The crown-of-thorns starfish, also known as Acanthaster planci, is a large starfish that eats hard coral polyps, which are part of the group called Scleractinia. This starfish gets its name from the sharp, spiky spines on its top surface, which look like thorns. It is one of the largest starfish in the world.

Acanthaster planci is found in many areas across the Indo-Pacific region. It is most commonly seen near Australia but can also be found in tropical and subtropical areas, such as the Red Sea, along the East African coast, throughout the Indian Ocean, and across the Pacific Ocean to the west coast of Central America. This starfish 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 special features, such as a disc-like shape, many arms, flexibility, the ability to grip, and a large stomach surface compared to its body size. Its ability to grip comes from two rows of tube feet that reach the tips of each arm. Unlike most starfish, which have five arms and five-part symmetry, the crown-of-thorns starfish has up to 21 arms and lacks this symmetry. It begins its life with five-part symmetry but loses it as it grows. This starfish has true vision that forms clear images.

Adult crown-of-thorns starfish are usually 25 to 35 cm (10 to 14 in) long and can have up to 21 arms. Although their bodies appear stiff, they can bend and twist to move around coral. Each arm has plates on the underside that form grooves leading to the mouth. Their color depends on diet and location, and may include purple, purple-blue, reddish grey, brown with red spines, or green with yellow spines.

The long, sharp spines along the arms and upper surface of the starfish create a crown-like shape, which gives it its name. These spines are 4 to 5 cm long, very sharp, and can pierce soft materials. Despite having sharp spines on the upper surface and blunt spines on the lower surface, the starfish’s body is soft and covered in a thin membrane. If the starfish is taken out of water, its body membrane breaks, causing fluid to leak out and the body to flatten. 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 has 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 other groups. 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 family from the Spinulosida group to the Valvatida group and placed it near the Oreasteridae, suggesting it may have evolved from that group. He linked the physical traits of Acanthaster 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 wave action is minimal.

A. planci has a long history in scientific studies, with much confusion about its name from the beginning. Many complex synonyms exist for its name. Georg Eberhard Rhumphius first described it in 1705, naming it Stella marina quindecium radiotorum. Later, Carl Linnaeus described it as Asterias planci based on an illustration by Plancus and Gualtieri (1743) when he introduced the system of binomial nomenclature. No type specimens are known; the specimen described by Plancus and Gualtieri no longer exists.

Other names used for the crown-of-thorns starfish included Stellonia, Echinaster, and Echinites, before settling on Acanthaster (Gervais 1841). Specific names included echintes, solaris, mauritensis, ellisii, and ellisii pseudoplanci (with subspecies). Most of these names arose from confusion in historical records, but Acanthaster ellisii was used for the distinctive starfish in the eastern Pacific Gulf of California.

The eastern Pacific Acanthaster is very distinctive, with a plump body, a large disc-to-total diameter ratio, and short, blunt spines.

Nishida and Lucas studied genetic differences at 14 genetic markers in 10 population samples of A. planci across the Pacific, including the Ryukyu Archipelago, Micronesia, the Great Barrier Reef, Fiji, Hawaii, and the Gulf of California. A sample of 10 specimens of A. brevispinus from the Great Barrier Reef region was included for comparison. Large genetic differences were found between A. brevispinus and A. planci (D = 0.20 ± 0.02). However, genetic differences between A. planci populations were small (D = 0.03 ± 0.00; Fsr = 0.07 ± 0.02) despite the long distances between them. A positive link was observed between genetic differences and geographic distance, suggesting that A. planci populations remain genetically similar due to gene flow from planktonic larvae. The effect of distance on genetic differences likely reflects reduced larval dispersal over long distances. Despite overall genetic similarity, significant differences were seen between nearby populations separated by about 10 km. The Hawaiian population was most different from others. Treating the morphologically distinct 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 discussed in the entry for A. brevispinus. These species are clearly closely related, and A. planci, the coral-feeding species, is suggested to have evolved from A. brevispinus, the soft-bottom species.

In a comprehensive study, Benzie examined genetic variation in 20 populations of A. planci across the Pacific and Indian Oceans. A major finding was a clear difference between Indian and Pacific Ocean 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, patterns within regions matched expectations of isolation by distance. Again, the pattern of reduced larval dispersal over long distances was observed. Benzie suggests the split between Indian and Pacific populations began at least 1.6 million years ago and may reflect changes in climate and sea level.

A more recent study by Vogler et al. used DNA analysis (one mitochondrial gene) to suggest A. planci is a group of four species or clades, defined by geography: Northern Indian Ocean, southern Indian Ocean, Red Sea, and Pacific Ocean. These data suggest the groups diverged 1.95 and 3.65 million years ago. (The divergence of A. planci and A. brevispinus is not included in this timeline.) The authors suggest differences in behavior, diet, or habitat between the groups may influence reef-conservation strategies.

