The crown-of-thorns starfish, often called COTS, is a large sea creature with the scientific name Acanthaster planci. It eats hard coral polyps, which are part of a group called Scleractinia. The starfish gets its name from sharp, venomous spines on its top surface, which look like the crown of thorns from religious stories. It is one of the largest starfish in the world.

A. planci is found in many areas across the Indo-Pacific region. It is most often seen near Australia but can be found in tropical and subtropical waters from the Red Sea and East African coast, across the Indian Ocean, and throughout the Pacific Ocean to the west coast of Central America. It lives in places where coral reefs or hard coral communities are present.

Description

The body shape of the crown-of-thorns starfish is similar to that of a typical starfish, with a central disc and arms that extend outward. However, it has unique features, such as a disc-like shape, many arms, a flexible body, the ability to grasp objects, and a large stomach surface compared to its body size. The ability to grasp comes from many tube feet that line the tips of each arm. Unlike most starfish, which have five arms and a five-fold symmetry, the crown-of-thorns starfish has up to 21 arms and lacks this symmetry. However, it starts its life cycle with five-fold symmetry. This animal has the ability to see clearly.

Adult crown-of-thorns starfish are usually between 25 and 35 cm (10 to 14 inches) long and can have up to 21 arms. Even though the body looks stiff, it can bend and twist to move around coral. Under each arm, there are plates that form a groove leading to the mouth. These starfish can be purple, purple-blue, reddish grey, brown with red spines, or green with yellow spines, depending on their food or location.

The long, sharp spines on the starfish’s arms and upper surface look like thorns and form a crown-like shape, which is how the animal got its name. These spines are 4 to 5 cm long, very sharp, and can easily pierce soft materials. Even though the upper surface has sharp spines and the lower surface has blunt ones, the starfish’s body is soft and covered in a thin, flexible membrane. When the starfish is taken out of water, the membrane breaks, fluid leaks out, and the body collapses and flattens. The spines also bend and flatten. If the starfish is still alive, it can return to its normal shape when placed back in water.

Taxonomy

The family Acanthasteridae contains only one genus. Its place within the group Asteroides is not clearly defined. It is generally seen as a group that is very different from others. Recently, paleontologist Daniel Blake studied the physical features of A. planci and found it shares many traits with members of the Oreasteridae. He moved the Acanthasteridae from the group Spinulosida to Valvatida and placed it near the Oreasteridae, suggesting it may have evolved from that group. He believed the physical features of Acanthaster may have developed due to its movement across uneven coral surfaces in high-energy environments. However, Acanthaster is not a single-species genus, so any study of the genus must also consider another species, Acanthaster brevispinus, which lives in a completely different environment. A. brevispinus lives on soft surfaces, sometimes buried in the substrate, at moderate depths where the surface is smooth and has little wave activity.

A. planci has a long history in scientific research, with much confusion about its name from the beginning. Many different names were used for it over time. Georg Eberhard Rhumphius first described it in 1705, calling it Stella marina quindecium radiotorum. Later, Carl Linnaeus named it Asterias planci based on an illustration from 1743 when he introduced the system of two-part scientific names. No original specimens are known, and the specimen used by Plancus and Gualtieri in 1743 no longer exists.

Other names used for the crown-of-thorns starfish included Stellonia, Echinaster, and Echinites, before the name Acanthaster was finally accepted in 1841. Other specific names included echintes, solaris, mauritensis, ellisii, and ellisii pseudoplanci (with a subspecies). Most of these names arose from confusion in older scientific writings, but Acanthaster ellisii became the name for a distinct starfish in the eastern Pacific Gulf of California.

The eastern Pacific Acanthaster is very different from other populations. It has a round body, a large disc-to-total diameter ratio, and short, blunt spines.

Nishida and Lucas studied genetic differences in 14 locations across 10 populations of A. planci using a method called starch-gel electrophoresis. Samples were collected from the Ryukyu Archipelago (four locations), Micronesia (two locations), and one location each in the Great Barrier Reef, Fiji, Hawaii, and the Gulf of California. A sample of 10 A. brevispinus from the Great Barrier Reef was also included for comparison. A. brevispinus and A. planci showed large genetic differences (D = 0.20 ± 0.02). However, genetic differences among A. planci populations were small (D = 0.03 ± 0.00; Fsr = 0.07 ± 0.02) despite being far apart. A positive relationship was found between genetic differences and geographic distance, suggesting that A. planci populations remain genetically similar due to larval dispersal through ocean currents. The Hawaiian population was most different from others. Treating the eastern Pacific Acanthaster as a separate species (A. ellisii) is not supported by these data. The lack of unique genetic markers in the central (Hawaii) and eastern Pacific (Gulf of California) populations suggests they originated from western Pacific populations.

