Everyone, aquarist or not, sees a healthy established marine reef aquarium as beautiful. It’s an advertisement for Mother Nature, and to humans an alien world. The corals sway in the currents displaying lovely colours and interesting shapes. So what is so important to corals in the aquarium and in the wild? Lighting.
Seawater quality is the number one requirement with the marine aquarium. There’s no argument, that’s it be it fish only or reef. With the reef aquarium lighting comes a close second. The interested newcomer discovers that the intensity or strength of the light must be as required and this depends on the depth of the aquarium. Not only that, the spectrum must be correct so that the corals can flourish. There is a choice of lighting types from fluorescent tubes through metal halides to LED’s (Light Emitting Diodes). They all have different abilities in power output and seawater penetration. The common ability is spectrum. The strength or intensity of the lighting is accepted as necessary as it has to penetrate the seawater sufficiently to be of use to the corals. What of spectrum? There isn’t any requirement for an aquarist to understand the scientific niceties but for the light to be of use the spectrum needs to be correct. The required technical qualities of lighting aren’t going to be discussed here, there’s plenty of information about that elsewhere. The question is ‘Why do corals need this lighting?’
Corals have developed over an extremely long period and require particular lighting. It is obvious how plants and trees gather light, they have leaves, but what of corals, do they just absorb it? There’s something inside the corals that make use of the light, these are known as zooxanthellae, an algae. They cannot be seen by the aquarist as they are extremely tiny. On a circle of say one millimetre diameter there would be around 10,000. So in a coral there would be many billions*. Quite a few then! They need the correct lighting to function.
The corals and the zooxanthellae are in a close partnership, without properly functioning zooxanthellae the coral is likely to die. This is because the zooanthellae are like tenants with the coral the landlord. The rent that is paid is food and other services. If there isn’t sufficient suitable light then photosynthesis cannot occur, which means the rent cannot be paid and the coral is likely to eject the zooxanthellae. Corals could capture food such as zooplankton and it could appear that they could survive on that, but on a reef there is insufficient and obviously much less in an aquarium. In the wild an experiment was completed when some stony corals were screened from the sun. It was known that zooplankton and dissolved nutrients were present. However the corals ejected their tenants and eventually died*.
The partnership is closer than just that of somewhere to live. The zooxanthellae are provided with types of food by the coral and vice versa. Wastes are also dealt with. Zooxanthellae are not present in all corals so there is a need for information on purchase, the dealer should be able to advise. If not some patience needs to be applied so research can provide the answer.
So let’s leave it at that point. There is a lot more that could be said about the zooxanthellae but it is not relevant to the basic needs of the reef aquarium. It could become relevant as time passes, not because of the corals but because the aquarist becomes intrigued. If more knowledge is desired well and good, if not it doesn’t matter. The main thing is that the lighting provided must serve the needs of the corals, or more accurately the zooxanthellae.
(*Ref: Borneman – Aquarium Corals)