Planarians are otherwise known as flatworms. They can be fairly colourful and a few in the aquarium could be accepted by the aquarist as another life form on the captive reef to create more interest.

Unfortunately this is not so. Flatworms can definitely be placed in the ‘pest’ category. Some are parasites on specific organisms such as corals and others, strangely enough and like many corals, have zooxanthellae within their bodies*. If the flatworms are coral parasites then it will soon become apparent what is being attacked. The parasitical types do not usually have any colour, in other words they are transparent, and can therefore be difficult to spot.

If the flatworms are the free living types, the ones with zooxanthellae, then they will be coloured a shade of red. They are properly called Convolutriluba. Probably because of their need for light, as with corals, they are more likely to thrive in a reef aquarium with its bright lighting.

Red flatworms can be seen anywhere in the aquarium, on the viewing glasses and rocks etc. Fish are generally not interested in them as food, though it is reported that the mandarin fish (Synchiropus splendidus) will eat them. Perhaps the psychedelic fish (Synchiropus picturatus) will as well, but this is a guess. They are both beautiful fish.

If the aquarist notices just a few flatworms then a mandarin fish could be released to clear them up. There is an important point to be made about this, quite apart from checking general compatibility of the mandarin with other stock before introduction. First, the mandarin will have sufficient food for a while as the flatworms can breed at a high rate. There will also be other life on the reef which is on the mandarin’s menu, such as tiny shrimps. However, this food will disappear if, as hoped, the mandarin does its job. To the aquarist’s dismay, many mandarins and sychedelic fish die months after introduction because of a lack of food. So they should only be released into a reef aquarium that has been up and running for around 18 months or preferably longer. In addition the reef should be constructed of live rock, which hopefully will mean that there will be plenty of minute reef life for the fish to hunt. A look at the reef at night will usually indicate the abundance of tiny creatures.

There is a sea slug that will prey on flatworms, and this is called Chelidonura varians. It is quite beautiful but where can the slugs be obtained? It is worth a check with the local retailer or a phone call to a large internet supplier, but it is likely that they cannot obtain them. However, some aquarists have got hold of them – a query on a busy internet forum may be of use. Of course the usual problem raises its head – what does the slug eat when all the flatworms have gone?

As already stated these red flatworms breed at a high rate. Just a few on the rocks can soon cover the rocks, then the rocks and glass, then the rocks, glass and substrate, then the rocks, glass, substrate and corals. Yes, they can be that bad! Anything living that is covered by them will perish in time by asphyxiation.

It isn’t any use introducing the fish mentioned above if the aquarium is in the described state, the fish will only be able to deal with some flatworms and the fish’s capacity will be outstripped completely. Though there have been one or two reports of flatworms suddenly dying out without interference from the aquarist (why they died out is not known), with a large population it is much more likely that to save the reef the aquarist is going to have to apply chemicals.

There is another danger too. Just to add insult to injury, when flatworm populations die they release toxins which can kill any fish present in a short time, a few hours. It is reported that invertebrates are also killed by the toxins. Thanks!

So if the aquarist is going to use chemicals to kill the flatworms then, subject to the instructions supplied by the manufacturer, the protein skimmer should be at peak efficiency and activated carbon should be used, discarded and a new batch used again, all immediately after the treatment. Also any mechanical filter(s) should be carefully cleaned. In addition, a supply of new seawater should be ready, and the amount should be sufficient to carry out a seawater change of 25% or even higher, and the aquarist should be prepared to do another very shortly after if any signs of distress are noticed.

Let’s finish on a happy note! In all the years (decades, good grief!) that I have been keeping fish and corals I have never faced a flatworm problem. This could be good luck of course, and so I’m keeping my fingers crossed.

(*Reference:  Marine Atlas. Helmut Debelius & Hans A. Baensch)