Written by Ernest Shipp, a marine hobbyist
Edited by Neale Monks

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photo by R2R member @swannyson7

There is a difference of opinion whether or not coral have, shall we say, ‘behavior.’

This idea has not sat well with me ever since I had listened to a podcast recording of a lecture about the decline of the world’s coral reefs. The author of the presentation decided that he was going to focus on the reef fishes as indicators of the health of the reef environments that he was studying. He argued that this worked well because they are active and animated, and therefore have obvious behaviors that might be expected to change under environmental stresses.

The lecturer was of the opinion that coral does not behave, but let me tell you, coral certainly does behave! They are just as animate as fish are; corals are active and if you watch them carefully, it doesn’t take long to see that they do indeed behave.

Corals behave!

Although they do not swim, they are hunters, using stinging cells called nematocysts to capture and kill their prey. But in a sense they are also food gatherers as well, because in building the reef they cause upwelling, and in doing so gather up nutrients that would otherwise not be available. Most remarkably of all, they are farmers, cultivating photosynthetic organisms (zooxanthellae) that are typically single-celled algae called dinoflagellates in their tissues.

Corals are also exhibit social and sexual behaviors. They are territorially aggressive, and will try to kill nearby corals that grow into their space. Individual coral polyps may be small, but they are able build some of the most spectacular monuments of nature anywhere in the world. Even their breeding behaviors can be remarkable and dramatic, most famously the synchronized spawning events tied to the phases of the Moon.

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photo by R2R member @swannyson7

What got to me while I was listening to this lecture was the implication that fish would be better indicators of the coral reef environment than the corals themselves, which I feel are more sensitive to their environment than fish and surely just as likely to show behavioral changes.

Not to discredit his findings—his conclusions are very well documented and he did prove his theory very well—but my question is whether he ever looked at a reef tank? Maybe; he did not say. But I’d argue that watching corals day-in, day-out within a captive environment would quickly remove any belief that corals lacked behaviors!

How I learned to love corals, and to watch them too…

I grew up swimming in the Gulf of Mexico. I fell in love with the South Pacific—thank you Uncle Sam—I then of course I started keeping a few freshwater fish. But one day I walked into the saltwater section of my local aquarium store, and that was that, I was hooked!

I practically dove into the marine aquarium scene, and right now I have two 150-gallon systems, one fish-only, the other a mixed reef. I also have a 60-gallon freshwater tank for plants and angelfish, and last but not least, a 30-gallon cube for cichlids. While I’m not an educated scientist or a marine biologist, I have educated myself by reading and doing research into the best ways to keep all these amazing animals. I also try to listen to educated professionals and scientists who know the most about aquatic life.

I am proud to call myself a marine hobbyist, a person as interested in marine stewardship and science as he is in keeping ornamental fishes and exotic corals. In the short six years I’ve been in the marine side of the hobby, I feel that I have only begun to explore the realms of the marine world, I feel I like I am learning every day and finding out that the more I learn about the realms of possibilities to explore, the less I think I know!

What I’m presenting in this article through simple observation of corals across six years in the controlled environment of a marine aquarium. There’s also that vast amount of ‘tribal knowledge’ in the hobby community to draw upon; one-off observations may not matter much, but if dozens or hundreds of hobbyists have seen the same thing, that’s an altogether more compelling set of data.

Different types of coral behavior

Coral reefs take centuries to develop, setting a time frame on coral biology that short-lived critters like us don’t necessarily understand. Coral reefs are built by countless generations of coral polyps growing and then dying, creating the base for the new generation of coral polyps to build upon, for the next hundred years or so. But while one individual polyp may not live for long, within a single colony all the polyps are clones, so the coral organism itself grows across a continuous process that spans centuries.

