Pretty obvious really! The weather is an example. But no, we’re concerned with marine aquariums of course.
In a fish only aquarium the changes are usually, strangely enough, to do with fish. New additions, growth, the cave/hole they live in and the like. In a reef system more changes are often noticeable.
On the wild reef changes occur slowly for the most part, though things could happen quickly. If a storm comes along then coral formation could change as corals are broken. Unfortunately change can also happen quickly (in terms of the life of the reef) caused by man-made pollution and interference. Normal reef changes are when one coral slowly dominates another in a long drawn out battle for space, or coral colonies spread over new areas.
So change is quite natural. In the aquarium changes could be unnatural, that is caused by the aquarist. An example is the cutting of corals because of growth. When corals grow they could shadow or at least reduce the light available for others and in addition the seawater flow could be altered, meaning that some corals do not receive the flow they require. So the aquarist cuts the corals to maintain the balance of light and/or flow. This is a quite drastic change that doesn’t occur in the wild of course. Overshadowing will occur to some extent though. Hopefully the aquarist having cut corals will ‘frag’ them so that additional good comes from the exercise.
Other changes can occur and this unfortunately includes coral failure. Nowadays with the greater amount of knowledge available failure in this area should be much reduced. However, failure can occur in more than the usual way, the usual way being that a coral is just not happy, closes and shrinks away quite rapidly.
In my soft coral reef there is a colony of green star polyps, which are sometimes called star polyps or daisy polyps. The proper name is Pachyclavularia purpurea (some call it Clavularia viridis but this is incorrect)*. Anyway, the thing is that this colony arrived six years ago on a rock which was completely covered. The rock would be generally about 6 inches across at the base and about 4 inches high. After a while the coral spread onto a neighbouring rock and completely covered that as well. There wasn’t anywhere else for it to go so it stopped spreading.
For a long while it just came out at ‘dawn’ and went in at ‘dusk’. It was perfectly healthy and also completely at home, happy with the lighting and seawater currents. It was attractive and added to the many colours of the display.
Eventually I noted that it was beginning to climb onto itself that is it had nowhere to go except to cover itself. So the mat was spreading and new polyps were opening on top of the original.
I was quite concerned about this as I thought first of all that the lower layer could lose adhesion – but it didn’t. The spread across a lower layer continued for a long while, years in fact and eventually the colony was higher than it had been originally because of the layering. I was pleased in a way as the shape of the colony had altered; it no longer followed the shape of the rock but had formed mounds and spires plus some flatter surfaces, making it more interesting.
This situation continued until I noticed that some of the colony, a small area only, didn’t have any extended polyps. Sometimes the colony had not expanded for a whole day or so on previous occasions, therefore I wasn’t concerned.
However, the polyp areas that failed to expand extended in area. Eventually I noticed a bare area of rock down near the base. This area extended until very nearly all of the rock was bare. Some of the remaining un-layered matt containing polyps was removed and placed elsewhere in the aquarium and this is opening normally, appearing to be beginning to form a new colony.
Apart from one area on the adjacent rock that was colonized there wasn’t anything left. The reduction and loss happened over about a period of two months, which isn’t very long.
It isn’t all bad news though. I’ve recently noticed that a few odd polyps have appeared on the rock and hopefully these will lead to a re-colonization. If this occurs then I’ll recover the original colony and have an additional transplanted one.
So why did this occur? All other corals are fine, expanded with proper polyp extension. It follows that seawater quality and lighting are also as they should be.
I reckon my original fear that the original matt could lose adhesion was not the reason for the die-back. Adhesion loss could of course have occurred but the colony remained in place.
It seems to me that as the colony for the most part actually disappeared then it must have ‘dissolved’, or rotted away. This could have started with the covered lower layers rotting, which would have caused the top layer to become affected.
What I have to do now is wait and continue observing the polyps that remain, both the two small colonies and the separate polyps. Hopefully the original colony will grow back to full size.
If full size is achieved then perhaps the colony will eventually begin to overgrow itself. This would presumably mean the colony will eventually fail again. The possible way out of this is to put a bare rock alongside the re-generated colony which can be grown over. This can be placed elsewhere in the reef or given away.
As in the wild changes occur with the captive reef. It all makes the hobby so interesting, don’t you think?
The photos (taken by me so not of professional standard!) show the area where the original colony existed alongside the rock that was colonized, where some of the coral remains. The other photo, taken closer in, shows the polyps that have appeared that give me hope that a new colony will re-generate.
(*Reference: Aquarium Corals. Eric H. Borneham)