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 Coral Aggression & Coral Placement Considerations



Just under a coral reef’s veneer of peace and tranquility lies a world of aggression and endless warfare between the constituent corals on the reef. In essence, everything on a coral reef is trying to battle for more real estate and the organisms on the reef developed specialized methods to wipe out their neighbors whether that be other corals, algae, or inverts. This article is all about coral aggression and the role it plays in how we place corals and build up our reef tank.

There are four major adaptations corals developed to combat one another. Corals use these methods to varying degrees and some use a combination of methods to further their competitive advantage. Let’s take a look at each one in turn.


 Nematocysts

First of all, corals possess nematocysts, or stinging cells. Nematocysts are used for both prey capture as well as defense. These venomous cells are not unique to corals as they are found in anemones and jellyfish as well. Some organisms developed adaptations to acquire nematocysts when they cannot produce the cells themselves. Zoanthid eating nudibranchs for example eat the polyps and store the nematocysts in these sacks on their backs. That is why often times the nudibranchs take on the coloration of the zoanthids they have been feasting on.

How they work is each nematocyst is a pressurized capsule that features a coiled thread with harpoon-like spines. These cells are under a lot of pressure so when they are triggered, this coil fires out and stabs into the target and injects its venom.

The power of the sting depends on the structure of the nematocyst as well as the toxicity of the proteins it fires into the target. Some stings are very mild while others are very intense. For example, the nematocysts from a Pseudocorynactis mushroom are very sticky and it is able to grab onto and hold large fish. In fact, those can easily stick onto your hand and get lifted right out of the water.

Most of the time our skin is thick enough that coral stings aren’t that noticeable, but sometimes people are more sensitive to a particular coral. For example, some people are very sensitive to frogspawn and immediately break out after handling a colony. In extreme cases, people can get very ill. Some people are allergic to the venom from carpet anemones and there have been reported deaths from allergic reactions.

How this relates to coral placement is that some corals are able to create hyper weaponized forms of tentacles called sweeper tentacles. A sweeper tentacle is a greatly elongated tentacle with a concentrated mass of nematocysts at the tip. These sweeper tentacles are most commonly found in large polyp stony corals such as euphyllia, certain brain corals, galaxea, the list goes on but they are also found in some small polyp stony corals such as Pavona. If you have a coral that is capable of sending out sweeper tentacles, it is important to give it a lot of room in your tank because they can cause a lot of damage to neighboring corals. This is often easier said than done. Some corals can extend sweeper tentacles up to a foot away.


 Mesenterial Filaments

Mesenterial Filaments are the inside guts of a coral that some species can expel onto nearby adversaries. Mesenterial filaments contain nematocysts as well as digestive enzymes and can do significant harm by coating the target for hours.

Eventually the coral retracts these filaments leaving behind a clearer substrate to grow upon. It is thought that mesenterial filaments used in this way are used both for aggressive expansion as well as nutrition for the coral when they are withdrawn back into the coral.

Often times this behavior happens at night so it is entirely possible to have two corals situated close to one another be fine but when you wake up the next day one of them is completely dead... a victim of a mesenterial filament attack.

Almost every type of coral is capable of extending mesenterial filaments but some are more aggressive than others in this regard. I see them commonly in favia, favites, pectinia, hydnophora, and many types of chalice corals.


 Mucus Coat

Mucus coats on various corals can cause serious damage to other corals even after brief contact. The aggressiveness of the mucus coat can vary greatly from fairly weak in the case of Xenia to hyper aggressive in the case of Acanthastrea echinata or certain types of chalice corals. The mechanism for this hostility isn’t well documented but it is pretty clear that whatever it is it happens quickly. It is not uncommon for a coral to fall off of the rock scape face down on another and both corals immediately start showing signs of damage. If left that way for a few hours both corals could die outright.