This is not a scientific paper, but rather an article to inform and entertain, so I will be using the common names of corals so far as possible, as well as some of the hobby lingo. In any case, hobbyists divide corals into three basic categories. The first are the “soft corals” or “softies” that are usually soft and spongy, as the name suggests. Next are the “large-polyp stony corals” or “LPS” corals that usually have a stony skeleton with big frilly polyps that are colorful and extend into the current. Finally are the “small-polyp stony corals” or “SPS” corals. This type is ridged, whether it is a plating-type or something like the endangered Elkhorn Coral of the Florida Keys.

One of my observations is that members of each of these three categories of corals behave in different ways. Since this isn’t a book or PhD dissertation I’ll limit myself to one or two observations, but I hope this article will prompt you to make your own observations!

Aggression among small-polyp stony corals

Let’s start with the SPS corals. They’re particularly important because they’re the ones that build the reef. But while they may look nice, they’re actually very dangerous, lethal even, and exhibit aggressive behavior towards any other corals that get too close to them.

Montipora capricornis, often called Monti Caps by hobbyists, is a celebrated example of the SPS group noted for its highly aggressive behavior. It competes strongly for the sun-drenched real estate it needs to grow well. It will stunt the growth of nearby corals by growing tall enough to block sunlight from reaching the other corals. I have also witnessed these corals using their harpoon-like nematocysts to grapple nearby corals a bit like a pirate using a grappling hook to board a seized cargo ship. But unlike a grappling hook, nematocysts are connected to thread-like structures that pump poisons into the rival coral, damaging its delicate skin tissue and sometimes even killing it outright.

Attacking corals like Monti Caps can get a surprise though; defending corals have nematocysts too, and they may send these back at the attacker, sometimes even killing it! Corals don’t have eyes so they can’t see what they’re attacking. When a coral initiates aggressive behavior, all it knows is that there is something there that might pose a danger.

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Another example of coral aggression by R2R member @lazylivin

How large-polyp stony corals feed

The next type of behavior I am going to give an example of is the hunting and gathering behavior of corals. Now who likes food? Phytoplankton anyone? I am going to use the group that is called the LPS corals. I am going to use this category of corals because they show gathering behaviors in a particularly clear and easy way to observe. All corals have polyps of course, but these corals have large polyps, so it’s easy to see them, and the larger the polyp, the large the food item the coral can catch and consume.

The coral species that I have picked for this is called the Grape Torch Coral, Euphyllia cristata, sometimes mislabeled as a Frogspawn Coral by new aquarist. This animal has a skeleton that houses green tentacles about an inch in diameter arranged around each branch. This constitutes the rather large polyp head, strikingly so considering that most coral polyps are less than a millimeter in diameter. From the skeleton there are what look like green strands about as thick as a piece of spaghetti; these are about 1 to 1.5 inches long. On the tips there are what look like little round balls that look like little white grapes. In the middle of the strands there is what is referred to as a “mouth-part”. Each branch of the coral colony has its own “mouth-part” to feed.

As the coral gets hungry or senses that there is food in the water near by, the tentacle-like strands reach out to anticipate the capture the food. In this case, we are going to use Mysis shrimp as an example of a food item, a small crustacean that this type of coral finds delicious. As these shrimp enter the domain of the torch coral in question, it extends its tentacles out as far as it can reach. Along with these tentacles, there are also specialized tentacles that are called sweeper tentacles. These reach out at a farther distance than the majority of the colony’s other tentacles. With its arsenal of stinging tentacles, it sits like a deer hunter, sitting and waiting, hunting in its own way. Waiting for the shrimp to come into contact with any of its tentacles.

As soon as this happens, the tentacle that the shrimp has come into contact with, stings it by a toxin that the corals skin contains as well as poisonous nematocyst. After the tentacle stings the prey item, it is delivered to the mouthpart for digestion and the tentacle once again extends out for the next food item. The coral does not rely on a single tentacle to supply its whole feeding. It gathers food items from all of its tentacles to feed the colony. Depending on the size of the colony, there may be as many as thirty to fifty tentacles per polyp head and up to twenty or so polyp heads in the colony. Each tentacle floating in the current, hunting as it waits for its next prey item, gathering it for the survival of the colony.

