Aquarium Corals

by Ross Gottlieb, DVM

The popularity of saltwater aquariums is undeniable, and one of the most attractive features of these systems is there ability to sustain beautiful and diverse corals. Corals are actually animals, and not plants. They are unable to make food for themselves, as plants are, and they rely on their symbiotic zooxanthellae algae to do that for them. They also have a gastric cavity consistent with animals and not plants. Coral polyps also possess stinging cells called nematocysts, which are used for both prey capture and defense.

Symbiotic algae

Corals possess symbiotic algae that live inside their polyp tissue known as zooxanthellae. The algae feed on the ammonia waste produced by the coral, while providing the polyps with nutrition from photosynthesis. Not all corals possess zooxanthellae, and these corals require strict and frequent target feedings, making them a difficult inhabitant to keep in home aquaria. Different species thrive at different wavelengths of light and water conditions. Corals may expel some algae every day- this is not a bleaching event- they are simply making room for a new population. This algae is responsible for giving coral its beautiful colors, so it’s important not only to keep your corals happy, but their zooxanthellae as well.

Diversity and Importance

Coral reefs are amazing in that there are 1-3 million species of invertebrates and fish that make their homes on coral reefs. However, they only occupy about 255,000 square kilometers of our earth. For such an amazing amount of biodiversity to be found in such a relatively small area of the world is pretty amazing, and we have only really explored and catalogued about 10% of our world’s reefs. Reefs have many benefits besides providing homes for many species. Coral reefs have given us the aquarium trade, which provides many jobs to people throughout the world, stimulating economy in even third world countries. They also provide protection to many coast lines from heavy wave action. Lastly, they utilize CO2 to produce carbonate for use in their growing skeletons. Because of this, corals can act as a CO2 sink in our oceans, and make a difference in global warming and ocean acidification. However, excessive atmospheric CO2 can hinder their ability to build their skeletons

What do captive corals need?

Different species of coral have different needs based on the ecological and physical niches they occupy in the ocean. Our goal as aquarists is to mimic the optimal environmental conditions for these animals to the best of our ability. Many factors go into consideration- we have to think about what type of water flow they would do best in, how close to the surface they generally sit on the reef, how close they can be to other coral species (we will talk about physical and chemical competition later), ideal water chemistry (some corals prefer pristine water, while others prefer “dirty” water with more nutrients), the necessity and frequency of target feedings needed, and perhaps most importantly, how much light they need to grow and thrive properly.

Water flow

Corals are sessile creatures and are not able to move to hunt for food. Thus, they rely on water flow to bring food TO them. They also rely on water flow to remove waste from their tissues, as they obviously can not clean themselves very well. In addition, more flow has been shown to increase the surface area of corals undergoing respiration, as well as photosynthesis and calcification, which is obviously beneficial. However, it is important to find a balance of beneficial flow and flow that is too powerful and blowing on the polyps too harshly. Too much flow can be a catastrophic disaster for coral, as it also seems to be for this poor woman. Powerful flow can cause trauma from the polyps scraping against the corals sharp skeleton, and introduce deadly infection. Water turnover rate in the aquarium should be high- a general rule is that pumps should be rated at 5x the number of gallons of water in your tank. Thus, if your aquarium is 90 gallons, pumps should be rated to turnover water at 450 gallons per hour. Also, it’s important to try and mimic the chaotic water flow on natural reefs. To do this, multiple pumps are needed, and pointed either at each other or against walls of the tank to create random, chaotic currents. It’s also necessary to strategically place pumps so that some areas of the tank have minimal flow, for corals that prefer those sorts of conditions. However, its important to not have areas in the tank with NO flow, called dead areas, as these can be areas of extreme bacterial buildup. Some corals prefer to be close to the surface, and others prefer to sit on the sand. Their preference is a combination of light, water flow, and prey preferences. It’s important when first introducing a new coral into the tank to acclimate it to your light conditions by starting it low in the tank, usually in the sand where the light intensity is lowest. You can then gradually move the coral up until its final comfortable position. However, it is important to touch corals as little as possible, as not to further stress them and predispose to disease. It’s also important when considering a spot for a new coral to measure distance from neighboring corals. As we will discuss, certain corals can be aggressive towards neighbors, even to the point of death if they are close enough. Once a coral has been in a spot for a while, and its polyps are extended, showing that its happy with where it is, you should secure it tightly to your rockwork to prevent falling, which can stress and kill coral quite fast.

Chemical warfare

Corals compete with one another on the reef to occupy similar ecological and physical niches. One possible mechanism of doing this is for one coral to grow over another, and block its light source, thus starving it to death. Corals can be quite mean to one another! They are even capable of expelling their digestive tracts onto nearby neighbors and eating their tissue! A very potent method of competition is the growth of sweeper tentacles. These are usually exhibited in large polyp stony corals, and are larger than normal tentacles with a higher concentration of stinging nematocysts. They are capable of sticking to neighboring corals and firing these repeated high potent stings, causing injury. Corals also excrete mucous in response to stress of neighboring corals. This mucous contains nematocysts and other bioactive compounds, and can be moved around by the current and drift into other corals causing injury as well.   Corals are amazing because they are capable of “detecting self vs non self,” and responding in defense to other corals to try and protect themselves. It’s important to maintain frequent water changes while corals are getting used to one another in order to remove these toxic and harmful substances from the water. You could also utilize carbon filtration and ozone media to remove them as well. Some aquarists are known to snip off sweeper tentacles with scissors, but this can be harmful and introduce infection. It is best when sweepers form to decrease the flow in the tank, and they will recede on their own.