However, the idea of cryptic speciation (cryptic species) has issues. The use of only one mitochondrial gene (mtDNA) as the sole basis for identifying species is disputed. Allozyme data should also be considered. Three locations sampled by Vogler et al.—Palau Sebibu, UEA, and Oman—showed two clades/sibling species living together. These cases are important for studying how these species coexist and avoid mixing genetic material. A. planci is a generalist, feeding on most hard coral species, reproducing during summer without a spawning pattern, and often participating in mass spawning events that trigger spawning in others. Considering two A. planci clades living together without habitat competition or genetic mixing is difficult.

Biology

• Broken and regenerating spines
• Swollen right hand after being pierced
• Frothing in water with A. planci
• Starfish handled carefully to avoid harm (spines on the underside are not sharp)

Starfish have special chemicals called asterosaponins in their bodies. These chemicals are found in many different forms, and scientists have studied them in at least 15 experiments. These 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, but when its spines pierce the skin of a predator or person, the tissue containing the saponins enters the wound. In humans, this causes a sharp, stinging pain that lasts for several hours, bleeding that continues for a long time due to the saponins’ effect on blood cells, and swelling and nausea that may last up to a week. 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 hiding under coral rubble
• Two starfish feeding on a coral, leaving white scars
• Feeding on branching Acropora coral
• Starfish competing for 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, which are found in special grooves on the starfish’s body. They press closely against the coral, even on complex surfaces like branching corals. Then, they push their stomach out through their mouth over the coral’s surface. The stomach releases enzymes that break down the coral tissue, allowing the starfish to absorb nutrients. This process leaves a white scar on the coral, which quickly grows algae. One starfish can eat up to 6 square meters of coral each year. Studies on two reefs in the Great Barrier Reef showed that large starfish (40 cm or larger) killed about 61 cm of coral per day in winter and 357 to 478 cm per day in summer. Smaller starfish (20–39 cm) killed 155 to 234 cm of coral per day in the same seasons. The amount of coral killed by large starfish is about 10 square meters. Differences in feeding and movement rates between summer and winter 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 per minute, which explains how they can damage large areas of reef quickly.

Starfish prefer to feed on branching and table-like corals, such as Acropora, Pavona, and Pocillopora, rather than rounded corals like Porites. They may avoid Porites and some other corals because of bivalve mollusks and worms living on the coral’s surface, which repel the starfish. Small crabs living in branching corals may also keep starfish away by blocking their ability to spread their stomach over the coral.

In areas with few hard corals, starfish may eat soft corals (Octocorallia) instead.

Starfish are usually hidden during their first two years, coming out at night to feed. As adults, they stay hidden when alone. The only sign of a hidden starfish may be white scars on nearby coral. However, their behavior changes in two situations:

• During the breeding season, which happens in early to midsummer, starfish gather on reefs and release eggs and sperm at the same time to increase the chance of fertilization. This synchronized behavior is common in many marine animals that do not mate. Solitary spawning would not allow eggs to be fertilized, and evidence suggests a chemical signal causes starfish to gather and release gametes together.
• When starfish are very dense, they may move day and night, competing for living coral.

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

The long, sharp spines on A. planci’s body protect it from large predators. These spines also contain saponins, which irritate predators when the spines pierce them, just as they irritate human skin. Saponins also taste bad, which may cause fish to reject the starfish. A study found that fish often tasted A. planci’s juveniles and then spit them out. These defenses make the starfish less appealing to predators. Despite this, A. planci populations often include individuals with arms that are regenerating.

About 11 species have been reported to occasionally eat healthy A. planci adults. These animals are not specialized to eat starfish but may hunt them. Some of these predators include:

• A type of pufferfish and two types of triggerfish in the Red Sea have been seen eating A. planci, but there is no evidence they control starfish populations. In the Indo-Pacific, white-spotted puffers and Titan triggerfish also eat A. planci.
• Triton’s trumpet, a large sea snail, eats A. planci in some areas. It uses a file-like structure to tear the starfish apart.
• The small painted shrimp Hymenocera picta eats A. planci in some places. A polychaete worm, Pherecardia striata, was seen eating A. planci with the shrimp on an east Pacific reef. About 0.6% of starfish in that area were attacked by both the shrimp and worm, leading to their deaths in about a week. This balance between deaths and new starfish may keep their population stable.
• Pherecardia striata only attacks damaged A. planci, so it may be considered a scavenger rather than a predator. Dead or injured A. planci attract scavengers like polychaete worms, hermit crabs, sea urchins, and small reef fish.

