This method of feeding is usually sustainable however due to a population explosion of Crown of Thorns starfish destruction of the reefs has become such as it can no longer be overlooked.
Before numbers of Crown of Thorns starfish became obviously dangerous there were approximately one per every km of reef or even less in some cases. Now the species is identified as an aggregative species (tend to gather in numbers). In some worst case scenarios significant numbers of starfish have been known to aggregate together, piling upon one another to get to the coral below.
Crown of Thorns tend to gather together in huge numbers on a reef system. The reason for aggregative behaviour is unsure as when the starfish aggregate extreme competition for food results.
Reef systems that have been extensively grazed by Crown of Thorns starfish take up to 15 years to recover depending on the distribution and species of new coral larvae in the area. Some corals grow faster however if for example a starfish plague returns to that reef in the next 5 years before area time to recover the slower growing corals have no chance to recover. This theory could mean that reefs would lose their biodiversity if every time the reef was destroyed faster growing coral became the dominant species due to the advantage they possess in the growth rate.
There are a few known predators of Crown of Thorns starfish: the starry pufferfish (Arothron hispidus), two species of trigger fish (Balistoides viridescens) and (Pseudobalistes flavimarginatus), one species of shrimp (Hymenocera picta), a species of worm (Pherecardia striata) and also the humphead maori wrasse.
The main predator of the CofT starfish adult is the Giant Triton Shell (Charonia Tritonis). The shell of this species is extremely beautiful and due to tourist interest known to be collected and sold by locals. Tritons are slow growing and the bigger they are the more eggs they lay yet the bigger the shell is the easier it is to be spotted by divers looking for an easy dollar. This market is thought by some as the reason for the increase in starfish numbers.
The Humphead Maori wrasse is an active CofT predators and is able to consume more than the Giant Triton. The Humphead Maori Wrasse has historically been taken in the Great Barrier Reef line fishery – and more recently in the live fish fishery – From December 2003, it became illegal to fish for the Wrasse under Queensland fisheries laws.
The swell in human activity along coastlines has caused an increase in fertiliser runoff. This fertiliser does exactly what it was designed to do, helps plants to grow. Algae growth has caused damage to coral and thus the damage is increased along with that already caused by the starfish. In some countries fishing methods are not regulated. Dynamite is often used to stun fish so that they can be easily plucked from the surface. This has devastating effects on the reef systems.
It is often thought that Crown of Thorns starfish are able to sense damaged coral and aggregate in that area as they know an easy meal can be obtained. This has not yet been proven yet if true it could vastly increase the damage already sustained by the area.
Crown of Thorns starfish are an example of exponential growth. They continue to multiply and unless there are no limiting factors (predators/prey, birth rate, disease, introduction of new species, immigration/emigration) the growth is unstoppable. It is obvious (due to the increase in numbers) that the limiting factors are limited or nonexistent. Crown of Thorns starfish are known to aggregate together for reproduction purposes. They spawn from December to April when the water temperature is about 28°C. Egg and sperm is released into the water where the mingling causes fertilisation to occur. The eggs then float around on the current for 2-4 weeks. This drifting period could be responsible for further distribution of the species. They then settle onto the reef where they stay till they are about 6 months old.
Crown of Thorns starfish are sexually mature at 2-3 years old and they breed for 5-7 years. Each female can produce 60 million eggs during one spawn season. The fertilisation rate is the highest measured for any invertebrate thus a small population of starfish can produce a very large number of offspring. This would not pose a problem if less juvenile starfish were surviving to become sexually mature. But the case is the increase of the nutrient flow into the sea has caused an increase of planktonic food for larvae (small Crown of Thorns).
Crown of Thorns starfish can currently be controlled in a few different methods and each has its own advantages and disadvantages.
Biological Control
The Triton most likely disappeared due to over-harvesting by pacific island villages
The Giant Triton has been favoured for use as a biological control as they are a known predator of the starfish.
Chemical Poisoning
Physical Removal
Introducing a Virus/Disease
The Crown of Thorns starfish are a natural part of the reef ecosystem however over the past 50 years numbers have boomed. This is thought to be due to a decrease in predators for all starfish ages and an increase in planktonic food available to larval starfish. The increase of starfish could not only affect habitats of many species of fish but it could also cause extensive loss to the tourism economy that depends heavily on the reef.
The current methods of eradicating the starfish (Physical Removal) are expensive and barely effective. The starfish move back just as fast as they are removed. They are short term solutions and unless a more successful control method is put in place reef systems around the world could be damaged permanently.
