Crown of Thorns Starfish (Acanthaster planci) is an echinoderm and is one of the few animals that can eat corals. It was first seen in 1957 and is a threat to the Great Barrier Reef. It is covered with venomous thorn-like spines. Its resemblance to the biblical crown of thorns gives its name “Crown of Thorns Starfish (COTS)”. It is native to Indo-Pacific region and is most commonly found in Australia.
COTS has a central disk and radiating arms just like any other starfish. Most starfish have only 5 arms, this Crown of Thorns Starfish can have as 13 – 23 arms.
It is the second largest fish in the world, only behind the Sunflower Starfish. They usually measure 25 – 35 cm in diameter but can also be as large as 80 cm.
Their bodies have good flexibility to bend and wrap around their food.
They can move at a speed of 20 km per hour.
They are usually of dull and subdued colors like brown and gray. Occasionally, one might come across a bright warning colored COTS.
Habits and behaviour
Crown of Thorns Starfish prefers calm waters like that found in the deep sea. They are found in seas surrounding Japan, Australia, Hawaii and, Red Sea.
They are nocturnal.
Threat to coral reefs
One COTS can eat up to 6 square meters of coral per year.
COTS can prey on almost all types of corals. They prefer to feed on branching and table corals which are also the most susceptible corals to bleaching.
It is called a COTS outbreak when the population of Crown of Thorns Starfish reaches 30 per hectare on a coral reef.
COTS advantage in diversifying corals
It is well-established that COTS outbreaks are damaging to the coral reefs. However, COTS existing in small numbers are actually good for diversifying corals since they eat the fast-growing corals and make space for the slow-growing corals.
What causes COTS outbreaks?
The outbreak in COTS population is highly debated, but some of the most accepted hypothesis are:
- Over-collecting of COTS predators like tritons
- Warmer sea temperature
- Phytoplanktons are an important food source for COTS larvae. Phytoplankton blooms cause an outbreak in COTS population. Phytoplankton blooms are caused by sewage or other nutrient releases into the sea water.
How is COTS outbreak controlled?
Methods of controlling a COTS outbreak on the coral reefs include:
- Taking the COTS to the shore and burying them or any other methods to kill them.
- Building fences in the sea to restrict their movement.
- Most effective of these is injecting the fish with bile salts. Bile salts kill them without harming the environment.
Crown of Thorns Starfish have oviparous fertilization, that is the male fertilizes the eggs outside the body of the female. Females produce 60 – 65 million eggs in warm temperatures, which is October to March.
Some quick interesting facts about COTS
- First recorded COTS outbreak took place in Ryukyu Islands, Japan in 1950.
- They a reproduce through sexual reproduction that takes place outside the female body.
- They can survive 9 months without eating.
- COTS have a peculiar feeding style. They feed by turning out their gastric sac and covering their meal by the sac. They then secrete digestive juices to digest it.
- Their favorite coral is scleractinian.
- COTS have been existing on earth for millions of years.
- When under stress, COTS can lose one of its arms, but can also regrow it within a span of 5 – 6 months.
- There is usually a 15-year gap between their population outbreak. The COTS outbreak is dangerous for the corals when the gap is less than 15 years.