunlight triggers photosynthesis in zooxanthellae. But that is only true for shallow corals.
Water naturally attenuates sunlight.
The deeper you go, the weaker the sunlight becomes. In fact, the sun’s rays typically disappear within the first two hundred meters below the sea.
And yet, some corals live at depths of up to 20,000ft below the surface. If you think this only applies to a handful of corals, you’re wrong.
Scientists have discovered more than 3,000 deep-sea corals thus far.[6] But how do those species survive?
Initially, experts assumed that deep-sea corals couldn’t photosynthesize. Instead, they relied on the food their tentacles caught to survive.[7]
However, recent studies have proven otherwise. First of all, sunlight isn’t as absent at the lowest depths as people think.
Colors with longer wavelengths will gradually disappear, leaving nothing but blue and violet because they have the shortest wavelengths.
If you think the smatterings of blue light at the bottom are insufficient for photosynthesis, you’re not wrong.
Joerg Wiedenmann has your answer. His team published a study that explained why some deep-sea corals glow so beautifully despite the absence of light on the ocean floor.
Apparently, deep-sea corals take the small amounts of blue light they receive, soak it in, and reflect red-orange light, which infiltrates the coral’s tissue, stimulating photosynthesis in the zooxanthellae.[8]
In other words, deep-sea corals have the ability to double the little sunlight they get.
This is similar to the way shallow corals use green as a form of sunscreen that repels intense sunlight. It also supports the idea that corals cannot survive without light.
Some scientists think shallow corals may survive the extreme heat originating from climate change by migrating to lower depths.
However, researchers don’t expect shallow corals to adapt successfully to low-light conditions at lower depths.
It is worth noting that some coral species can live without light. A prominent example that I mentioned earlier is the sun coral, which doesn’t contain zooxanthellae.[9] It survives by catching food with its tentacles.
However, most of the corals you find in a home aquarium require light. They cannot survive without it.
How Long Can Corals Survive Without Light?
Corals can survive the temporary absence of light. In fact, it is crucial to provide them with at least six hours of darkness during the night.
But besides that, the sun doesn’t shine every day in the wild. Corals must tolerate days, possibly even weeks, of cloudy weather.
Although, even on a cloudy day, shallow water corals have sufficient ambient light to photosynthesize.
Many aquarists deprive their reef tanks of light for three days at a time to combat algae, and their corals survive.
You can trust a healthy coral to go without light for a week. But the aquarium conditions play a significant role.
You need a well-maintained aquarium with the correct ammonia, nitrate, nitrite, magnesium, and calcium levels.
You should also maintain the appropriate temperature and carbonate hardness for the coral species in your tank.
What Are The Cons Of Keeping Corals In Low-Light Environments?
It depends on the coral species.
Some aquarists have theorized that low-light conditions can starve the corals because they compromise zooxanthellae’s ability to photosynthesize.
However, corals have different levels of tolerance to light. They can also acclimate to nonconducive lighting conditions.
Therefore, you can’t predict the response you will observe.
Although, your corals will probably turn brown because their zooxanthellae populations will increase to compensate for the low-light intensity.
Pro Tip: Feeling uncertain about how to light your coral tank? Don’t worry, I’ve got you covered with my ultimate guide on the topic.
Conclusions
If you’re in a hurry, here’s a quick rundown of the key points mentioned above:
- Most corals require light for their survival as zooxanthellae, the microorganisms living in their tissues that provide 90% of their nutrients, require light to photosynthesize.
- Some corals can survive without light, but they are azoxanthalete corals, which do not have zooxanthellae, and their exposure to light can result in tissue damage, reduced feeding, and altered behavior.
- Deep-sea corals can survive with little or no light, and LPS corals (Large Polyp Stony) are better suited to low-light conditions, requiring low to moderate light.
References
- https://www.epa.gov/coral-reefs/basic-information-about-coral-reefs
- https://www.whoi.edu/know-your-ocean/did-you-know/are-corals-plants-animals-or-rocks/
- https://www.sciencealert.com/corals-at-the-bottom-of-the-sea-produce-their-own-sunlight-to-survive
- https://www.piecesoftheocean.com/blogs/poto-articles/how-to-care-for-lps-corals
- https://www.smithsonianmag.com/smart-news/deep-water-corals-glow-their-lives-180963967/
- https://ocean.si.edu/ecosystems/coral-reefs/deep-sea-corals
- https://www.fisheries.noaa.gov/national/habitat-conservation/deep-sea-coral-habitat
- https://www.rjeint.com/deep-sea-coral-create-light/
- https://www.aquariumofpacific.org/onlinelearningcenter/species/sun_coral