Lifecycle

• A stained cross-section showing a mature ovary filled with eggs
• A stained cross-section of a testis, with sperm appearing blue
• The process of releasing eggs and sperm into the water
• Early cell divisions in fertilized eggs, each about 0.3 mm in size
• A free-swimming gastrula stage, about 0.5 mm long

As the starfish grow and become sexually mature, their gonads (reproductive organs) grow larger and fill their arms and the disk-shaped body. The ovaries and testes can be easily seen and differ in appearance. Ovaries are usually more yellow and have larger sections, while testes are filled with sperm, which are mostly made of a nucleus and a tail. Female starfish produce more eggs as they grow larger. For example:

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

In the Philippines, female starfish have been found with a gonadosomatic index (the ratio of gonad weight to body weight) as high as 22%, showing how much energy they use for reproduction. Studies from 1990 to 1992 on Davies Reef showed that starfish spawn (release eggs and sperm) between December and January, with most activity in January. However, their reproductive success and fertility (ability to fertilize eggs) were highest early in the season and declined later. In the Northern Hemisphere, starfish reproduce in April and May, and in the Gulf of Thailand, they spawn in September. High egg fertilization rates may happen when starfish spawn at the same time and place. Spawning is influenced by environmental factors like water temperature and the moon’s phases.

After fertilization, the embryo begins to develop about 1.5 hours later, with early cell divisions (cleavage). By 8–9 hours, the embryo has 64 cells.

Some evidence suggests that the starfish species Acanthaster cf. solaris may be hermaphrodites (able to produce both eggs and sperm).

• A bipinnaria larva
• A scanning electron micrograph of a bipinnaria larva
• A brachiolaria larva
• A late brachiolaria with early signs of a starfish body
• A scanning electron micrograph of brachiolaria arms

By day 1, the embryo becomes a ciliated gastrula, which can swim. By day 2, the larva is called a bipinnaria and uses ciliated bands to swim and eat tiny algae. By day 5, it becomes a brachiolaria larva, with longer arms and structures forming in the back. By day 11, the larva is about 1.0–1.5 mm long and has three arms at the front. It begins to sink and test the ocean floor with its arms.

This description is based on laboratory studies under ideal conditions. However, real-world conditions can affect growth and survival.

• A brachiolaria larva settling on the ocean floor
• A five-armed juvenile starfish after changing from a larva
• A young starfish eating coralline algae, leaving white scars
• A very young starfish with all arms and a structure called a madreporite
• A young starfish feeding on coral

The brachiolaria larva searches for surfaces to settle on, often choosing coralline algae. After metamorphosis, the larva becomes a five-armed juvenile starfish, about 0.4–1.0 mm in size. These juveniles eat thin layers of algae on dead coral and other surfaces. They digest the algae by extending their stomach over the surface. The algae are pink to dark red, and feeding leaves white scars. Over months, the juveniles grow, adding arms and structures called madreporites until they reach adult size in 5–7 months. When they are ready, they begin eating hard corals like Pocillopora damicornis and Acropora acunimata.

Juveniles that start eating coral were raised in a controlled environment with plenty of food. Their growth followed an S-shaped curve, common in many marine animals. They grew slowly while eating algae, then quickly once they started eating coral, reaching sexual maturity at about 2 years old (around 200 mm in size). They continued growing rapidly, reaching about 300 mm, but growth slowed after 4 years. Gonads developed more strongly after 3 years, and spawning followed a seasonal pattern linked to water temperature. Most starfish died from old age between 5.0–7.5 years, losing their ability to eat properly.

These findings come from laboratory studies, which are easier to conduct than field studies. However, they match limited observations of starfish life cycles in nature.

In the wild, young starfish (less than 20 mm in size) were found on coralline algae (Porolithon onkodes) on Suva Reef in Fiji. They lived in hidden places, such as under coral rubble, on dead coral bases, and in narrow reef areas. Growth rates there were 2.6, 16.7, and 5.3 mm per month before they started eating coral.

Ecology

A. planci is one of the most effective predators of scleractinian corals, which are also called stony corals or hard corals. Most animals that eat coral only harm small parts of the coral, but adult A. planci can destroy entire coral colonies.

In the 1960s, marine scientist Robert Endean observed very high numbers of A. planci on the Great Barrier Reef. This, along with a book from the 1970s called Requiem for the Reef, which suggested that the damage caused by A. planci was being hidden, led people to worry that the coral and the reefs might be destroyed. However, A. planci eats coral by breaking down the living tissue on the coral’s surface. The coral skeletons remain, along with coralline algae, which are important for the reef’s structure. The first change caused by A. planci is the loss of the thin layer of living coral tissue.

A. planci is found on most coral reefs, and its impact on reefs depends on how many starfish are present. At low numbers (1 to about 30 per hectare), the coral grows faster than A. planci can eat it, so the area covered by living coral increases. However, A. planci may still influence the types and sizes of coral that grow on a reef. This can be seen by comparing reefs where A. planci is absent to those where it is present.

