The crown-of-thorns starfish is a marine creature living in the Indo-Pacific region. These starfish have sharp, venomous spines. They come in various colors, like purplish-blue and greenish-gray, making them a sight to behold. But beware! Their beauty can be deceiving. They can cause severe damage to coral reefs.
But what makes these starfish such a threat to coral reefs? Well, they play a crucial role in maintaining coral diversity by munching on the faster-growing corals. However, when their population exceeds the threshold, they can munch on more coral than can be replaced, leading to long-term damage to the reef.
Shockingly, a single Crown-of-thorns Starfish can eat around 10 square meters of living coral in a year.
Despite their destructive habits, Crown-of-thorns Starfish are incredibly interesting creatures with unique behaviors and characteristics. So let’s take a deep dive into the world of the Crown-of-thorns Starfish and discover what makes them so fascinating!
About Crown-of-thorns Starfish – A Quick Biology
First, take a quick look at this amazing fish.
|Scientific name||Acanthaster planci|
|Habitat||Coral reef regions|
|Size||Average 25 to 35 cm in diameter|
Taxonomy and Classification of Crown-of-thorns Starfish
The World Register of Marine Species classifies the Crown-of-Thorns Starfish as a member of the phylum Echinodermata. Specifically, it belongs to the Asteroidea class, which includes about 1,500 starfish species. The COTS is the only member of the Acanthasteridae family and the Acanthaster genus.
In Latin, the COTS’ scientific name is Acanthaster planci, which translates to “spiny star.” There are four recognized subspecies of COTS, each with subtle differences in distribution, shape, and coloration.
- Acanthaster planci planci: The most widespread subspecies in the Indo-Pacific region.
- Acanthaster planci Solaris: A smaller and more colorful subspecies found in the eastern Indian and western Pacific oceans.
- Acanthaster planci kuroshioensis: A subspecies with longer and more slender arms found in the northwestern Pacific Ocean.
- Acanthaster planci verweyi: A subspecies with shorter and thicker arms found in the Red Sea and the western Indian Ocean.
Geographical Distribution of Crown-of-thorns Starfish
COTS lives in the tropical and subtropical waters of the Indo-Pacific Ocean. It can be found from the east coast of Africa all the way to the Pacific Ocean. You can find this starfish in various places, such as
- Indian Ocean
- Central and Western Pacific Ocean
- Papua New Guinea
- French Polynesia
- Hawaii in the United States.
Fun fact alert! The Crown-of-thorns starfish is particularly prevalent in Australia’s Great Barrier Reef, where its insatiable appetite for coral polyps has caused a significant loss of coral.
Habitat and Range – Where Do Crown-of-thorns Starfish Live in?
COTS is a versatile creature living in various coral reefs, like fringing reefs, barrier reefs, and atolls. They like to live in calm waters, such as lagoons and deep water.
Warm and shallow waters with coral reefs or hard coral communities are their favorite places to be since they give them plenty of food and shelter. These starfish can live from 1 to 70 meters but are most common at depths of 10 to 30 meters.
Physical Characteristics/Anatomy – What Do Crown-of-thorns Starfish Look Like?
Body and Size
This starfish has a disc-shaped body with multiple arms radiating from it. It can have up to 21 arms. The average size of an adult COTS is about 25 to 35 cm in diameter. But, some individuals can reach impressive sizes, growing up to one meter in diameter. The species can range from 0.5 to 5 kg in terms of body weight.
COTS comes in different colors. They can be
- Reddish gray
- Green with yellow spine tips.
- Brown with red spine tips
Interestingly, their color can change depending on their diet and habitat.
The most distinctive feature of COTS is its long, sharp spines covering its upper surface and sides. These spines are 4 to 5 centimeters long. They serve as a defense mechanism against predators. They also help the starfish grip onto corals while feeding.
The underside of each arm has a string of plates that make a groove with rows of tube feet that stretch to the tip of each arm. These tube feet help in movement, feeding, and sensing.
Mouth, Jaw, and Anus
The mouth of this starfish is on its underside at the center of its body. It has no teeth but a powerful jaw that can open wide enough to engulf large pieces of coral. The anus is located on its upper surface near one of its arms.
The crown-of-thorns starfish has no brain or central nervous system. But, it has a nerve ring around its mouth that connects to radial nerves along each arm. It also has eyes at the end of each arm that can detect light and darkness.
