The first thing that needs to be done after writing the above is to ensure the spelling is correct. It is a strange word to us ordinary mortals, but came into being with marine biologists.

In the marine world one of the most fascinating double acts to watch is that of a clownfish and an anemone. The clown swims to the anemone and gains protection from the stinging tentacles, and, though there is argument, the anemone may gain by the clown chasing off unwanted threats to the anemone or dropping bits of food on it. To my knowledge anyway the anemone’s advantage is not fully clear.

The above example illustrates how two different life forms can be of use to one another. There is another that is much more common in marine aquariums. This one is the relationship between corals and zooxanthellae. Zooaxanthellae are in fact single celled algae that live within the tissue of a coral, often called symbiotic algae. Why should this be?

It is thought that the coral uses the oxygen and carbohydrates manufactured by the zooxanthellae. In turn, the zooxanthellae utilise coral waste products and assist in obtaining trace elements from the seawater. It is theorised that up to 90% of the corals required food may be supplied by the zooxanthellae.

For the algae light is very important, as it is required for the photosynthesis process. This is why reef aquariums are brightly lit, some with T5 fluorescent tubes and others with metal halide bulbs. Power output and spectrum need to be correct (power output for the depth of the aquarium). It is thought that the ‘actinic’ fluorescent tube is very beneficial to the zooxanthellae.

A clue as to whether a coral has zooxanthellae or not is in the colour. Generally, it is likely that corals coloured yellow, purple, red, and orange do not have the algae, and corals coloured blue, beige, brown and green do.

Both soft and hard corals may have zooxanthellae. In addition, there are others, such as the aforementioned anemones and clams such as Tridacna.

Problems can arise with this strange relationship but this is not the text to discuss that. Provided the aquarist provides adequate lighting and water quality, then all should be well.

(Reference: Marine Invertebrates. Martin Haywood and Sue Wells)