More details about the genetic relationship between A. planci and A. brevispinus are described in the entry for A. brevispinus. These species are closely related, and A. planci, which feeds on coral, likely evolved from A. brevispinus, which lives on soft ocean floors.

In a large study, Benzie examined genetic variation in 20 A. planci populations across the Pacific and Indian Oceans. A major finding was a clear difference between Indian and Pacific populations. However, populations near northern Western Australia showed strong ties to Pacific populations. Except for southern Japanese populations, which were closely related to Great Barrier Reef populations, genetic patterns within regions followed a pattern of decreasing genetic similarity with increasing distance. Benzie suggested that the split between Indian and Pacific populations began at least 1.6 million years ago, likely due to climate and sea level changes.

A more recent study by Vogler et al. used DNA analysis (one mitochondrial gene) and found A. planci may actually be a group of four distinct species or clades, each linked to a geographic region: Northern Indian Ocean, southern Indian Ocean, Red Sea, and Pacific Ocean. These groups are thought to have split 1.95 and 3.65 million years ago. (The split between A. planci and A. brevispinus is not included in this timeline.) The researchers suggested differences in behavior, diet, or habitat among these groups could help design better reef conservation strategies.

However, this idea of hidden species (cryptic speciation) has issues. Using only one mitochondrial gene (mtDNA) to identify species is debated, and other genetic data, like allozyme studies, should also be considered. Vogler et al. found two clades or sibling species coexisting in the same area at Palau Sebibu, UEA, and Oman. These findings are important for understanding how these species coexist and avoid genetic mixing. A. planci is a generalist, feeding on many coral species, reproducing in summer without a set spawning pattern, and often participating in large spawning events that trigger others to spawn. It is challenging to explain how two A. planci species could coexist without competition or genetic mixing, especially since they share the same habitat.

Biology

• Broken and regenerating spines
• Swollen right hand after being punctured
• Frothing in water containing A. planci
• Starfish handled carefully to avoid damaging it (spines on the underside are blunt)

Starfish have special chemicals in their tissues called asterosaponins. These chemicals are found in different forms, and scientists have studied them in at least 15 research projects. The chemicals act like detergents, and when starfish are kept in small amounts of water with air bubbles, large amounts of foam form on the surface.

A. planci does not have a way to inject its toxin. However, when its spines pierce the skin of a predator or person, the saponins in the tissue enter the wound. In humans, this causes a sharp, stinging pain that lasts several hours, bleeding because the saponins break down blood cells, and swelling and nausea that may last a week or more. The spines, which are fragile, may also break off and get stuck in the skin, requiring surgery to remove.

Saponins are present throughout the life of the crown-of-thorns starfish. The saponins in the eggs are similar to those in adult tissues, and likely pass to the larvae. Observations of predators rejecting juvenile starfish suggest that the juveniles also contain saponins.

• Juveniles hidden under coral rubble
• Two starfish feeding on a coral, leaving white feeding scars
• Feeding on branching Acropora coral
• Starfish competing for remaining live coral

Adult crown-of-thorns starfish are predators that eat coral polyps, other hard corals, and dead animals. They climb onto a section of a living coral colony using many tube feet located in special grooves on their body. They press closely to the coral surface, even on complex, branching corals. Then, they push their stomach out through their mouth over the coral surface. The stomach releases enzymes that break down the coral tissue, allowing the starfish to absorb nutrients. This leaves a white scar on the coral that quickly grows algae. One starfish can eat up to 6 square meters (65 square feet) of coral each year. Studies on two coral reefs showed that large starfish (40 cm or more in size) can kill about 61 cm (9 inches) of coral per day in winter and 357 to 478 cm (55 to 74 inches) per day in summer. Smaller starfish (20–39 cm or 8–15 inches) kill 155 to 234 cm (24 to 36 inches) of coral per day in the same seasons. The area killed by large starfish is about 10 square meters (108 square feet) per year. Differences in feeding and movement rates between seasons are due to the starfish being cold-blooded, meaning their body temperature and energy use depend on the water temperature. In tropical reefs, starfish move at about 35 cm (14 inches) per minute, which explains how they can damage large areas of reef quickly.

