Corals belong to a group of animals called Cnidarians. The Phylum Cnidaria consists of over 9,000 species of animals which are found exclusively in aquatic environments, and then mostly in marine habitats. Other types of Cnidarians are Jellyfish, Anemones, Sea Pens, and Box Jellies.
The simplest form of a coral is a polyp, which consists of a basal plate that is used to attach the coral, a digestive sac, and a mouth surrounded by tentacles with cnidocytes (or nematocysts) – stinging cells used to catch prey (see below diagram). These polyps reproduce asexually to form colonies of genetically identical individuals. These colonies are what is normally found in the home aquarium and can be extremely varied in both shape and colour, even within the same species of coral.
Corals, like all other animals, are either herbivorous (plant eating) or carnivorous (animal eating) and prey primarily on plankton (ie those organisms that are too small to swim against the currents and tides). Plankton can be separated into 3 main groups – Zooplankton, which consists of animals like copepods, amphipods, rotifers, and also larval forms of fish, crabs, and other corals; Phytoplankton, which consists of plants and algae like Tetraselmis, Nannochloropsis, Isochrysis; and Bacterioplankton, which consists of those bacteria which are responsible for breaking down organic material.
Some corals also host within their cells a symbiotic (meaning “living together”) algae called Zooxanthellae. This algae is responsible for the primary brown colour attributed to a lot of corals. The more zooxanthellae a coral is hosting, the browner the coral becomes. This relationship is a mutualistic one, meaning that both organisms benefit from the association. The algae receives a fairly safe place to grow and reproduce, with a steady supply of “fertiliser”, while the coral receives a portion of the energy that the algae converts from the surrounding light.
It is this relationship which has led many aquarists into believing that corals only require a decent light for them to obtain the necessary energy to survive. Recent reports and studies have shown however that by providing a range of both phytoplankton and zooplankton to your reefs, not only will corals become more colourful (as they are reducing the zooxanthellae in their tissues and utilising the energy in the food they are hunting on themselves), they will grow faster, be more resilient to disease and predation, and we have also heard reports of corals actually spawning once plankton has been introduced into the home aquarium.
Obviously there is no miracle fix for growing corals. A wide variety of both abiotic and biotic factors influence how healthy corals will be. However if you do plan on keeping corals in your aquarium, you will be amazed at the difference the addition of plankton will have.
There are a variety of readily available plankton specifically designed for reef aquariums. Live zooplankton such as rotifers, copepods, artemia (newly hatched and also ranging through to adults), and amphipods are commonly available. Phytoplankton like Nannochloropsis, Tetraselmis, Isochrysis, Pavlova and Thalassiosira weissflogii are also available in concentrated amounts that are stored in the fridge, making it extremely easy to keep a variety of highly nutritious algae to feed to your animals. Products like Phyto Diet are an accumulation of individual species, allowing a well rounded diet to be easily added to the aquarium. Also products like Coral Diet mimic the zooplankton allowing aquarists to feed their reef without the need for culturing live plankton.