Most aquarists know they have to acclimatise their new charges. This involves slowly adding tank water to the transport water in an effort to gradually shift the chemical and physical environment of the organism from that in the bag to that in the tank. What may not be obvious, however, is that light requiring organisms also need to be acclimatised to the new lighting regime. This article seeks to show why it is important that all lighting environment changes shouild be gradual for corals.

Zooxanthellae

The basic anatomy of a coral polyp shows the importance of the capture and digestion of food. The main body of a coral polyp is called the gastrovascular cavity and this is where digestion takes place. Coral also have tentacles with cnidae (nematocysts, spirocysts) for the capture of prey.

Despite their ability to capture their own food, many corals have evolved a symbiotic relationship with a dinoflagellate alga, the zooxanthellae. This relationship works largely due to usual low concentrations of dissolved nutrients in waters around coral reefs. The polyps share some of their waste with the zooxanthellae in exchange for the products of photosynthesis of the algae. This benefits both parties in a relationship called mutualism. The zooxanthellae not only receive nutrients, such as ammonia, in an otherwise relatively nutrient free environment, they are also protected from predation inside the coral tissue. The polyp benefits from the carbon rich compounds produced by photosynthesis.

While it was once believed there was only one species of algae responsible for symbiosis in corals and other invertebrates, it has now been determined that there are more than 80 different strains of symbiotic dinoflagellates from at least 4 orders, seven genera (Borneman, 2001). Different strains have been found to have different capabilities in relation to photosynthesis and corals may be able to switch from one species to another to adapt to specific conditions.

The coral can control the populations of zooxanthellae in their tissues by controlling the amount of waste they release to the algae. If the zooxanthellae population gets too large, the coral can expel them. This is a normal process for corals but if more than 50% of the zooxanthellae are expelled it is considered bleaching (Borneman, 2001). After a bleaching episode, if the coral is still alive, it will increase the release of waste to the remaining zooxanthellae in an attempt to have them reproduce.

Plants and algae are able to harness the energy of light by using a number of pigments. Chlorophyll a is the most widely used pigment for "harvesting" light and is found in zooxanthellae. Chlorophyll a has two light absorption pigments at around 440nm and 675nm. Wavelengths near these peaks are captured directly by chlorophyll a but other light is either captured by some other pigments such as chlorophyll c2 or peridinin or the light energy is transferred by further pigments such as beta-carotene and diadinoxanthin and this results in a near 100% utilisation of all wavelengths of visible light (Borneman, 2001).

The corals polyps themselves also produce pigments to protect themselves and the algae from damaging ultraviolet radiation. There are two main types, mycosporine-like amino acids (MAAs) which are largely clear and fluorescent pigments which reflect UV radiation as visible light. In shallow water corals, both pigments are found above the zooxanthellae. In deeper water corals the pigments are found beneath the zooxanthellae to enable the zooxanthellae to use longer wavelengths of UV radiation (Salih et al, 1997).

Photoadaptation and Photoacclimation

Photoadaptation is an ability corals have to alter the efficiency of the photosynthesis within their tissues to suit changing lighting conditions. They do this through various photoacclimative processes. These processes include increasing of decreasing the number of zooxanthellae and increasing or decreasing the amount of UV protecting pigment. Borneman (2000) list a more complete set of processes.

If the amount of light reaching a colony or part of a colony decreases, for example due to shadowing by another coral, the colony will photoadapt to the new conditions by increasing the efficiency of the total photosynthetic ability. Some changes may be quite quick, such as the zooxanthellae increasing their pigments and some may be slower such as increasing the number of zooxanthellae or even by changing the strain of zooxanthellae. If the coral is to survive it must either maintain the photosynthetic ability it had or increase its feeding.

If the amount of light increases, for example by a shading organism being broken in a storm, the coral will reduce the photosynthetic efficiency of the zooxanthellae to avoid overproduction by the zooxanthellae.

Acclimatisation

Under normal wild conditions changes are quite gradual and the difference in lighting that a single colony experiences is not all that great. Under aquarium conditions, the change in lighting can be quite drastic. In most cases we do not know at what depth a coral is collected and even if we did, the coral is likely to be subjected to low lighting from time of collection to the time it reaches our aquarium.

If the coral has been kept under low lights, it will have adapted to those conditions to increase its photosynthetic abilities. If the coral is then placed under intense or very intense lighting, the rate of photosynthesis will be much higher then before. This can cause the polyp to be supersaturated with free oxygen and oxygen radicals which is toxic to the coral. The coral produces enzymes to destroy the oxygen but this in turn produces hydrogen peroxide which itself is dangerous and must be removed. If the coral is quick enough it will bleach (expel the zooxanthellae) and even then the coral may later die through lack of nutrition. If the coral is not quick it will most probably die. Additionally, the coral may be exposed to more UV radiation than it was previously exposed and this radiation may damage coral tissue.

To avoid the above situation, it is always best to slowly acclimatise all new corals to the new aquarium lighting conditions (and this is also true when changing the lighting on a tank with existing corals). Light intensity should be reduced for a period of a few days to a week and gradually increased until the coral receives the full intensity intended for it. The amount of time taken should be dependent on the previous lighting conditions, if known, and the intensity of the lighting of the tank. It is always safer to take longer rather than shorter.

Light intensity can be reduced through a number of means:

• Raising the lamps
• Placing a filter between the light and the new coral
• Placing the new coral deep in the tank or in a more shaded area.

For new corals, I prefer placing the coral on the bottom of the tank and then placing a piece of shadecloth on the tank lids between the light and the new coral. This means that only the new coral is affected. After a week I remove the shadecloth. After further time, I move the coral to a place closer to the lights and eventually to its final position.

There may be some corals which have come from comparable lighting conditions. If it is known for sure the lighting conditions are close, acclimatisation may not be necessary, but if there is any doubt it should be performed. Exposing corals to lower light for these short times will have no detrimental effect.

References

Borneman, E H (2001). Aquarium Corals: Selection, Husbandry, and Natural History. Microcosm Ltd, Charlotte, VT 464pp

Salih A, Hoegh-Guldberg O and Cox G (1997). Photoprotection of Symbiotic Dinoflagellates by Fluorescent Pigments in Reef Corals. In: Proceedings of the Australian Coral Reef Society 75th Anniversary Conference, Heron Island October 1997 (Greenwood, J.G. & Hall, N.J., eds 1998). pp 217-230 School of Marine Science, The University of Queensland, Brisbane.

Last updated: June 4, 2006