Starfish prefer certain types of coral. They often eat branching or flat corals, such as Acropora, Pavona, and Pocillopora, but avoid rounded corals like Porites that have less exposed surface. They may also avoid Porites and other corals because of bivalve mollusks and worms living on the coral surface, which repel the starfish. Small crabs living in the structures of branching corals may also keep starfish away by preventing them from spreading their stomach over the coral.

In areas with low coral cover, starfish may eat soft corals (Octocorallia).

Starfish are hidden during their first two years, feeding at night. As adults, they stay hidden unless they are in groups. Signs of a hidden starfish may be white scars on nearby coral. However, their behavior changes in two situations:

• During the breeding season, usually in early to midsummer, starfish gather on reefs and release eggs and sperm at the same time to increase the chance of fertilization. This behavior is common among marine animals that do not mate. Spawning alone would waste eggs, and evidence suggests a chemical signal causes starfish to gather and release gametes together.
• When starfish are very common, they may move day and night, competing for coral.

A. planci eats coral quickly, especially when their numbers are high, which harms reef habitats and reduces the variety of species. Studies show that when large numbers of these starfish are present, they can destroy coral and change reef structures, affecting the marine life that depends on the reef.

The long, sharp spines covering the upper body of A. planci protect it from large predators. The spines also have a chemical defense. When the spines pierce a predator or human, the saponins cause irritation, similar to how they affect human skin. Saponins also taste bad. A study found that fish often bite Acanthaster juveniles, taste them, and reject them. These defenses make the starfish less appealing to predators. Despite this, some Acanthaster populations have individuals with regenerating arms.

About 11 species are known to occasionally eat healthy adult A. planci. These are generalist feeders, but none prefer the starfish as a main food source. Some of these predators include:

• A pufferfish and two triggerfish in the Red Sea have been seen eating A. planci, but there is no evidence they significantly control its population. In the Indo-Pacific, white-spotted puffers and Titan triggerfish also eat the starfish.
• Triton's trumpet, a large sea snail, is a known predator of Acanthaster in some areas. It uses a file-like structure to tear the starfish apart.
• The small painted shrimp Hymenocera picta and a polychaete worm, Pherecardia striata, have been observed eating A. planci on some reefs. About 0.6% of starfish in these areas were attacked by both the shrimp and worm, leading to their death in about a week. This may help balance the starfish population.
• Pherecardia striata only attacks injured A. planci, so it may be considered a scavenger rather than a predator. Dead or damaged starfish attract other scavengers, including two polychaete worms, a hermit crab, a sea urchin, and seven species of small reef fish.

Lifecycle

• A stained cross-section of a mature ovary filled with eggs
• A stained cross-section of a testis (sperm appear blue)
• Spawning
• Early cell divisions in fertilized eggs, about 0.3 mm in diameter
• Free-living gastrula stage, about 0.5 mm long

As the starfish grow and become sexually mature, their gonads (reproductive organs) increase in size and fill their arms, extending into the disk region. Mature ovaries and testes are easily identified, with ovaries appearing more yellow and having larger lobes. When viewed in cross-section, ovaries are densely packed with nutrient-rich eggs, while testes are densely filled with sperm, which consist mainly of a nucleus and a flagellum. The number of eggs a female starfish can produce depends on her size. For example:

• A female with a 200-mm-diameter body produces 0.5–2.5 million eggs, which make up 2–8% of her wet weight.
• A female with a 300-mm-diameter body produces 6.5–14 million eggs, which make up 9–14% of her wet weight.
• A female with a 400-mm-diameter body produces 47–53 million eggs, which make up 20–25% of her wet weight.