COTS has several behavioral adaptations that help it survive and thrive in its coral reef habitat. Some of these adaptations are
They are highly mobile. According to the Great Barrier Reef Foundation, they can move up to 20 meters (66 feet) per hour using tube feet. They can also use their spines to cling to rocks or corals when needed. (Source)
This starfish is naturally nocturnal. It is more active at nighttime than in the daytime. During the day, you can’t really spot it as it hides under rocks or crevices. It might even bury itself in sand or rubble to avoid detection.
At night, it emerges to feed on coral polyps by crawling over them with its tube feet and extruding its stomach out of its mouth. It then wraps its stomach around the coral and digests its tissues with enzymes. (Source)
Starfish possess a unique defensive mechanism to defend against predators. They release a poisonous secretion, plancitoxins, that coat their spines, which can harm the liver.
Similarly, during times of perceived threats, they emit saponins – substances that can destroy red blood cells. The combination of these toxins can inflict significant distress on any creature, including humans, who come into contact with them.
Feeding Habits and Diet – What Do Crown-of-thorns Starfish Eat?
COTS are opportunistic carnivores that feed on corals, invertebrates, and dead animals.
According to Scientific Reports,
“COTS are voracious feeders that can consume up to 10 square meters of coral annually. They have been known to feed on over 50 species of coral. They can quickly consume large amounts of coral tissue, significantly damaging coral reefs.”
These starfish target hard or stony corals, preferring fast-growing varieties like Acropora and Pocillopora. But it also eats other types like Porites, Favia, and Goniopora.
COTS has a unique way of eating – it pushes its stomach out of its mouth and coils it around the coral polyps. By releasing particular enzymes, it liquefies the coral tissues. It obtains the essential nutrients before withdrawing its stomach back into its body. Interesting, isn’t it?
It prefers specific colors and shapes of corals, depending on their availability and nutritional value. However, it avoids corals with chemical defenses, such as fire and soft corals.
Did you know? COTS have a true image-forming vision, which is rare among starfish.
Reproduction and Lifecycle
COTS reproduces through spawning. Males and females discharge their gametes into the water for fertilization. Unlike some starfish species, which can reproduce asexually via somatic fission or arm autonomy, the crown-of-thorns starfish can only reproduce sexually.
Scientific evidence indicates that this species discharges chemicals that stimulate spawning in nearby individuals. Still, not all individuals in a particular population reproduce simultaneously.
When this starfish reproduces, it reaches a high point on a coral and twists its body. It then releases gametes through five small openings on its upper surface while quickly moving its arms and tube feet.
COTS are dioecious, meaning they have separate sexes. They can lay millions of eggs or sperm at a time, usually during the summer when the water temperature is high. Populations in the north lay eggs between May and August. Whereas those in the south lay eggs between November and February.
The fertilized eggs grow into planktonic larvae that float in the ocean for 10-30 days. During this time, they undergo several stages of development, including a bilateral stage, a brachiolaria stage, and a bipinnaria stage.
The larvae then settle on the reef and metamorphose into juvenile starfish. The juveniles are tiny (about 1 mm) and cryptic, hiding in crevices or under algae. They feed on microorganisms until they grow large enough to eat coral. They reach maturity at about two years. (Source)
Predators – What eats Crown-of-thorns Starfish?
The crown-of-thorns starfish have few predators due to its size and venomous spines. But there are a few creatures that have learned to prey on it. These include:
- The giant triton snail can grow up to 50 cm long and has a powerful toothed tongue that can drill through the starfish’s spines and shell.
- The titan triggerfish can grow up to 75 cm long and has solid jaws and teeth that can bite off chunks of the starfish’s arms.
- The starry pufferfish can grow up to 120 cm long and has sharp teeth that can pierce through the starfish’s spines and skin.
- The humphead Maori wrasse can grow up to 230 cm long and has thick lips and teeth that can crush the starfish’s spines and shell.
- The yellow-margin triggerfish can grow up to 60 cm long and has solid jaws and teeth that can bite off pieces of the starfish’s arms.
- The guard crabs use their claws to grip the starfish’s tube feet and even damage its stomach lining to protect their homes.
- The harlequin shrimp can grow up to 5 cm long and has pincer-like claws that can sever the starfish’s tube feet and arms.
- The lined worm can grow up to 3 m long. It can cut off the starfish’s arms with its sharp jaws.
Do you know these predators are becoming scarce in many areas of the Indo-Pacific? Yes, that’s true! That is due to human activities like overfishing, habitat loss, pollution, and climate change. And as a result, these predators are less able to control the starfish population.
