Belonging to the phylum Echinodermata, the crown-of-thorns starfish, Acanthaster planci, is a radial marine animal. It is related to sea urchins, sea lilies and sea cucumbers, and is very poisonous. It is found in tropical shallow waters in association with coral reefs. It reaches a diameter of 60 cm and may live for eight years. It feeds on coral polyps. In the late 1960s the crown-of-thorns appeared in increasing numbers on the Great Barrier Reef in north-eastern Australia. By 1968 the area from Gladstone to Cooktown in the north had been attacked, and by the next year more than 250 km2 of reef were dead and the tropical fish had disappeared.

Scientists from many nations collaborated in research to determine the cause of the infestation. Among other things, theories advanced have suggested that humans had been the catalyst, either by polluting the water and killing the organisms which had once fed on starfish larvae or by overfishing the marine species which had preyed on adult starfish, such as the triton trumpet shell, trigger fish, puffer fish and the painted shrimp; that warming of the water had encouraged the population upsurge; and that it was a natural phenomenon occurring at irregular intervals. Examination of material from the ocean bed supported this last theory.

The crown-of-thorns population did not maintain its high level and by the 1970s the Commonwealth and Queensland governments concluded that the starfish did not constitute a threat to the existence of the Great Barrier Reef. Regeneration of damaged coral is possible but in many areas the reef-building stony corals have been replaced by soft corals.

The Crown-of-Thorns starfish (Acanthaster planci)

What do they look like?

An adult Crown-of-Thorns starfish (often abbreviated as COTS) is a large flattened asteroid typically 25 to 35 cm across (COTS over 70 cm have been reported). The body of the COTS is hamburger bun shaped and makes up between a third and a half of the overall disk. Attached all the way around the circumference of the body and filling out the disk in a radial pattern are about fifteen legs. Each of the legs is about one cm in diameter and around 10 cm long. The body and legs are a somewhat dirty rusty red. The body and all of the legs are covered in spines or thorns which are a darker rusty red in colour, three to five cm long and two to three mm in diameter. The end of each spine is needle sharp and contains a toxin.

FAQ: Just how poisonous are the spines?

Speaking from personal experience, if you jab your finger onto one of the spines, the toxin will leave you with a sharp burning sensation which gradually fades away after a few hours. Symptoms including nausea, vomiting and swelling are fairly common reactions to the poison if you get enough of it (a poke from a single spine generally just leaves you with a burning sensation). People who manage to get "poked" fairly often tend to develop a progressively stronger reaction to the poison.

As for what would happen if you were to say, step on a COTS? Well, it is unlikely to kill you unless you suffer a severe allergic reaction but you will probably remember the event for the rest of your life and your foot is going to hurt a LOT for quite some time! With due apologies to the reader, I can't offer any observations drawn from personal experience.

Fortunately(?), walking on a coral reef with bare feet is a pretty foolish thing to do even if there aren't any COTS nearby so actually stepping on a COTS is rather unlikely (you'll probably step on some other nasty thing first). On the other hand, brushing your hand or some other part of your body against one as you swim by while scuba diving or snorkeling is a distinct possibility and falls firmly under the category of "something which is to be avoided"!

FAQ: Why are there about fifteen legs?

The number of legs varies as it isn't all that unusual for a leg to be torn off in the day to day hustle and bustle of COTS life. According to one fairly authoritative reference, the maximum number of legs seems to be 21. The same reference, the monograph listed at the bottom of this writeup, says that adult COTS have been found to possess from one to six anuses. Go figure!

The net effect is of a rusty red coloured vaguely starburst shaped disk which is covered in nasty looking spines. Trust me - you won't reach over and grab one on purpose and you'll regret lightly brushing your hand over it to see how sharp the spines are!

Types of coral

Before we continue, we need to be clear on a couple of points related to coral . . .

Although there are two kinds of coral (hard and soft), it is the hard corals which most of us associate with a coral reef and which are relevant to the COTS topic.