There are some studies that indicate the mucus coats of corals provide all kinds of benefits such as UV protection, microscopic prey capture, and protection from detritus settling on the coral by periodically shedding the coat away. On the flip side however there are studies showing this mucus coat is also a dense breeding ground for bacteria and viruses.

The chemical mechanisms behind the effectiveness of the mucus coat of a coral are still murky, but in practical terms, it is important to place corals in such a way that you minimize the likelihood of contact. Anticipate future growth and make sure to place corals securely so they don’t lean into or worse yet fall on top of each other. Right now at Tidal Gardens the most likely reason any coral dies is because one colony fell into (or got pushed into) another one.


 Chemical Warfare

The last coral aggression adaptation I will discuss in this video is chemical warfare. Some corals don’t necessarily pack a mean sting or barf up their guts and dissolve their neighbor. Corals such as these toadstool leathers can secrete toxic compounds into the water that slowly poisons everything around it.

How this manifests is that other corals around it especially stony corals simply stop growing.

The secreted toxins are metabolites which may include terpenoids, steroids, acetates, and polyamide compounds.

Terpenoids are a super diverse group of natural compounds that are often associated with protective oils. For example some plants have an oily coat that makes them toxic to insects and other herbivores or termites have terpenoid-based oils that make them unappetizing to potential predators.

These terpenoids are also precursors of polyhydroxylated steroids that have been detected as well from certain leather corals. There are also Acetates and Polyamine compounds that are all potentially bio active. In fact, several of these compounds are being investigated for therapeutic uses because some are inhibitors of chemical pathways which are very interesting in drug development circles.

To sum that all up, corals are capable of excreting a cocktail of bio active compounds in an effort to wipe out neighboring organisms. The corals associated most with these chemical toxins are leather corals like sarcophyton, lobophytum, lemnalia, etc.

So what can we as home hobbyists do about corals that engage in chemical warfare?

Some aquarists chose to avoid these corals altogether especially those that are trying to create an SPS dominated tank. A tank full of acropora can be challenging enough without adding large corals that can stunt their growth into the mix.

Having said that, there are plenty of people that keep stony corals with leathers. It may not be ideal but it can be done. There have been plenty of times that I have kept leather corals and acropora in the same tank. If you decide to try something like this in your reef aquarium, there are three techniques that can help.

The first technique is to perform more water changes. By changing out water, this will serve to dilute the concentration of toxins that build up in the tank.

Second method is running activated carbon. Activated carbon works by permanently binding up compounds through adsorption which happens at the surface of the media. As the media fills up it will have to be replaced, but activated carbon is inexpensive and readily available.

The third and by far the least commonly used method is to run ozone in the tank. Ozone is O3 a highly reactive molecule that has powerful oxidizing properties. In an aquarium application it is generated by an ozone machine, usually by passing oxygen through an electrical field splitting the O2 molecule and allowing the oxygen atoms to re form as ozone.

In the aquarium, ozone has a variety of effects such as clearing up the water as the ozone breaks down tannins that give our aquariums a yellow cast. It also virtually eliminates any tank odors. The effect we are interested in for this discussion is how affects the toxins that corals can emit. The extreme reactivity of ozone causes these toxins to break down upon contact to less harmful compounds with little to no bioactivity.

There are severe downsides to ozone, such as safety risks if there is a malfunction. If you are interested in trying ozone, as I am, be aware of the risks and have security measures in place to detect possible leakage of ozone into the air.


 Summary

That pretty much does it for our discussion on coral aggression. To sum things up, corals developed all kinds of adaptations to gain a competitive advantage in the battle for real estate on the reef. In our home aquariums we have to be conscious of these in order to create the best environment for them long term. Spending extra time to consider placement of corals to minimize risk of two corals battling or falling into one another can save you a massive headache down the road. Also being aware of potential chemical aggressions and dealing with them proactively can be the difference between a tank that is just holding on vs. one that is thriving.

Best of luck in your own tank and I hope to see you all next time. Happy reefing.

Than Thein

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