Fijiguide.com's Roberta Davis, owner of Makaira, has been called the Master Coral Gardener of Taveuni. Perhaps no one has done more to replant coral than Roberta, who hails originally from Honolulu. This was evident after Cyclone Tomas, which devastated coral the length and breadth of the island.
Today we begin a three part series on everything you ever wanted to know about the Crown of Thorns and how to remove it. Fiji Resort owners and Fiji dive operators take note!
Crown of Thorns, the Cancer on Fiji's Reefs
For the coral gardener the worst plague that can occur on a reef is the Crown of Thorns Acanthaster planci) in human terms these gnarly creatures would be considered the cancer of the reef.
The Crown of Thorns is the second largest Starfish; the largest is the Sunflower Starfish. It receives its name from the venomous thorns that cover its body. They are endemic to the Red Sea and the Indian and Pacific Ocean. In short they can be found wherever there are tropical reef systems.
The crown-of-thorns is a corallivore or a carnivorous predator that preys on reef coral polyps. They crawl onto reef structures, and then extrude their stomach onto the coral. They release a digestive enzyme that liquefies the coral tissue enabling the starfish to absorb nutrients from the liquefied coral tissue, leaving a dead white coral skeleton in its wake.
Most references claim they are nocturnal or night time feeders. From our observations, they may feed more at night but they certainly are not missing meals in the day either.
Those venomous spines they brag about can easily pierce through gloves and wetsuits. To make matters worse, those spines are like hypodermic needles filled with a venomous neurotoxin that can cause nausea, vomiting, swelling in the area.
They are from the same Echinodermata family as Sea Urchins, but the pain from their spines is more severe and longer lasting since the their thicker thorns tend to break off under the skin and throb for weeks.
There are some that claim that their venom is a cumulative poison, which means it never completely leaves your body. After repeated exposure one will achieve the point of critical mass that could cause death or grave illness.
Scientists don’t agree as to how long Crown of Thorns have existed, but it is safe to say they have existed for a very long time and during that time have evolved some interesting mechanisms to survive as a species and individually.
When spotted they appear sedentary. According to Scientists they can lift all those 17-23 legs and run an incredible 20 miles an hour over a sandy bottom and move a bit slower over the reef. One visiting scuba diver got quite a shock when she saw one stand up and make a mad dash. Before realizing this on one occasion while coral gardening, we spotted two and marked the spot. No more than 10 minutes later, they were long gone and nowhere in the surrounding area to be found.
Once they find a nice feeding area they emit a chemical to alert other Crown of Thorns to congregate to the area. Although they normally get no closer than 6 feet from each other that happens to be the perfect distance for breeding with a better than 90% rate of sea borne fertilization.
One Crown of Thorn can devour 65 sq. ft. of reef per year. However, they can store food and survive for up to 6 months on their reserves. They have an average life span of 4 or 5 years. They can also transmit coral diseases throughout the reef system. Although they don’t have eyes they can chemically sense predators. There is no doubt even with these mechanisms they are a highly evolved reef predator.
However, the most alarming survival mechanism they possess is when they are stressed out; they immediately start their fertilization cycle and simultaneously release a pheromone or chemical hormone that creates a chain reaction with every Crown of Thorns in the area to trigger their fertilization mechanisms. So you seriously do not want to get them excited and inadvertently create legions of new Crown of Thorns. Nor do you want to leave them alone to devour your favorite reef in epidemic proportions.
Top photo courtesy of Roberta Davis
Middle photo taken near Qamea Island by Matt Wright
Bottom photo courtesy of Paddy Ryan