The crown-of-thorns starfish has both good and bad effects on coral reefs and human lives.
On one side, it plays a vital role in keeping coral diversity and resilience. By consuming fast-growing corals, it allows slower-growing corals to have a chance to compete for space and resources. This enhances the complexity and diversity of coral reefs. This starfish is also a food source for predators important for eco-tourism and fisheries.
However, on the other side, when the starfish population grows too large, it becomes trouble for coral reefs. It consumes massive amounts of coral, destroying the reef’s structural complexity. This, in turn, impacts the ecosystem’s diversity and resilience. It also affects the livelihoods of people who depend on tourism and fishing in the area.
In some places, the uncontrolled growth of these spiky creatures threatens coral reef survival. However, by adopting innovative management strategies, scientists and conservationists can control their numbers and limit their impact on the reef ecosystem and local communities.
Conservation Status – Are Crown-of-thorns Starfish Threatened or Endangered?
Crown-of-thorns starfish are not considered threatened or endangered. In fact, they are quite the opposite. COTS are known for their population outbreaks, which can significantly damage coral reefs. This happens when their numbers suddenly increase, upsetting the balance of the ecosystem.
Therefore, instead of being protected, they are seen as a problem. They are often targeted for control and management efforts. The main goal of conservation efforts is to keep coral reefs healthy. This helps to maintain the fragile balance of the marine ecosystem.
Exciting Crown-of-thorns Starfish Facts
COTS is a fascinating marine creature that has both beauty and danger. Here are 10 exciting facts about this spiny starfish:
- COTS is the second largest starfish on the planet, reaching up to 1 meter and weighing up to 5 kg.
- It can remarkably regenerate lost arms and even grow new ones from severed pieces in around 6 months.
- A large female COTS can release 200+ million eggs annually!
- Juvenile starfish can be quite elusive. They are experts at hiding, making them hard to spot. They love munching on crustose coralline algae.
- COTS can live up to 15-17 years.
Watch this video:
Crown-of-thorns Starfish Destroying the Great Barrier Reef
The Great Barrier Reef has suffered a significant coral loss due to crown-of-thorns starfish outbreaks since 1962. An outbreak occurs when 15 or more of these fish are found in a one-hectare area. Currently, a fourth outbreak is occurring within the World-Heritage Area.
These outbreaks usually begin offshore from Cairns or farther north and gradually expand southward along the Reef, usually lasting for about a decade. According to research, the reefs affected by these outbreaks may experience coral mortality rates as high as 90 percent. (Source)
FAQs on Crown-of-thorns Starfish
Why was the Crown-of-thorns Starfish introduced to Australia?
The COTS was not intentionally introduced to Australia. It is a native species of Australia. It has been present in the Great Barrier Reef for countless years.
Can a Crown-of-thorns Starfish kill a human?
The Crown-of-thorns Starfish has venomous spines. These spines can cause
- Severe pain
- Allergic reactions
However, it’s highly unlikely for this starfish to kill a human. The only exception is if someone has a pre-existing medical condition or suffers from anaphylactic shock.
How do Crown-of-thorns Starfish destroy coral?
Crown-of-thorns starfish destroy coral by feeding on the polyps that make up the coral structure. They enter the coral and remove the polyps using their elongated arms, leaving only the colorless coral skeleton.
What causes Crown-of-thorns Starfish outbreaks?
The exact causes of Crown-of-thorns Starfish outbreaks are not fully understood. However, some possible factors include
- Overfishing of their natural predators,
- Increased nutrient levels in the water due to agricultural runoff and sewage,
- Changes in ocean temperature and currents due to climate change.
Are Crown-of-thorns Starfish poisonous?
COTS is not poisonous but venomous. This means that it does not secrete or inject poison into its prey or predators but rather has spines that contain venom glands. The venom can harm humans and animals if they touch or step on the starfish.
What is the common name for the Crown-of-thorns Starfish?
The Crown-of-thorns Starfish are also called Acanthaster planci or COTS.
Crown-of-thorns starfish are marine invertebrates known to consume coral and thrive on Indo-Pacific reefs. Their population can quickly spiral out of control, causing significant damage to hard coral communities. The consequences of COTS destruction extend beyond marine life, impacting humans who rely on coral reefs for food, tourism, and coastal protection.
Despite employing several population control methods, including manual removal, chemical injection, and biological predators, we still face limitations and challenges that require further research for sustainable and effective solutions.