In the Philippines, female starfish have been found with a gonadosomatic index (the ratio of gonad mass to body mass) as high as 22%, showing their high egg production. A study by Babcock et al. (1993) observed changes in egg production and fertility (fertilization rates) during the spawning season of the crown-of-thorns starfish on Davies Reef, Great Barrier Reef, from 1990 to 1992. Spawning occurred from December to January (early to midsummer), with most observations in January. However, both the gonadosomatic index and fertility peaked early in the season and declined by late January, indicating that most successful reproduction happened early in the spawning period. In the Northern Hemisphere, starfish reproduce in April and May, and spawning has also been observed in the Gulf of Thailand in September. High egg fertilization rates may occur when starfish spawn closely together and at the same time. Spawning is influenced by environmental factors, such as changes in water temperature and lunar events.

Embryonic development begins about 1.5 hours after fertilization, with early cell divisions (cleavage). By 8–9 hours, the embryo reaches the 64-cell stage.

Some molecular and histological evidence suggests that hermaphroditism (the ability to produce both eggs and sperm) may occur in Acanthaster cf. solaris.

• Bipinnaria larva
• Scanning electron micrograph (SEM) of bipinnaria larva
• Brachiolaria larva
• Late brachiolaria with starfish primordium
• SEM image of brachiolarian arms

By day 1, the embryo hatches as a ciliated gastrula stage. By day 2, the gut is complete, and the larva is now called a bipinnaria. It has ciliated bands along its body, which it uses to swim and filter feed on microscopic particles, especially unicellular green flagellates (phytoplankton). The SEM image shows the detailed ciliated bands of the bipinnaria larva. By day 5, the larva becomes an early brachiolaria. The arms of the bipinnaria grow longer, and two stump-like projections appear at the front (not visible in the photograph). Structures begin to develop in the rear of the larva. By day 11, the larva is a late brachiolaria. Its arms are elongated, with three distinct arms at the front and small structures on their inner surfaces. The larva was previously transparent, but the rear now becomes opaque as the starfish begins to develop. The late brachiolaria is 1.0–1.5 mm long. It tends to sink to the bottom and test the substrate with its brachiolar arms, including bending the front body to position the

Ecology

A. planci is one of the most effective predators of scleractinian corals (stony corals or hard corals). Many coral-eating animals only harm small areas of coral, but adult A. planci can destroy entire coral colonies.

In 1960–1965, marine scientist Robert Endean observed very high numbers of A. planci on the Great Barrier Reef. This, along with a 1970s book titled Requiem for the Reef, which suggested that the damage caused by A. planci was being hidden, led to concerns that the starfish were destroying coral and entire reefs. However, A. planci does not destroy coral by breaking it apart. Instead, it eats the living tissue on the surface of coral skeletons. These skeletons remain, along with coralline algae, which are important for reef stability. The first noticeable change is the loss of the thin layer of living coral tissue.

A. planci is found on most coral reefs, and its impact depends on how many starfish are present. At low numbers (1 to about 30 per hectare), the rate at which A. planci eats coral is slower than the rate at which coral grows. This means the surface area of living coral increases over time. However, A. planci can still affect the types of coral and their sizes. For example, reefs with A. planci may have different patterns of coral species and colony sizes compared to reefs without the starfish.

Some scientists believe A. planci plays a role in keeping coral reef biodiversity healthy. Before overpopulation became a problem, A. planci helped prevent fast-growing corals from outcompeting slower-growing ones.

At high numbers (called outbreaks or plagues), A. planci can cause coral cover to decrease. When there are too many starfish, they must eat different types of coral, including those they prefer less. During these outbreaks, starfish often gather in large groups, and the areas they clear of coral become nearly continuous. These large areas of damaged coral lead to several effects:

• Bare coral skeletons are quickly covered by filamentous algae.
• Large groups of staghorn coral (Acropora species) may collapse into rubble, reducing the reef’s complexity.
• Sometimes, macroalgae, soft corals, and sponges take over the damaged areas. These organisms can stay on the reef for long periods, making it harder for hard corals to grow back.

Even though the reef surface may look less attractive than living coral, it is not completely dead.

A third effect occurs when filamentous algae invade the reef. Animals that rely on hard corals for food or shelter may decline, while herbivores and other less specialized feeders may increase. This change is most noticeable in fish populations, and long-term studies of reef fish support this pattern.

Large A. planci populations, sometimes called plagues, were recorded at 21 reef locations between the 1960s and 1980s. These locations ranged from the Red Sea to French Polynesia. At least two major outbreaks were confirmed at 10 of these locations.