Some scientists believe A. planci plays an important role in keeping coral reef biodiversity healthy and helping ecosystems change over time. Before A. planci became too common, it helped prevent fast-growing coral species from taking over slower-growing ones.

At very high numbers (called outbreaks or plagues), A. planci can cause coral cover to drop. When there are too many starfish, they must eat different types of coral, and large areas of coral may be completely cleared. These large areas of damaged coral lead to changes such as:
– Bare coral skeletons quickly become covered by filamentous algae.
– Large coral structures, like Acropora species, may collapse into rubble, reducing the reef’s complexity.
– Sometimes, other organisms like macroalgae, soft corals, and sponges take over the reef surfaces, making it harder for hard corals to grow back.

Although these changes make the reef less visually appealing, the reef is not completely dead.

A third change happens when filamentous algae take over. Animals that depend on hard corals for food or shelter may decline, while animals that eat plants or are less picky about food may increase. This is especially noticeable in fish populations, and long-term studies of reef fish support this.

Large groups of A. planci have been recorded at 21 reef locations between the 1960s and 1980s. These locations range from the Red Sea to French Polynesia. At least two major outbreaks were confirmed at 10 of these locations.

Starfish numbers between 140 and 1,000 per hectare are considered outbreak levels, while numbers below 100 per hectare are considered low. However, even at low numbers, A. planci may eat more coral than it grows, causing a net loss.

Surveys of many reef locations show that large numbers of Acanthaster spp. can be grouped into:
– Primary outbreaks, where starfish populations increase rapidly without being linked to previous outbreaks.
– Secondary outbreaks, which may be caused by starfish from a previous group moving to new areas.
– Chronic situations, where starfish remain at high numbers on reefs with little coral due to ongoing feeding.

The Great Barrier Reef (GBR) is the world’s largest and most diverse coral reef system. When high numbers of Acanthaster were first seen near Green Island in 1960–1965, it caused concern. Later, high numbers were found on many reefs in the central GBR. Some books suggested the entire reef might die, which increased public worry about the reef’s future.

Many studies have tried to understand the causes of A. planci outbreaks on the GBR. During this time, Australian and Queensland governments funded research and created advisory groups. Scientists were criticized for not providing clear answers, and disagreements about the causes of outbreaks were common, sometimes called the “starfish wars.”

Some theories suggest that changes in the survival of young or adult A. planci may cause outbreaks. These include:
– Overfishing of predators like tritons or Humphead wrasse.
– Habitat destruction reducing predator numbers.
– Warmer ocean temperatures helping starfish larvae grow.
– Human activities, such as adding nutrients to the water.

Reports about fish eating A. planci are often based on single observations or assumptions. For example, Humphead wrasse may eat starfish, but no evidence shows they control starfish populations. Studies of fish stomachs in the Gulf of Oman found no starfish remains.

A problem with the idea that predators kill A. planci is that the starfish can regenerate lost body parts and avoid being eaten. Many A. planci have missing or regenerating arms, showing they survive attacks. If a starfish loses a large part of its body, it may not fully recover.

Another theory is the “aggregation hypothesis,” which suggests that large groups of A. planci appear as outbreaks because they have eaten all the nearby coral.

Population control

Population numbers of crown-of-thorns starfish have been growing since the 1970s. However, historical information about their numbers and where they lived is limited. This is because SCUBA technology, which is needed to count them underwater, was developed only in the previous few decades.

To stop crown-of-thorns starfish from overpopulating and damaging coral reefs, humans have used several methods. Manually removing them works, but it requires a lot of effort. Injecting sodium bisulfate into the starfish is the most effective method. Sodium bisulfate kills the starfish but does not harm the reef or ocean ecosystems. In areas with many starfish, teams of divers have killed up to 120 starfish per hour per diver. Another method, cutting the starfish into pieces, only killed 12 per hour per diver. This method is discouraged because it is less efficient, not because of myths about the starfish regrowing.

Another method is burying the starfish under rocks or debris. This is only used in areas with few starfish and when materials are available without harming corals.

A 2015 study by James Cook University found that household vinegar is also effective. The acid in vinegar causes the starfish to break down within days. Vinegar is safe for the environment and not limited 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 thiosulfate-citrate-bile salts-sucrose agar (TCBS) into the starfish. One injection causes the starfish to die in 24 hours from a disease that leads to discolored and necrotic skin, sores, loss of spines, and open wounds exposing internal organs.

An autonomous robot called COTSBot was developed to kill starfish. It uses a vision system to find starfish and injects them with bile salts. After the robot reduces starfish numbers, divers can remove the remaining ones. Field tests of the robot began in Moreton Bay in 2015 to improve its navigation. The robot will be used on the Great Barrier Reef once its navigation is refined.

Research in Indonesia is exploring the use of ground crown-of-thorns starfish remains as a supplement in feed for whiteleg shrimp.