Corals are vicious animals

Many people still don’t know that corals are animals, alive and adhering to the Darwinian principle of survival. Since Victorian times, or perhaps even before, many corals were confused for plants. To this day I sometimes have trouble getting guests to comprehend that they are alive. But this fact is vitally important to the aquarist. Corals are downright violent and will aggressively defend against and attack other corals. Because of this it’s important to research the corals you buy, understand how they attack and defend, as well as which other corals. The below article is a primer on this topic, you will have to research each of your corals in order to learn specifically about that species. The intent here is to explore the ideas behind coral on coral violence.

There are four basic forms to look out for which we will discuss below. This isn’t going to be a scientific discussion so I’m using terms you are likely to hear from other aquarists rather than marine biologists.

• Overgrowth (Invasive Tactics)
• Chemical Warfare (Alleopathy)
• Stinging (Nematocysts/Sweepers)
• Consuming (Mesenterial Filaments)

Overgrowth (Invasive Tactics)

I’ve experienced overgrowth in two forms, one from shading, and the other literally growing over top of another coral. Shading is common when you have a coral like mushrooms, zoanthids or other low growing corals and taller corals, especially Xenia, grow in and around them to the point where they occlude the light causing harm to the shorter corals. This isn’t so much an attack as it is a natural result of the type of corals involved.

Other corals will just grow right on or over your prized corals. Paly are a prime example of this. Some species will grow right onto stony corals. They will fight to defend themselves from the attack but in the end the faster growing species of paly overwhelms the SPS.

This can easily be avoided by careful placement of corals, or if you wish by coral gardening, which is the process of trimming back the offending coral. Corals like xenia are easy to remove, green star polyps can be removed with a razor blade, but paly are a different matter and require some precautions.

Palythoa (and zoanthids) are highly poisonous. They can kill fish, and humans alike. I’ve personally been made very ill from them on two occasions. After the last I decided not to keep them anymore and I’ve lost fish to them in the past. Removing paly with tweezers is a pretty easy way to take care of them but if you squish them or damage them it releases the poison, known as palytoxin, into the water. This is one of the most deadly poisons on the planet (CDC, 2015). What I found was if I removed only a few polyps a day and let things alone for a day I noticed no ill effects. Of course this can be painstaking if you aren’t the patient type. I want to stress, even if this process annoys you, it is worth taking the time. Releasing palytoxin into the water can kill all of your fish quickly and make you quite sick, even to the point of needing to be hospitalized.

Chemical Warfare (Alleopathy)

Various corals will send out toxins into the water to fight off other corals. In some cases this is just a sort of invisible barrier that other corals won’t cross, and in other cases the nearby coral will be killed. Soft corals most often use this method which is especially effective against stony corals. Your prized acropora might be in danger as a lobo is too close or as the sinularia spreads.

Unless the tank is very small the symptoms are likely to be noticed in one or two corals that are close to the offending soft coral. If you notice all the corals having problems it’s most likely going to be a general water issue like alkalinity or even high nutrients. If you do notice an issue there are a few things you can do.

• Move the offending coral or the affected coral (whichever works best for your tank)
• Run activated carbon (this helps to remove the toxins)
• Increase skimming (if you have a good skimmer just giving it a good clean can help)
• Do a water change (you should be doing this anyway but if you notice a problem do a larger one than normal)

In my experience this is one of the easier issues to deal with for the aquarist because most soft corals are quite easy to trim and remove and the above mentioned steps are quite effective. Also, depending on the type of coral the amount of toxin might be low creating a barrier between corals that actually helps the aqarists maintain a nice tank. Just be aware that when a coral dies it does release its nutrients back into the water so if the die off is large enough this could affect overall water quality.

Stinging (Nematocysts/Sweepers)

We all know that anemone sting but they aren’t the only corals that do that. Many corals have stinging cells of various power. Some are even more potent than anemone. For example galaxia sends out sweeper tentacles with stinging cells on the ends. These are very strong and have been known to really irritate fishkeepers who come in contact with them and certainly they will harm fish. But what they also do as it relates to this article is to sting corals that get in the way. Even your beautiful little zoanthids sting (Ryaland & Lancaster, 2003). This is a method of attach by soft corals and large polyp stony corals, and of course anemone.

Consuming (Mesenterial Filaments)

This is the example shown in the video above. One coral gets too close to the other, feeling threatened it attacks the other coral by spewing out what are known as mesenterial filaments, or basically its guts. It then proceeds to slowly digest the other coral. This video clearly shows this happening. My mama clown fish likes to keep her area clean, she even keeps the sand away, so she moved the fungia and it ended up too close to the other one.  I got to it in time but there is clear damage to the fungia. This is a method used by stony corals to attack. I’m not aware of any soft corals that do this.

Conclusion

Corals don’t know they are being vicious but the end result of their attacks and defenses is violent and damaging. You spend lots of money on your corals and you care for them as creatures. For an aquarist they have emotional and monetary value. Careful placement and preventative research will help preserve this investment from being killed needlessly.