Starfish densities from 140 to 1,000 per hectare are considered outbreak levels, while densities below 100 per hectare are considered low. However, even at low densities, A. planci may still cause a net loss of coral.

Surveys of many reef locations show that large numbers of Acanthaster spp. can be grouped into three categories:

• Primary outbreaks, where starfish populations increase rapidly without a clear connection to previous outbreaks.
• Secondary outbreaks, which may follow earlier outbreaks through the reproduction of previous starfish populations. These can appear as new starfish on reefs downstream from existing outbreaks.
• Chronic situations, where starfish remain at moderate to high numbers on reefs with little coral due to ongoing feeding.

The Great Barrier Reef (GBR) is the world’s most famous coral reef system because of its length, number of reefs, and variety of species. In the 1960s–1965, high numbers of Acanthaster were first observed near Green Island, off Cairns, causing concern. Later, high-density populations were found on many reefs to the south of Green Island in the central GBR region. Some publications claimed the entire reef was at risk of dying, which increased public worry about the GBR’s future.

Studies have modeled Acanthaster outbreaks on the GBR to better understand the phenomenon.

During times of high concern about Acanthaster outbreaks, the Australian and Queensland governments funded research and formed advisory committees. Scientists were criticized for not providing clear answers about the causes of the outbreaks, leading to disagreements sometimes called the “starfish wars.”

Serious discussions about the causes of Acanthaster outbreaks include the “predator removal hypothesis,” which suggests that changes in the survival of juvenile and adult starfish may be linked to:

• Overharvesting of tritons, a predator of Acanthaster.
• Overfishing of predators like the humphead wrasse.
• Declines in predator populations due to habitat loss.
• Warmer ocean temperatures that help starfish larvae develop.
• Human activities, such as nutrient runoff from land.

Reports about fish eating Acanthaster are often based on single observations or assumptions. For example, the humphead wrasse may eat Acanthaster as part of its diet, but there is no evidence that it controls starfish populations. Studies of the stomach contents of large predatory fish found no evidence of Acanthaster in their guts.

One challenge with the idea that predators control Acanthaster populations is that starfish can regenerate lost body parts and avoid being eaten. Many Acanthaster have missing or regenerating arms, showing they often survive partial attacks. If a large part of the starfish’s body is damaged, it may not fully regenerate.

Another idea is the “aggregation hypothesis,” which suggests that large groups of Acanthaster appear to be outbreaks because they have eaten all the nearby coral. This implies that the starfish may gather in large numbers after consuming surrounding coral.

Population control

Population numbers of crown-of-thorns starfish have grown since the 1970s. However, records about their past numbers and locations are limited because SCUBA technology, needed to count them underwater, was developed only in the decades before.

To stop crown-of-thorns starfish from damaging coral reefs, humans have used several methods. Removing them by hand works but requires a lot of effort. Injecting sodium bisulfate into the starfish is the most effective method. This substance kills the starfish without harming the reef or ocean life. Divers in areas with many starfish can kill up to 120 per hour. Another method, cutting the starfish into pieces, only kills about 12 per hour. This method is discouraged because it is less efficient, not because of myths about the starfish regrowing.

Another method, burying the starfish under rocks or debris, is more time-consuming but safer for divers. It is only used in areas with few starfish and where materials to cover them are available without harming coral.

A 2015 study by James Cook University found that household vinegar can also kill starfish. The acid in vinegar causes the starfish to break down within days. Vinegar is safe for the environment and not restricted by rules about animal products. In 2019, divers used a 10% vinegar solution to reduce starfish numbers in the Raja Ampat Islands.

A new method involves injecting a substance called thiosulfate-citrate-bile salts-sucrose agar (TCBS) into the starfish. One injection causes the starfish to die in 24 hours from a disease that harms their skin and body. Symptoms include discolored skin, open sores, and loss of body structure.

A robot named COTSBot has been created to kill starfish. It uses a camera system to find them and injects bile salts to kill them. After the robot reduces starfish numbers, divers can remove the remaining ones. Testing of the robot began in Moreton Bay in Brisbane in 2015 to improve its movement. The robot will be used on coral reefs once its navigation is fully tested.

In Indonesia, researchers are studying whether crushed starfish remains can be used as a supplement in food for whiteleg shrimp.