Chemistry

Corals are sentinels of water quality, meaning they will let you know when one of your parameters is off, by not fully extending its polyps and acting “angry.” Ammonia is extremely caustic to corals and fish, and should ideally be kept at 0 in the aquarium. Any detectable ammonia should be removed as soon as possible with a water change. Nitrites are slightly less toxic, but still very dangerous to corals. Levels should always be at 0 as well. Nitrates are not quite as damaging, and some corals and other animals prefer them to be relatively high for a “nutrient rich” environment. High nitrate also contributes to increased growth of nuisance algae. However, most corals prefer cleaner water and minimal nitrates, and thus they should be no more than 10ppm ideally. Too many nitrogenous compounds in the water will negatively impact coral respiration, photosynthesis, and calcification. We ideally like to see marine aquaria pH to sit between 8.1 and 8.4. However, there will be fluctuations throughout the day. Also, maintaining pH a bit higher is not necessarily a bad thing, and will inhibit the growth of nuisance algae. Alkalinity is constantly used up in the aquarium to resist changes in pH, and so must usually be dosed as a two part additive along with calcium. Alkalinity can be measured in dKh (degrees of carbonate hardness) or meq/L (which is dKh/2.8).  We like it to sit between 7-12 dKh. Temperature is preferred to remain in the zone of 75-80 degrees F. A new school of thought is to maintain corals at higher temperatures, from 80-84 degrees F, which promotes faster coral growth. However, this is risky as higher temperatures can lead to coral bleaching, so it’s important to find a happy medium. Make sure you have a good hydrometer or refractometer, keeping marine aquaria at or near 34-37 ppt or 1.024-1.027. Corals will not be tolerant of salinity outside this range, while fish will be fine. Additive such as iodine are not substantiated, although many hobbyists have claimed to see better coral growth.

Feeding

Corals are mixotrophic feeders, meaning they get their energy from products of photosynthesis, as well as meaty foods such as zooplankton. They all have different nutrient requirements, and thus some will feed more than others. Some corals will not feed at all, and thrive solely on products of photosynthesis. Corals will trigger a feeding response when even a small amount of food as added to the water. They have many different methods of feeding, but the general idea is that they capture the food items within their polyps and bring it to their mouth. Corals out on the reef will generally filter feed at night, but home aquarists often feed during the day so they are able to watch the process. Corals can be trained to be “out” in preparation for their regular feeding. After digesting their prey, their waste is often recycled by their symbiotic algae. Waste can be further be removed by protein skimmers and adequate filtration.

Lighting

We do our best to mimic the natural sunlight that the corals experience out on the reef. The sun is very powerful- difficult to mimic these conditions in home aquariums.  This is not an area to cut corners on in your aquarium! There are many good options today for home aquaria lighting, with each having certain advantages and disadvantages. Metal halides lights, an option used for many of the reef tanks here at the aquarium, are a very good choice. However, they give off lots of heat and use a lot of energy. One usually needs a chiller to keep temperatures down if using halide lights in their tank. However, halide bulbs last a long time, usually over a year. T5 Hi-output lights are a nice option due to their minimal heat and energy usage. However, you often need multiple bulbs, between 4-8 generally to generate enough light. Also, intensity decreases fairly quickly, and bulbs must be replaced every 6 months. LEDs are fairly new, and people are having great success with them. They put off minimal heat, and are very long lived. However, they are an expensive initial investment, usually between 500-800 dollars to light your tank. All of these options are good, but you have to make sure the light penetrates through the tank enough to hit all corals, even ones sitting on the sand bed. A general rule has usually been 5 Watts per gallon, but this isn’t really a good rule as no matter how big the aquarium is, corals still require the same, intense lighting.

Quarantine

Quarantine tanks are very easy to set up, and they can prevent catastrophic disasters in your aquarium. A tank as small as a simple 10 gallon will do. Don’t use substrate- just use a few rocks, and a simple heater/filter/light. Animals should be quarantined for a minimum of 30 days to monitor for signs of disease or parasitism and treat appropriately.

Natural Defense

Corals have many natural defenses against disease. Most corals primarily secrete a mucous from their tissues, which is full of anti-bacterial compounds. The composition of each coral’s mucous is different- they are resistant to different things. Their immune system is similar to that of vertebrates, in that they have phagocytes that are capable of ingesting and breaking down bacteria. However, they don’t possess the humoral immunity of vertebrates. Corals also produce heat shock proteins, which help them to resist bleaching events from heat stress. However, the composition and effectiveness of these proteins is altered when corals are stressed by disease, making them more prone to bleaching. Corals also utilize prophenoloxidase- a compound that aids in wound healing, encapsulation, parasite and disease recognition, and resistance. Corals are pretty amazing at protecting themselves, as you can see.

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