Photo-adaptation

[Gouldian]

Photo-adaptation

In the context of this article, the term photo-adaptation is used to describe the ability of the organism to alter its structure and function in response to the characteristics of the light in its environment. When a coral from the wild or another tank environment is placed in a reef aquarium it has to photoadapt to the various aspects of light — the intensity, spectral distribution and direction of light. The basic adaptation mechanisms employed are various combinations of change in the chlorophyll content per unit surface area, change in the number and size of zooxanthellae, changes in coral morphology to increase the surface area available for light capture, and changes in respiration rate. More recent research is indicating that the corals may, in fact, harbor different types of zooxanthellae and the corals may be able to change the mix of the zooxanthellae as a photo adaptation response.

It has been shown that most of our aquariums are limited in light when compared to light in the natural reef environment. Often the light intensity may be one-third to one-quarter of what the coral receives in the wild. Yet we find that corals live and prosper (as far as growth is concerned) under these conditions.

The scientific literature has some interesting results on photoadaptation. For example, Porter et al. (1984) have provided data for Stylophora pistillata adapted to high and low irradiance levels, and examined photosynthetic parameters of intense light-adapted colonies at high and low light levels and low light-adapted colonies at high and low light levels. The results showed that low light-adapted colonies have much higher photosynthetic capacity at low light than the intense light-adapted colonies. Corals growing at higher light irradiance are often much lighter in color than colonies of the same species in lower irradiance regions. Interestingly, Falkowski and Dubinsky (1981) found that the difference in pigmentation in S. pistillata was not due to the density of zooxanthellae, but due to the average pigment content of the zooxanthellae. Another interesting result that could explain why corals live and grow well in our poorly lit aquariums is the research finding that the coral adaptation to low light was faster than adaptation to intense light.

Corals may also show morphological changes in response to irradiance. In the wild, colonies that are often hemispherical in shape in shallow waters become increasingly plate-like in deeper waters. In aquariums, most hobbyists have noticed that when fragments of the same coral are planted in different locations in a tank, they may often grow in different shapes. These morphological differences may not be completely attributable to irradiance, but may be a combination of several factors that include irradiance and water motion.

Corals in the wild are subject to light from various directions, and different parts of the coral may be photoadapted to different light characteristics based on its position and relative exposure to the light. When placed in an aquarium, the light is typically unidirectional (from the top), and parts of the coral that were once subjected to light may now be shaded, whereas parts that were shaded may be exposed to more light. Hence, different parts of the coral may have to photoadapt differently. Often we find that the underside or shaded portions of the coral colonies will bleach, and this is due to the change in the light reaching the bottom of the coral colony. Sometimes the tips may bleach, and this may be due to coral tips receiving too much light and its failure to adapt rapidly.

Source: www.fishchannel.com