The hard corals come in an amazing variety of colours and shapes. Some of the basic types are:

staghorn coral:

Probably what most people think of when they think of coral. This style of coral looks like a nest of fingers or horns sticking up in all directions.

fan coral:

Looks much like staghorn coral except that it is two dimensional (i.e. flat) and tends to look a lot like an oriental hand fan. Fan corals range in size up to several meters in diameter (the largest one that I've ever seen "in the wild" was about five meters or fifteen feet across).

massive coral:

Tends to look like a lump and typically has some sort of bumpy maze-like pattern on it (brain coral falls into this category). These massive corals can get to be quite large. For example, massive corals that are roughly spherical and four or five meters in diameter are not unheard of (it takes over a thousand years for a coral to grow this large).
Important point - even if you go diving on a coral reef, you are unlikely to see a hard coral. Rather, what you will see are colonies of billions of very small individual "creatures" living on the surface of a dead calcium "skeleton". As the tiny "creatures" die, they leave behind a small amount of calcium. Over a period of years, it is this calcium skeleton which gives the staghorn coral (or any other hard coral) its shape.

Note: understanding that a clump of staghorn coral is actually a colony of creatures living on a dead calcium skeleton is necessary if the essence of the COTS story is to make sense.

The soft corals are just that - soft. These corals tend to attach themselves to something and just sit there swaying in the breeze current. Soft corals aren't really relevant in a COTS discussion so that's all that I'm going to say about them here.

One final piece of trivia: there are thousands upon thousands of different species of coral. Inspecting a piece of coral and determining which species it is can be a task which requires considerable expertise. It might even require laboratory analysis (i.e. two species might look absolutely identical to the naked eye).

Ok. Onwards . . .

What do they eat?

The adult and juvenile COTS diet is pretty simple - they eat the soft fleshy outer layer of most hard corals (COTS start out life as plankton sized creatures which feed on truly microscopic bits of whatever happens to come their way). In other words, they eat the billions of tiny "creatures" that live on the calcium skeleton which gives the hard coral its shape. Obviously, if they eat (i.e. kill) the part of the hard coral that is alive then what is left is dead. What happens next depends on the kind of coral. If it is a massive coral then you're initially left with a bright white lump. If it was a staghorn or a fan coral then you're left with a bright white staghorn or a bright white fan coral. They actually look quite pretty (if you don't think about what they are - dead coral skeletons). Within a few days they start to darken and eventually disintegrate.

If enough COTS eat enough coral then you're left with a field of rubble. Not a pretty sight!

How much damage can a single COTS do?

A single COTS can consume about six square meters of coral per year. This isn't enough coral to have any meaningful impact on the reef. Whatever coral is killed by the COTS is fairly quickly replaced by new coral and it is unlikely that anyone other than an expert could tell that the COTS was ever there six months or so later (the coral won't have grown back that fast but the dead patches won't look much different than any of the other spots on the reef that also don't happen to have any live coral on them).

Where are COTS found?

COTS are found in tropical coral reef systems all over the world. There may be tropical coral reef systems which are totally COTS-free but most tropical coral reef systems seem to have COTS populations. They prefer to live in sheltered parts of the reef away from the wave action.

It's important for the discussion which follows that you be clear on the difference between a coral reef and a coral reef system. Please read the respective writeups, if necessary, before preceding.

How many COTS might live on a single reef?

On a typical coral reef, there might not be any COTS. In fact, if you went diving on a random coral reef, your chances of seeing a COTS are really quite poor. There might be COTS present but there are probably only a few dozen or maybe a few hundred and your chances of finding them on a coral reef that is five kilometers in diameter (i.e. about 20 square kilometers) are pretty much zero (your chances of seeing the bright white (dead) coral that they leave behind aren't much better).

COTS Outbreaks

As it turns out, sometimes there are more than a few dozen or a few hundred COTS on a reef. In fact, sometimes there are a few million COTS on a reef! Trust me - if you go diving on a reef that has a few million COTS on it, you WILL see COTS! Lots and lots of COTS!

This is what is called an outbreak of COTS. When a reef is suffering from an outbreak of COTS, the COTS end up eating a great deal of coral. In a large outbreak (i.e. a few million COTS on a 5km diameter reef), the COTS will eat essentially all of the coral on the reef! What happens next is pretty predictable:

  1. all the COTS on the reef eventually starve to death (the individual reefs in a reef system tend to be at least a few kilometers apart which is way too far for an adult or a juvenile COTS to travel without food).

