Island blog
A Coral Reef’s Nemesis – The Crown of Thorns
Perhaps no other star fish receives more attention than Crown of Thorns. This attention is not necessarily born from love. The insidious Crown of Thorns (COTs) are voracious predators of hard corals, with their ability to reproduce rapidly and literally mow down large tracks of coral within a short period of time.
The Crown of Thorns population swung way out of balance decades ago when their main predator disappeared. This predator of the COTs was the “Conch” the beautiful and large seashell found in tropical waters (the same conch used by Polynesians as a hand help horn for signaling others). When Conches were plentiful they enjoyed eating the COTs, poison spines and all, by climbing out of their big seashell home and enveloping the COT completely within the conch body. After swallowing a few thousand COTS this way the two species reached a neutral balance and coexisted together. Humans broke this balance when they discovered the muscular conch body was delicious. When tourists began to arrive they wanted to buy conch shells as souvenirs. When faced with this two-pronged offence the Conch disappeared quickly and the COT faced a world without predators and the COT population exploded.
The method of destruction employed by the COTs is a gruesome sort of “beauty and the beast” story. Like all sea stars, the bottom of the COT is made up of hundreds of suction tentacles. As the COT moves across the hard coral surface (covered with thousands of holes each being the home built by one small coral animal called a polyp) the COT adroitly
aligns each suction tentacle over one coral home and sucks the little coral polyp out of its home and eats it. An efficient COT can get hundreds of its tentacles working in unison on individual holes (homes) and wreak havoc on a large coral structure in just a few minutes leaving behind an empty and dead coral carcass.
Protecting the corals
Fortunately the NoaNoa reef has never suffered from a huge infestation of COTs, just a few COT visitors here and there. Since the local government appointed me as the guardian of the NoaNoa Marine Sanctuary, I snorkel around and about the reef on a daily basis and am always on the lookout for anything amiss. When I spot a COT, I am determined to remove it from the reef but that is not always easy to do.
Swimming down deep to reach the COT is the easy part. Once there I can’t just grab the COT because the top side of the COT is lined with poisonous thorns while the hundreds of suction tentacles keep it firmly glued to the coral surface. Tempting as it may be, I can not smash the COT with a rock or cut it with a knife. If I did the COT would instantly release thousands of eggs into the sea. The COT must be treated gently, taking care not to hurt or alarm it. Sometimes, if I am close to shore, I can find two large, dead, plate or antler corals with which I can use one piece to slide underneath the COT while using the other piece to trap the COT from the top. Then, very carefully swim towards the shore while the COT squirms and wiggles its many legs covered with poisonous thorns towards my hands. Inevitably I have to release the COT and get a new grip on it every few minutes to prevent getting stung by the thorns. Once on the shore the COT dies quickly.
Most of the time I am far from the shore and have to mentally mark the underwater location of the COT and then swim a few kilometers back to the Beach Bar where I can arm myself with long cooking tongs and an old rice sack before swimming back to the COT’s location. Usually the COT is still in or near the same spot, sometimes the COT
can’t be found.
An an ongoing battle
The Great Barrier reef has been battling COTs for years and the Australian marine biologists invented a devise which looks like a cattle prod (long narrow tube, pointed at the end) whereby the swimmer can simply push the sharp end of the tube into the COT thereby injecting a dose of acid into the COT. The acid effectively prevents the COT from releasing eggs and renders death shortly thereafter. In locations infected by thousands of COTs this sort of device came in very handy with a quick kill and no need to remove the dead COTs. Numerous other nations purchased these devices as part of their effort to control the COTs but some of the Australians marine biologists started noticing a troublesome pattern.
The troublesome pattern was this: numerous, seemingly spontaneous, releases of eggs from thousands of COTs. As the biologists searched for an explanation the pattern became clear. As large populations of COTs were attacked by the acid injection tools, nearby COT populations that were not under attack would release their eggs. Long story short: Somehow the COTs could sense the destruction of their species nearby which caused the unharmed COTs to release their eggs. Needless to say the acid method is not used much anymore. The best method of COT control has reverted back to tongs and a sack. Many Marine Biologists have explained to me that COTs in a reef is not really a bad thing, it is just a fact of a natural eco system. I’m not comfortable with this explanation. The Conch is the primary natural predator of the COT and Conch are now very rare. Too many tourists are willing to buy the beautiful Conch shells and too many locals are happy to capture the Conch to make a nice soup and then sell the shell for a good chunk of cash. The main natural predator of COTs is gone but the COT has a new predator in the NoaNoa reef: Me.
Years ago, I would collect four to six COTs per month in the NoaNoa reef. Now I can only find about one every three or four months. For now, it appears I have the situation under control, but I know that close observation must continue since the reproduction abilities of the COT are so effective and rapid. Don’t worry about me though, I enjoy my work.
-Andy Bowers