Marine fishkeeping is an increasingly popular field, nano-reefs being the new craze, but how many reef keepers actually know what coral is? Let me introduce you to a world of co-operation and feuding, of epic building and catastrophic collapse.
The basic coral is not unlike an anemone, and can be either solitary or colonial animals. The coral polyp is essentially a small ‘sac’ surrounded by a ring of retractable tentacles. The tentacles come complete with nematocysts - small stinging cells used in the capture of prey or in defence. Colonial polyps secrete a calcium carbonate ‘skeleton’, and it is this which eventually builds the coral reef - many species of coral appear on a reef but it is these that largely form the massive structures, and which are known in the aquarium trade as the ‘hard corals’.
One of the most interesting aspects of the reef building corals, and a matter of key importance for reef keeping enthusiasts when considering their lightning arrangements, is the nutrient requirement of corals. The almost plant-like dependency of these corals for light (remember corals are animals) can be attributed to the presence of symbiotic photosynthetic zooxanthellae. These are basically algae living within the polyp in a mutually beneficial situation - the coral provides a safe situation for the algae to photosynthesise without being grazed upon.
Corals benefit from the carbon products of photosynthesis produced by the zooxanthellae (essentially this is a method of getting carbohydrate for the coral!). This, combined with the nitrogen polyps get from feeding on zooplankton, means the corals have the basis for amino acids - and thus have the building blocks for proteins. Those corals utilising zooxanthellae are known as hermatypic corals
Zooxanthellae can also increase the calcification of hard corals - by removing carbon dioxide the zooxanthellae can aid the coral in its production of calcium carbonate, thus those corals which have greater numbers of zooxanthellae will often be some of the most important reef-builders.
As a result we can see the importance of light as the ‘limiting factor’ in the growth of corals. It comes second only to temperature for the reef-builders. Here we also remember that corals are animals, and actively catch zooplankton form the water column.
However, while the hard corals (the Scleractinia) are often referred to as the ‘true corals’, the soft corals (Order Alcyonacea) are perhaps more important in the aquarium world, providing a variety of form and colour, often starkly contrasting to that of the hard corals. Whilst these corals do not excrete calcium carbonate in the same manner as the reef builders they do feature sclerites - internal calcium based structures. These tend to be fleshy and flexible. These corals can be small and encrusting, or incredibly huge, but are probably easier than hard coral to maintain within the aquaria. They are also colony dwellers and like the hard corals can withdraw their feeding tentacles. As with hard corals, soft corals catch zooplankton from the water column, but not being rigid they can exist in areas of relatively strong current, and thus areas of high food influx. Soft corals can also feature zooxanthellae - some in fact rely entirely on their colonies - with concentrations of these appearing in the tentacles (often giving them a brownish hue). Soft corals have often been known to go for a wander too - however this is no simple shuffle along the substrate, for a soft coral it means excreting toxins to clear the way as they extend their bases, leaving some considerable destruction in their (albeit quite small) wake. However, many corals will never move - it is often out of necessity, given a safe and nutrient rich spot movement is unlikely.
While many people may be familiar of some aspects of corals from television - things such as the mass spawning of corals, the impressive size of reefs and the diversity of life they support - there are things about a coral’s ‘private life’ that may not be as well known. For example, ‘love thy neighbour’ is not a coral philosophy! Your neighbour is your competitor, competing with your colony for space, food and precious light. So then, as an ambitious coral, what is your solution to this problem? Simple, ‘digest thy neighbour’! Some species of slow-growing hard coral will exhibit ‘interspecific digestion’, to out-compete their neighbour. Some prefer a less subtle method - they are armed with ‘sweeper tentacles’. These longer tentacles will sting and damage neighbours within reach. Both soft and hard corals will exhibit this territorial nature! In contrast two colonies of the same species meeting will often result in a fusion of the two. However, even so armed, one coral species rarely dominates; there are too many other factors in this competition for reef space.
So, what of the catastrophic collapse? Well this may be overstating things slightly, but there are very real environmental threats to coral reefs worldwide - trawling destroys huge expanses of coral, the devastation left behind has been compared to the surface of the moon. Coral reefs are not quick to appear, this damage is not easily recovered in nature. However I suspect few of you will be trawling in your tanks unless you’re either very lucky or have been gifted with ‘trawler-like’ skills with your aquarium nets.
So how about eutrophication? A danger in the environment from nutrient spikes - for example phosphates running off agricultural land and causing algal blooms that swamp corals, out-competing coral for both light and nutrients - nutrient spikes like this are also bad news in marine aquaria, especially in a small ‘nano’ system. Building work and deforestation can increase sediment loads in the water reducing light levels and harming coral growth, increased water temperatures as a result of global warming or El Nino effects can result in the coral expelling their zooxanthellae, and reef tourism brings its own dangers, destroying the reefs the tourist has paid to admire.
Corals are stunning animals, outstanding in their diversity, elegant in their simplicity. Keeping them successfully in aquaria produces impressive displays, but should not be done with out considering this little animal as a whole, and being aware of the threats it faces in the world today.