  2. the ecosystem of the reef (not the reef system, just the reef) collapses. The creatures capable of going somewhere else leave and most of the rest which depend on the live coral in any way die.
Fortunately, the reef eventually recovers (in a sense). Coral larvae drift over the reef from other reefs. Some of these coral larvae will settle on the devastated reef and, over a period of years, the reef recovers (in a sense). In fact, about twenty years after the outbreak, anyone other than a coral reef expert might not be able to tell that the reef had ever suffered an outbreak.

It's the phrase "in a sense" that we need to consider: Before the COTS outbreak, the reef probably had a very diverse ecosystem consisting of thousands of different kinds of coral and many species of plants and other animals. Each of these species had found a niche somewhere in the reef where they could survive if not thrive. Many of these species interacted in various ways. In fact, a coral reef that hasn't suffered something like a COTS outbreak for a century or two will have a truly astounding diversity of life.

Some people have used the analogy of prairie fires or forest fires to describe COTS outbreaks. In a forest fire, a large expanse of forest catches fire and burns. When the fire is out, what's left looks pretty dead. Fortunately, nature takes over and it doesn't actually take all that many years before the forest has recovered. There are even species which require fires to survive. For example, the lodgepole pine has pine cones which will only release their seeds when exposed to fire. i.e. lodgepole pines require at least a small forest fire as part of their lifecycle.

Back to coral reefs, the ecosystem on a coral reef twenty five years after a COTS outbreak is quite different than it was before the outbreak. It is unclear whether or not the "reboot" that a coral reef goes through after a COTS outbreak is natural and simply the way nature works (scientists understand that an outbreak is a major event, the question is "how major?"). If the reef doesn't suffer another outbreak then it almost certainly eventually recovers completely although, if it does, full recovery will probably take centuries.

What's the COTS life cycle like?

When an adult female COTS is ready to reproduce (an annual event after it reaches the ripe old age of two years), it produces about ten million eggs. Once fertilized, the COTS drifts as plankton in the ocean currents for about twenty days (suffering a mortality rate of around 50 percent per day). Once the COTS reaches the age of about twenty days old, if the COTS happens to be still alive and over a reef then it descends/settles onto the reef and starts to grow. The roughly 50% per day mortality rate that COTS suffer while drifting as plankton means that COTS which don't happen to drift over a reef between the ages of twenty and thirty days do not survive.

The first year of a COTS's life is spent as an algae-eating juvenile. Life is tough on a coral reef and most of the algae-eating juvenile COTS will die before they are a year old.

By about their first birthday, COTS are coral-eating juveniles. They are about a centimeter or slightly more in diameter and really aren't capable of eating enough coral to matter (a three year old COTS has been shown to be capable of eating more coral in a day than it consumed in its entire first year of life). The mortality rate for coral-eating juvenile COTS remains high and most of these COTS do not reach the age of two.

By about their second birthday, the surviving COTS are coral-eating young adults which are also capable of reproducing. The normal lifespan of a COTS seems to be about four or so years.

Waves of outbreaks

Let's now consider a few facts:
  • In some reef systems, like the Great Barrier Reef (GBR), there are reasonably regular ocean currents that move through or along the reef system. In the case of the GBR, the current generally flows in a southerly direction down through the 2,000+ mile long reef system.
  • If a COTS outbreak occurs on a reef then the COTS on that reef are likely to produce a vast number of COTS plankton.
  • If the ocean currents are flowing in the right direction and the COTS plankton have a reasonable amount of luck (i.e. they drift over another reef on or fairly soon after their 20th day of life) then there's likely to be quite a few COTS on the downcurrent reef in a couple of years (i.e. another outbreak is likely).
  • Since the COTS plankton from the outbreak reef don't travel as a tight group, they could get spread over multiple downcurrent reefs (i.e. outbreaks can spread to more than one downcurrent reef).
  • This process is likely to repeat itself with more outbreaks somewhere downcurrent a couple of years later.
This can result in a wave of outbreaks traveling down the reef at a speed of a few hundred kilometers every two to three years.

Note that a reef can suffer another outbreak while it is recovering from an earlier outbreak in the sense that it can end up with a huge population of COTS that eats all the coral. The catch is that until it has recovered sufficiently, few of the second "crop" of COTS will survive long enough to produce a vast quantity of plankton (i.e. most will starve to death before it becomes time for them to reproduce). It takes about twelve years of uninterrupted recovery time for a coral reef to have enough coral to support enough COTS long enough for them to produce another vast quantity of COTS plankton.