I just want to make it clear that the LPS coral that I described above take unusually large prey by the standards of corals generally. Almost all coral have much smaller polyps and feed on what is called phytoplankton or zooplankton, both of which are much smaller food items, indeed, in many cases they are microscopic. Most of the corals that feed on these planktonic foods hunt them in the same way as I described above, but on a much smaller scale.

Even though the polyps of such corals are smaller, a lot of them belong to species that as colonies grow much bigger than the Torch Coral that I described. That said, although these corals still have the ability to hunt food items, this behavior provides only a supplemental nutritional source for them. Indeed, for most reef corals their primary sources of nutrition are the sugars produced by the symbiotic algae that live within them.

Time lapse video of a plate coral exhibiting eating behavior

The sex lives of corals

Corals can reproduce asexually, such as through fragmentation. Stony corals may break by accident, maybe after being hit by a falling piece of coral from elsewhere on the reef. Many pieces will end up somewhere too deep or dark to be hospitable, but water currents may sweep along luckier fragments to favorable spots where they can start another colony. Hurricanes and violent storms are particularly important sources of the sorts of physical damage that can fragment colonies and thereby spread them out across the reef.

But other way corals get fragmented is through a kind of self-mutilation; and it is the soft corals that show this type of behavior particularly well. When a mature colony has reached its maximum potential within the spot where it lives, it sometimes pinches itself in two almost as if a tourniquet was being wound around it. When the pinching is complete, the part of the coral that had been pinched off from the colony floats away to another part of the reef, hopefully to settle somewhere amenable to its existence and grow into a new, though genetically identical, colony.

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Leather coral self-fragging
photo by R2R member @Electrobes

Now, for the part that we are all waiting for, the conception of new little baby corals, through the dramatic process of spawning! This is when corals release eggs and sperm into the water column. This is sexual reproduction, allowing genes from different colonies to mix and introduce diversity into the gene pool. As many readers will be aware, these spawning events happen are synchronized so that all the colonies of a given species on the reef shed their eggs and sperm at the same time. That corals are able to do this is remarkable enough, but even more impressive is how many pelagic fish are able to predict when these spawning events will happen, visiting the reef just in time for this all-you-can-eat smorgasbord of coralline caviar. Scientists haven’t figured out how the fish know when this happens, but they do, and it’s uncanny watching them moving in from their open ocean home onto the reef, waiting for the feast.

Once the coral begin to release their eggs and sperm into the water, the water becomes so milky that you cannot see through it. That’s when the feeding frenzy begins. Only those few fertilized eggs that happen to make it past the predators become coral larvae, ready to develop into the polyp that will settle down onto the substrate to start a new colony.

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Torch coral spawning
photo by @BeakerBob

There isn’t much known about this phenomenon, but the little bit we do know about this event is intriguing. For example, we do know that it coincides with the lunar phases, and thanks to observational records across many years we can now predict when it will happen, to the hour, at least for the Great Barrier Reef. This has allowed scientists to collect live coral eggs and sperm so that they can study this little-known part of the coral life cycle.

What makes the Great Barrier Reef interesting is that the whole reef seems to spawn at the same time, once a year, but we are finding that other tropical reefs around the world seem to spawn in the same sort of way. On the other hand, we have found deep-water coral species that spawn on a nightly basis. Marine aquarists are adding to what we know as well; home aquarium spawning of corals is rare, but happens, and these can help us build up a better picture of coral sexual behavior.



The above are only a very few of the different ways in which coral behave, and I believe a thorough review would easily fill a book. My hope is that I have prompted you to find out more yourself, perhaps by watching your own collection of corals a little bit more carefully. So while I’m not disagreeing with the notion that fish exhibit interesting behaviors that can tell us something about the state of the world’s reefs, I think there’s plenty of science to back up a deeper study of the behavior of corals as well.