This conjures up thoughts of sharks cruising menacingly over the reef looking for lunch, or perhaps just smaller fish arguing over a morsel. This all happens but in this case its corals.
Corals could be aggressive, a known fact. This is how they defend their space and obtain more. Some corals are more aggressive than others as is the usual case with Mother Nature.
As with fish, corals don’t change their habits in the aquarium. All things being equal, the corals are placed into a well suited environment well spaced from other corals. They settle, the time needed for this varies, and once the settling period is over they begin to grow.
In a reef aquarium the aquarist watches for growth because in the first place it’s pleasing – it indicates that the conditions are to the corals liking. From this growth the aquarist may well take cuttings (‘frags’) which are offered to others thus reducing to an extent the pressure on the wild reefs. Growth could cause problems which need to be watched for. For example a reduction in seawater flow could cause problems for any coral downstream which needs higher flow. Another example is light – a coral may extend enough to throw another coral into shade, meaning the other coral suffers.
Aggression should not be overlooked either. It’s said that if a reef looks overcrowded it probably is, though this is general and confusing particularly to a beginner. When a reef is set up corals should be placed so that they do not touch taking expansion (if any) into account. This applies particularly to unrelated species. This is because corals are able to fight for territory, some extending stinging tentacles to damage their neighours. Even though a coral is considered to be benign overall, there could well be other means that territory is gained, such as creeping forward on a mat.
If signs of aggression are noticed, which could be a clearly damaged coral with one side withering or losing colour, then something needs to be done. Perhaps cutting (‘fragging’) could take place, the cut piece being grown on somewhere else or handed on to another aquarist or dealer. Perhaps the offending coral could be moved entirely, though care needs to be taken that the coral doesn’t lose the required intensity of light and seawater flow.
In my aquarium I fairly regularly need to do some work because of coral growth. Perhaps a species is becoming too overcrowded or too large. Normally very sharp scissors plus yours truly deals with the ‘problem’. It’s interesting to see the different ways corals can be aggressive and spread.
I’ve taken three photographs of corals on my reef (I apologise for any quality shortcomings). There’s nothing particularly special about them but they show territorial behavior generally, though there aren’t any signs of serious damaging aggression as the corals are considered benign.
The first shows green star polyps (Pachyclavularia sp) and mushrooms (Rhodactis sp?) that have bumped into each other. This collision happened over a quite lengthy period of time, though green star polyps do spread quickly on a mat. Where the two types are together there doesn’t appear to be much movement, but the stars polyps are beginning to go around the outside edges. Is this an overpowering or avoidance strategy I wonder? There doesn’t seem to be any damage at the adjacent site to either coral. I suspect, but don’t know as yet, that the star polyps will attempt to overgrow the mushrooms. We’ll see.
The next photo doesn’t really show aggression as such, its more taking advantage of additional territory that’s available. The rock used to be covered in green star polyps but, as is sometimes the case, these died back leaving bare rock (they have now started re-growth). Toadstool corals (Sarcophyton sp) have taken hold on the bare surface and are spreading. The tiny corals appeared fairly quickly, perhaps in two or three weeks after the rock area became available and grew quickly. This is not aggression as generally understood, but perhaps could be considered aggressive opportunism. If the green star polyps grow back over the remaining rock area I don’t expect any aggression problems between the two. I do expect to have to interfere with the toadstool’s growth and number at some point.
The final photo shows mushrooms (Rhodactis sp?) and cabbage coral (Sinularia dura). The mushrooms have nearly encircled the cabbage coral and touch on occasion when seawater flow causes it. Neither of the two types is showing any sign of distress. The cabbage coral is a well known very hardy coral – it could be called tough as old boots! Maybe it shrugs off as inconsequential the presence of the mushrooms. The mushrooms for their part may not perceive any threat whatsoever and continue to colonize territory unperturbed.
There are many examples of coral aggression that are not as mild and well-mannered as those given. Many will have seen time-lapse photography showing battles in progress.
All the aquarist needs to do is be aware, which is the watchword for marine aquarium keeping.