The result is that if a reef system suffers successive waves of outbreaks then the waves tend to be at least twelve years apart.

Of course, not all reefs in an area suffer an outbreak when a wave comes through. Enough reefs in any given area do generally suffer an outbreak when a wave comes through that there are usually not enough reefs with enough food left in the area to support a second wave (i.e. to produce the quantities of plankton required for the wave to continue beyond the area).

Outbreak pulses

There is, unfortunately, another possible scenario. The ocean currents are not the only factor distributing COTS plankton through the reef system. For example, although the currents in the GBR cause the COTS plankton to drift in a generally southwards direction, other effects, like storms and tidal action, can cause the COTS plankton to drift in other directions. If the number of outbreaks in a reef system is low then these other effects don't tend to have much impact. If, on the other hand, there are enough outbreaks in a reef system then the system could, at least theoretically, shift into a quite different state.

Instead of waves of outbreaks moving through the reef with a reasonable period of time between waves (imposed by the "recovery time immunity" described above), outbreaks and smaller but still serious COTS infestations start to occur randomly throughout the reef. The presence of recovering reefs throughout the system inhibits outbreak waves but there are always enough reefs with reasonable coral populations to keep the COTS population thriving.

Whether or not a reef system in "the real world" has ever transitioned into this "outbreak pulses" state is a matter of some debate:

  • Some researchers argue that outbreak pulses are merely theoretical and cannot happen in "the real world".
  • Others argue that they are clearly possible but have not yet occurred.
  • Still others argue that the GBR has already made the transition and is currently suffering outbreak pulses instead of outbreak waves.
(this is NOT the only area of controversy in the COTS story but that's a topic for another writeup)

A personal note

The Crown-of-thorns starfish is a fascinating creature found in tropical coral reef systems all over the world. The chance to become involved in the COTS research community was, quite literally, one of those "once in a lifetime" opportunities. Here's how it came to pass:

I was working as a software developer for Myrias Research Corporation in Edmonton, Alberta, Canada. In the summer of 1989, Dr. Roger Bradbury of the Australian Institute of Marine Science (AIMS) happened to be on sabbatical at the University of Alberta. He heard about what we were doing at Myrias (trying to build a large scale parallel computer system) and asked if we'd be interested in helping him to develop a research simulation on our system. The company agreed and I was asked to help Roger to develop the simulation.

Over the next few months, Roger and I became close friends and I went from knowing absolutely nothing about COTS (I'd never heard of them before) to being fairly familiar with them and what they'd been doing to the GBR.

I went to Australia the next April to spend a few weeks working on a research paper. My visit was timed to allow me to go on a five day research trip to Davies Reef and Myrmidon Reef for the purpose of doing a survey of the coral. The weather didn't cooperate so the trip was cut short (we never made it to Myrmidon) but it was still the experience of a lifetime.

In the end, life got in the way and the research paper was never published. Roger had been working on other COTS related research topics and he was able to publish results which certainly agreed with what he and I had done although I never did get to see my name in a COTS journal (because none of my research got published - i.e. I'm not complaining!).

My COTS story isn't over yet. There's lots more to tell and, hopefully, I'll find the time to tell it someday. That said, it's time to take a break from noding and go out there and earn a living or something like that.


A COTS book

I recently came across a book on the COTS phenomenon. I have not read it yet but I'm told that it's quite good. Here's the info:
What Is Natural?: Coral Reef Crisis
by Jan Sapp
published by Oxford University Press
ISBN 0 19 512364 6


Sources:

  • THE ACANTHASTER PHENOMENON by P.J. Moran (Australian Institute of Marine Science Monograph Series; volume 7); copyright © 1988 Australian Institute of Marine Science; ISBN 0-642-13250-X.
  • A parallel computer simulation of the large scale dynamics of the Acanthaster pheomenon on the Great Barrier Reef by Roger Bradbury and Daniel Boulet; 18 December 1989 draft; unpublished.
  • The AIMS web site at http://www.aims.gov.au/
  • personal recollections and experiences

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